THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 


W- 


fit- 


SYSTEM 


OF 


DISEASES  OF  THE  EYE, 


BY 


AMERICAN,   BRITISH,   DUTCH,   FRENCH, 
GERMAN,  AND  SPANISH   AUTHORS. 


EDITED    BY 

WILLIAM  F.JJORRIS,  A.M..  M.D..  AND  CHARLES  A.  OLIVER,  A.M.,  M.D. 

OF    PHILADELPHIA,    PA.,    U.S.A. 


VOLUME  I. 
EMBRYOLOGY,  ANATOMY,  AND  PHYSIOLOGY  OF  THE  EYE. 


WITH    TWENTY-THREE    FULL-PAGE    PLATES    AND    THREE     HUNDRED    AND 
SIXTY-TWO    TEXT  ILLUSTRATIONS. 


PHILADELPHIA  : 

J.  B.  LIPPINCOTT    COMPANY. 

1897. 


COPYRIGHT,  1896, 

BY 
J.  B.  LIPPINCOTT  COMPANY. 


PRINTED  BY  J.  B.  LIPPINCOTT  COMPANY,  PHILADELPHIA,  U.S.A. 


100 


PREFACE. 


THIS,  the  first  "  System  of  Diseases  of  the  Eye"  written  in  the  English 
language,  embraces  the  most  advanced  theoretical  and  practical  views  on 
the  subject  that  could  be  systematically  grouped  in  a  single  publication. 

The  editors  believe  that,  by  a  careful  selection  of  material  and  by  the 
aid  of  many  and  able  collaborators,  a  work  has  been  produced  which  will 
take  a  place  in  the  English  language  similar  to  that  occupied  by  the 
"Handbuch"  of  Graefe  and  Saemisch  in  German  and  by  the  "TraitS 
complet"  of  de  "Wecker  and  Landolt  in  French,  and  which  will  be  of 
service  not  only  to  ophthalmologists  and  special  students,  but  also  to  the 
medical  profession  at  large. 

It  is  with  regret  that  they  make  note  of  the  death  of  their  first  con- 
tributor, Dr.  John  A.  Ryder.  To  Dr.  John  Green,  of  St.  Louis,  they 
are  under  obligation  for  much  kindness  and  assistance. 


iii 


CONTRIBUTORS  TO  VOLUME  I. 


JOHN  A.  RYDER,  Pn.D PHILADELPHIA,  PA.,  U.S.A. 

THOMAS  DWIGHT,  M.D.,  LL.D BOSTON,  MASS.,  U.S.A. 

FRANK  BAKER,  M.D.,  Pn.D WASHINGTON,  B.C.,  U.S.A. 

GEORGE   A.  PIERSOL,  M.D PHILADELPHIA,  PA.,  U.S.A. 

ALEX  HILL,  M.A.,  M.D CAMBRIDGE,  ENGLAND. 

WILLIAM   LANG,  F.R.C.S.E LONDON,  ENGLAND. 

E.  TREACHER  COLLINS,  F.R.C.S.E LONDON,  ENGLAND. 

EDWARD  JACKSON,  A.M.,  M.D PHILADELPHIA,  PA.,  U.S.A. 

J.  McKEEN  CATTELL,  Pn.D NEW  YORK  CITT,  N.Y.,  U.S.A. 

EUGEN  BRODHUN,  M.D BERLIN,  GERMANY. 

WILLIAM  THOMSON,  M.D PHILADELPHIA,  PA.,  U.S.A. 

CARL  MAYS,  M.D HEIDELBERG,  GERMANY. 


ASSISTANT  CONTRIBUTOR. 


CARL   WEILAND,   M.D PHILADELPHIA,  PA.,  U.S.A. 


TRANSLATORS. 


CHRISTINE   LADD  FRANKLIN BALTIMORE,  MD.,  U.S.A. 

JAMES  A.  SPALDING,  A.M.,  M.D PORTLAND,  ME.,  U.S.A. 


CONTENTS  OF  VOLUME  I. 


EMBRYOLOGY,    ANATOMY,    AND   PHYSIOLOGY   OF 

THE    EYE. 

PAOB 

DEVELOPMENT  OF  THE  EYE.  By  JOHN  A.  KTDER,  Ph.D.,  Philadelphia, 
Pa.,  U.S.A.,  Professor  of  Comparative  Embryology  in  the  University  of  Penn- 
sylvania   7 

THE  ANATOMY  OF  THE  ORBIT  AND  THE  APPENDAGES  OF  THE 
EYE.  By  THOMAS  DWIQHT,  M.D.,  LL.D.,  Boston,  Mass.,  U.S.A.,  Parkman 
Professor  of  Anatomy  at  Harvard  University 69 

THE  ANATOMY  OF  THE  EYEBALL  AND  OF  THE  INTRA-ORBITAL 
PORTION  OF  THE  OPTIC  NERVE.  By  FRANK  BAKER,  M.D.,  Ph.D., 

Washington,  D.C.,  U.S.A.,  Professor  of  Anatomy  in  the  University  of  George- 
town ;  Honorary  Curator  of  Anatomy  in  the  U.S.  National  Museum 109 

THE  MICROSCOPICAL  ANATOMY  OF  THE  EYEBALL.  By  GEORGE 
A.  PIERSOL,  M.D.,  Philadelphia,  Pa.,  U.S.A.,  Professor  of  Anatomy  in  the 
University  of  Pennsylvania 217 

ANATOMY  OF  THE  INTRA-CRANIAL  PORTION  OF  THE  VISUAL 
APPARATUS.  By  ALEX  HILL,  M.A.,  M.D.,  Cambridge,  England,  Master 
of  Downing  College ;  late  Hunterian  Professor  at  the  Koyal  College  of  Surgeons 
of  England 383 

CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE 
HUMAN  EYE.  By  WILLIAM  LANG,  F.K.C.S.E.,  London,  England,  Sur- 
geon to  the  Royal  London  Ophthalmic  Hospital ;  Ophthalmic  Surgeon  to  and 
Lecturer  on  Ophthalmology  at  the  Middlesex  Hospital;  and  E.  TREACHER 
COLLINS,  F.K.C.S.E.,  London,  England,  Curator  and  Librarian  to  the  Eoyal 
London  Ophthalmic  Hospital 417 

THE  DIOPTRICS  OF  THE  EYE.  By  EDWARD  JACKSON,  A.M.,  M.D.,  Phila- 
delphia, Pa.,  U.S.A.,  Professor  of  Diseases  of  the  Eye  in  the  Philadelphia  Poly- 
clinic  ;  Special  Lecturer  on  Physiological  Optics  in  the  University  of  Colorado  .  459 

THE  PERCEPTION  OF  LIGHT.     By  J.  McKEEN  CATTELL,  M.A.,  Ph.D., 

New  York  City,  N.Y.,  U.S.A.,  Professor  of  Experimental  Psychology  in  Co- 
lumbia College 605 

BINOCULAR  VISION,  CONFLICT  OF  THE  FIELDS  OF  VISION, 
APPARENT  AND  NATURAL  SIZE  OF  OBJECTS,  ETC.  By  EuGEN 
BRODHUN,  M.D.,  Berlin,  Germany,  Assistant  in  the  Frederick  William  Uni- 
versity. Translated  by  CHRISTINE  LADD  FRANKLIN,  Baltimore,  Md.,  U.S.A.  539 


Vlll  CONTENTS   OF   VOLUME   I. 

PAGE 

NORMAL  COLOR-PERCEPTION.  By  WILLIAM  THOMSON,  M.D.,  Phila- 
delphia, Pa.,  U.S.A.,  Professor  of  Ophthalmology  in  the  Jefferson  Medical 
College;  Attending  Surgeon  to  the  Wills  Eye  Hospital.  Assisted  by  CARL 
WEILAND,  M.D.,  Philadelphia,  Pa.,  U.S.A.,  Clinical  Assistant  in  the  Eye 
Department  of  the  Jefferson  Medical  College  Hospital 681 

PHOTO-CHEMISTRY  OF  THE  RETINA.  By  CARL  MAYS,  M.D.,  Heidel- 
berg, Germany,  Assistant  in  the  Physiological  Laboratory  of  Heidelberg. 
Translated  by  JAMES  A.  SPALDINQ,  A.M.,  M.D.,  Portland,  Me.,  U.S.A.,  Oph- 
thalmic Surgeon  to  the  Maine  Eye  and  Ear  Infirmary,  and  to  the  Maine 
General  Hospital 617 


LIST  OF  ILLUSTRATIONS  TO  VOLUME  I. 


PLATES. 

PAGE 

Oblique  longitudinal  section  through  the  external  geniculate  body  and  base  of  optic 

tract  of  a  human  embryo  of  36-38  weeks 60 

Horizontal  section  through  the  optic  thalamus  and  deeper  parts  of  the  optic  tract  of 

a  34-36  weeks'  human  embryo 62 

Horizontal  section  through  the  thalamus,  inner  geniculate  body,  and  optic  tract  of  a 

34-36  weeks'  human  embryo 64 

A  Eoman  female  skull  with  a  high  orbital  index.     A  Caucasian  skull  with  a  low 

orbital  index 71 

The  position  of  the  lids  in  the  open  eye.  The  position  of  the  lids  in  the  closed  eye. 
The  position  of  the  lids  when  the  eye  is  turned  in.  The  position  of  the  lids 
when  the  eye  is  turned  out 81 

Insertion  of  the  ocular  muscles  upon  the  sclera  of  the  right  eye.     Insertion  of  the 

ocular  muscles  upon  the  sclera  of  the  left  eye  as  seen  from  the  front 128 

The  origin  of  the  chorioidal  veins.     The  chorio-capillaris 165 

Images  of  Purkinje  formed  by  reflections  from  the  surfaces  of  the  eye.  Double 
images  of  Purkinje  used  for  showing  the  curvature  of  the  lens  during  accommo- 
dation. Action  of  the  ciliary  muscle 172 

Iris  and  sclera  viewed  from  the  side.     Development  of  the  eye  shown  diagrammati- 

cally.     The  vascular  tunic  of  the  lens 176 

Diagram  of  a  radial  section  of  the  iris.     Horizontal  section  through  the  ciliary  zone 

of  the  iris 178 

Segment  of  the  anterior  surface  of  the  iris  X  20 ;  pupil  contracted.  The  same  sur- 
face with  pupil  somewhat  dilated 180 

Segment  of  the  posterior  surface  of  the  iris.     Keproduction  of  photomicrograph 

showing  a  portion  of  a  radial  section  of  the  iris  after  bleaching  with  chlorine  .    .     182 

The  optic  disk  viewed  with  the  ophthalmoscope 194 

Vessels  of  the  retina 195 

Blood-vessels  of  the  yellow  spot  injected.     The  hyaloid  artery 197 

Concentric  layers  of  lens-stars.     Appearance  of  fibres  in  the  adult  lens 204 

Attachment  of  the  suspensory  ligament  of  the  lens.    The  suspensory  ligament  of  the 

lens 208 

Lymph-spaces  of  the  pig's  cornea  injected  with  asphalt-chloroform.     Lymph-spaces 

of  the  sclerotic  of  pig  injected  with  asphalt-chloroform 236 

Diagrammatic  representation  of  the  visual  cells.     Section  of  the  human  retina  .    .    .     300 

The  elements  of  the  mammalian  retina  based  on  the  investigations  of  Ramon  y 

Cajal 308 

Photomicrograph  of  a  transverse  section  of  the  optic  nerve  of  a  calf.     Mid-brain 

of  ox , 388 

Child,  aged  nine  weeks,  with  congenital  anophthalmos.    Portion  of  the  eye  showing 

neck  of  cyst  with  retinal  tissue  passing  through  it  from  the  eyeball  into  the  cyst  .     420 

Coloboma  of  the  chorioid  in  the  macular  region.     Unusual  appearance  of  the  optic 

disk,  due  to  the  abnormal  direction  of  the  head  of  the  optic  nerve 445 

ix 


X  LIST   OF   ILLUSTRATIONS   TO   VOLUME   I. 

FIGURES. 

PAGE 

Side  view  of  anterior  end  of  central  nervous  system  of  a  young  Amphioxus  ....  10 

Head  of  embryo  chick  of  twenty-three  hours,  viewed  from  above 12 

Head  of  embryo  chick  of  twenty-five  hours,  viewed  from  above 12 

Head  of  embryo  chick  of  twenty-six  hours,  viewed  from  above 13 

Head  of  embryo  chick  of  twenty-seven  hours,  viewed  from  above 13 

Head  of  embryo  chick  of  twenty-nine  hours,  viewed  from  above 14 

Anterior  end  of  rabbit  embryo  of  eight  days  and  fourteen  hours,  viewed  from  above  14 

Head  of  embryo  chick  of  thirty-two  hours,  viewed  from  above 15 

Head  of  embryo  chick  of  thirty-eight  hours,  viewed  from  above 15 

Oblique  view  of  head  and  forepart  of  trunk  of  a  human  embryo  of  twelve  to  fif- 
teen days 16 

Side  view  of  the  head  of  a  human  embryo  of  about  the  same  age  as  the  preceding  .  16 
Side  view  of  the  head  of  a  human  embryo  of  the  same  age  as  the  two  preceding,  but 

with  the  outer  ectoderm  and  mesoderm  of  the  head  removed 16 

Head  of  human  embryo  of  eighteen  to  twenty  days,  viewed  from  the  side 17 

Side  view  of  the  head  of  a  human  embryo  of  about  the  same  age  as  the  foregoing, 

with  the  ectoderm  and  mesoderm  of  the  head  removed 17 

Cross-section  through  the  forepart  of  the  head  of  an  embryo  chick  of  fifty-two 

hours 18 

Diagram  showing  relations  of  the  ectodermal  rudiments  of  the  eye  in  an  embryo 

chick  at  the  end  of  the  second  day 19 

Enlarged  model  or  reconstruction  of  secondary  optic  cup  of  a  vertebrate  embryo  .  .  20 

Optic  stalk  of  eye  of  mouse  embryo  in  cross-section 22 

Side  view  of  head  of  an  embryo  chick  of  fifty-two  hours 23 

Side  view  of  head  of  an  embryo  chick  of  sixty-eight  hours 23 

Head  of  an  embryo  chick  eighty-two  hours  old,  viewed  from  above 25 

Front  view  of  head  of  a  human  embryo  of  four  weeks 26 

Front  of  head  of  chick  embryo  of  six  days 27 

Front  of  head  of  chick  embryo  of  seven  days 27 

Front  view  of  head  of  eight-day  chick  embryo 27 

Front  view,  slightly  oblique,  of  head  of  an  embryo  chick  of  nine  days 27 

Side  view  of  head  of  human  embryo  of  thirty-seven  to  thirty-eight  days 29 

Side  view  of  head  of  human  embryo  of  thirty-nine  to  forty  days 29 

Side  view  of  head  of  human  embryo  of  forty  to  forty-five  days 80 

Side  view  of  head  of  human  embryo  of  fifty-eight  to  sixty-two  days 31 

Profile  view  of  head  of  human  embryo  of  about  seventy-five  days 31 

Lacrymal  gland  of  four  months'  human  embryo 32 

Horizontal  section  of  the  eye  of  an  embryo  of  the  domestic  ox 33 

Anterior  half  of  the  eyeball  of  a  human  embryo  of  four  weeks 35 

The  hinder  half  of  the  same  eye  viewed  with  reflected  light 35 

Distribution  of  the  hyaloid  artery  on  anterior  wall  of  the  capsule  of  the  lens  of  a 

newly-born  kitten 86 

Distribution  of  the  hyaloid  artery  on  posterior  wall  of  the  same 36 

Vertical  section  through  the  eye  of  an  embryo  mouse 39 

Section  through  a  part  of  the  lens  and  rim  of  the  optic  cup  of  a  mouse  embryo 

somewhat  more  advanced  than  the  preceding 40 

Diagram  showing  the  arrangement  and  mode  of  convergence  on  the  anterior  and 

posterior  aspects  of  the  lens  of  a  very  young  mammal  of  the  fibres  developed 

from  the  cells  of  its  posterior  wall 41 

Vertical  section  through  the  optic  axis  of  the  eye  of  a  recently  hatched  salamander 

larva 43 

Series  of  figures  from  successive  sections  of  the  retina  of  an  embryo  torpedo  near 

the  point  where  the  retina  joins  the  optic  stalk 46 


LIST   OF   ILLUSTRATIONS   TO   VOLUME   I.  XI 

PAGE 

Section  through  the  margin  of  the  optic  cup  of  an  advanced  embryo  of  a  thrush  .    .  62 

Transverse  section  through  the  head  of  a  chick  embryo  at  the  end  of  the  sixth  day  .  65 

Side  of  part  of  head  of  an  Acanthias  embryo  six  millimetres  long 66 

The  four  anterior  head-cavities  of  an  embryo  of  Acanthias  twelve  millimetres  long  .  67 
The  four  head-cavities  of  an  embryo  of  Acanthias  sixteen  millimetres  long  ....  67 
Still  more  advanced  condition  of  the  head-cavities  of  an  Acanthias  embryo  ....  68 
The  transformation  of  the  head-cavities  nearly  completed  in  an  embryo  of  Acan- 
thias fifty-five  millimetres  long 69 

Diagram  showing  mode  of  preparation  of  three  frontal  sections  of  the  skull  ....  74 

Frontal  section  of  the  skull  at  the  middle  of  the  orbital  opening 75 

Frontal  section  of  the  skull  at  the  middle  of  the  orbit 76 

Frontal  section  of  the  skull  near  the  apex  of  the  orbit 77 

Diagram  showing  profile  view  of  the  eyelids  when  eye  turned  in 81 

Diagram  showing  position  of  the  eyelids  when  eye  turned  out 82 

Diagram  showing  position  of  the  eyelids  when  eye  looks  up 83 

Asymmetry  of  the  head  and  face  as  seen  through  a  screen  of  wires  at  right  angles  .  84 

Arteries  and  veins  of  the  eyelids  and  adjacent  parts  of  face 85 

Orbicularis  palpebrarum 87 

Microscopic  section  of  the  upper  lid  and  front  of  the  eye 89 

Coronal  section  showing  reflection  of  conjunctiva 90 

Coronal  section  showing  reflection  of  conjunctiva 90 

Coronal  section  showing  reflection  of  conjunctiva '.    .  90 

Coronal  section  showing  reflection  of  conjunctiva 90 

Microscopic  section  through  the  upper  lid  of  a  negro  infant 91 

Lacrymal  apparatus  and  Meibomian  glands 93 

Tear-sac  from  a  metal  cast 93 

Horizontal  section  through  inner  half  of  right  orbit 94 

Frontal  frozen  section  of  left  orbit,  seen  from  before 95 

Section  about  five  millimetres  behind  globe        95 

Section  about  three  millimetres  in  front  of  back  of  globe 96 

Section  near  equator  of  globe 96 

Dissection  of  capsule  of  Tenon 99 

The  orbital  arteries 102 

The  orbital  veins 103 

The  orbital  nerves,  seen  from  above 105 

The  orbital  nerves,  seen  from  the  outside 106 

Camera  obscura  of  a  photographic  apparatus 109 

The  eye  as  a  camera 109 

Diagram  of  a  horizontal  section  of  the  left  eye,  drawn  to  scale 110 

The  eyeball  compared  to  a  sphere Ill 

An  emmetropic  or  normal  eye 114 

A  myopic  or  short-sighted  eye 114 

A  hypermetropic  or  far-sighted  eye 114 

Sagittal  section  of  the  orbit  and  eyeball 118 

Position  of  the  eyeball  in  the  orbit 118 

Frontal  section  of  the  orbit  and  eyeball  just  anterior  to  the  equator 119 

The  eye  and  optic  nerves  seen  from  above  after  removal  of  the  roof  of  the  orbit  .    .  119 

The  palpebral  opening 121 

Sagittal  section  of  the  eye  . 124 

Transverse  section  of  sclera  and  chorioid 127 

Insertion  of  the  ocular  muscles  upon  the  sclera  of  the  right  eye,  as  seen  from  the  rear  129 

Intervals  in  the  sclera 130 

The  junction  of  the  sclera  and  the  cornea 131 

Posterior  view  of  right  eye,  showing  entrance  of  optic  nerve 131 

Section  through  the  optic  nerve  entrance 132 


iii  LIST  OF   ILLUSTRATIONS   TO   VOLUME    I. 

PAGE 

Fibres  of  the  sclera  blending  with  those  of  the  cornea 138 

Examples  of  the  arcus  senilis 140 

Transverse  section  of  the  cornea 141 

Section  of  the  cornea  of  an  ox 143 

The  "corneal  tubes" 144 

Cornea  of  a  frog  stained  so  as  to  show  the  "  negative"  picture 145 

Cornea  of  a  frog  stained  so  as  to  show  the  "  positive"  picture        146 

Meridional  section  showing  the  connection  between  the  cornea  and  the  middle  coat 

of  the  eye 151 

A  preparation  made  parallel  to  the  surface  of  the  annular  ligament 161 

Meridional  section  of  rabbit's  cornea,  showing  strands  of  the  pectinate  ligament  .    .  152 

Section  of  a  preparation  made  by  injecting  India  ink  into  the  anterior  chamber    .    .  154 

Nerve-plexuses  of  the  cornea 156 

Section  of  the  human  chorioid 160 

Vessels  and  nerves  of  the  middle  coat 101 

Vessels  of  the  chorioid 162 

The  ciliary  body  seen  from  behind        165 

Segment  of  the  ciliary  body  and  of  the  iris 167 

Vascular  plexuses  of  the  ciliary  processes 167 

Varieties  of  the  ciliary  muscle 171 

Diagram  showing  the  prevalence  of  different-colored  eyes  among  European  peoples  .  178 
Diagram  showing  the  prevalence  of  different-colored  eyes  among  soldiers  native  to 

the  United  States 179 

Effect  of  local  stimulation  of  the  sclera  in  the  cat 189 

Arteries  of  the  iris 190 

Model  showing  formation  of  optic  cup 192 

Radiation  of  the  optic  nerve  fibres  upon  the  retina 194 

Blood-vessels  of  the  retina  injected 196 

Lymphatic  spaces  of  the  eyeball 210 

Diagram  of  supposed  structure  of  the  vitreous  body  as  shown  by  an  equatorial  sec- 
tion    211 

Vertical  section  of  the  cornea 218 

Vertical  section  of  the  anterior  epithelium 220 

Endothelioid  markings  on  the  anterior  limiting  membrane 222 

Interlacing  lamellae  composing  the  substantia  propria  of  calf  s  cornea 223 

Surface  view  of  silvered  cornea 225 

Corneal  spaces  of  ox  after  interstitial  injection  of  argentic  nitrate 226 

Surface  view  of  silver  preparation  of  corneal  stroma  of  ox 227 

Corneal  corpuscles  as  seen  in  surface  view  in  gold  preparations 228 

Vertical  section  of  the  posterior  lamellae  of  the  cornea 231 

Endothelium  covering  the  posterior  limiting  membrane 232 

Vascular  net-work  at  the  corneal  limbus 234 

Perineural  lymph-sheath  lined  by  an  imperfect  endothelial  layer 235 

Medullated  nerve-fibres  from  an  anterior  corneal  nerve-trunk 238 

Termination  of  the  nerves  in  the  anterior  part  of  the  cornea 239 

Special  endings  of  corneal  nerves .    .  240 

Portion  of  fundamental  plexus  from  the  periphery  of  the  anterior  layers  of  the 

cornea 240 

Portion  of  fundamental  plexus  of  anterior  layers  of  the  cornea 241 

Section  of  sclera  showing  the  component  fibrous  tissue 243 

Section  through  the  adjacent  parts  of  the  sclera  and  the  chorioid 244 

Meridional  section  of  ciliary  region 247 

Section  through  the  lateral  wall  of  the  anterior  chamber 247 

Section  through  the  sclero-corneal  junction 249 

Section  of  the  chorioid  with  portions  of  the  adjacent  coats 254 


LIST   OF    ILLUSTRATIONS   TO   VOLUME   I.  xiil 

PAGE 

Surface  view  of  a  fragment  of  the  lamina  suprachoroidea 256 

Surface  view  of  a  portion  of  the  stroma  of  the  chorioid 257 

Surface  view  of  the  injected  chorioid 258 

Surface  view  of  the  injected  chorioid,  showing  the  dense  net-work  of  the  chorio- 

capillaris 259 

Surface  view  of  the  chorioid  seen  from  the  inner  side 260 

Posterior  view  of  iris  and  ciliary  bodies 262 

Section  through  the  ciliary  ring 262 

Injected  ciliary  processes  viewed  from  behind 263 

Meridional  section  of  the  ciliary  processes 264 

Section  through  the  posterior  part  of  the  ciliary  processes 264 

Meridional  section  of  the  ciliary  region 265 

Part  of  a  meridional  section  through  the  ciliary  region 267 

Diagram  of  vascular  supply  of  anterior  segment  of  eye 270 

Nerve-terminations  within  the  ciliary  muscle 271 

Radial  section  of  iris 273 

Tangential  section  of  iris 275 

Radial  section  of  pupillary  zone  of  iris 276 

Radial  section  of  iris  of  rabbit , 277 

Surface  view  of  dilator  fibres  of  human  iris 278 

Radial  section  of  posterior  portion  of  human  iris 282 

Arterial  supply  of  the  iris 283 

Distribution  of  the  motor  nerves  of  the  iris 284 

Surface  view  of  sphincter  muscle  of  iris  of  rabbit  after  gold  staining 285 

Horizontal  section  of  rabbit's  iris  after  gold  staining 286 

Surface  view  of  sensory  nerves  of  iris  of  rabbit  after  gold  staining 286 

Diagram  illustrating  the  relation  of  the  retinal  elements 290 

Surface  view  of  pigmented  retinal  epithelium 292 

Surface  view  of  pigmented  retinal  epithelium  from  an  aged  subject 293 

Sections  of  frog's  retina 295 

Sections  of  retinae,  showing  effect  of  light  and  darkness  on  the  pigment  and  cones  .  296 

Outer  layers  of  frog's  retina,  showing  effect  of  exposure  to  light 297 

Outer  layers  of  frog's  retina,  showing  effect  of  darkness 297 

Section  of  human  retina 298 

Semi-diagrammatic  view  of  a  rod  and  a  cone  from  the  human  retina 299 

Diagrammatic  view  of  twin  cones 302 

Surface  view  of  retina,  showing  disposition  and  relative  number  of  rods  and  cones  .  302 

Surface  view  of  horizontal  cells  from  retina  of  ox 305 

Horizontal  cell  from  retina  of  ox 306 

Nerve-cells  from  retina  of  ox  309 

Vertical  section  of  retina  of  ox 311 

Supporting  fibres  of  Miiller,  from  peripheral  area  of  retina  of  ox 320 

Supporting  fibres  of  Muller,  from  retina  of  ox,  in  the  vicinity  of  the  papilla  .  .  .  321 

Silver  markings  of  surface  of  human  retina 323 

Superficial  surface  markings  from  silvered  human  retina 324 

Section  of  human  retina  through  the  macula 324 

Portion  of  bundle  of  fibre  layer  of  retina  in  the  vicinity  of  the  optic  papilla  ....  326 

Surface  view  of  macular  area  of  human  retina 327 

Diagrammatic  section  of  the  human  fovea 330 

Section  of  human  retina  at  the  ora  serrata 333 

Section  of  optic  entrance 335 

Diagram  of  the  blood-vessels  of  the  human  retina 337 

Accurate  drawing  of  the  blood-vessels  supplying  the  macular  region  of  the  human 

retina 339 

Retinal  blood-vessel  surrounded  by  perivascular  lymph-sheath 340 


XIV  LIST   OF   ILLUSTRATIONS   TO   VOLUME   I. 

PAGE 

Transverse  section  of  optic  nerve 341 

Longitudinal  section  of  optic  nerve 342 

Longitudinal  section  of  optic  nerve  stained  by  Golgi  method 343 

Section  of  the  lamina  cribrosa 344 

Longitudinal  section  through  the  optic  entrance 346 

Transverse  section  of  optic  nerve  with  its  sheaths 347 

Meridional  section  through  human  lens   .       350 

Vertical  section  through  the  eye,  at  an  early  stage,  of  an  embryo  mouse 352 

Section  of  crystalline  lens  embracing  capsule 354 

Meridional  section  through  equatorial  region  of  young  lens 355 

Fragments  of  isolated  lens-fibres 856 

Lens-fibres  seen  in  transverse  section 357 

Central  part  of  the  anterior  lens-star 358 

Central  part  of  the  posterior  lens-star 358 

Crystalline  lens  of  new-born  child,  seen  from  the  side 359 

Adult  crystalline  lens,  showing  lens-stars 359 

Diagram  showing  course  of  lens-fibres  from  anterior  to  posterior  stars 360 

Meridional  section  of  equatorial  region  of  crystalline  lens  of  a  woman  aged  seventy- 
five  years 361 

Portion  of  vitreous  substance  of  six  months'  human  foetus 365 

Portion  of  vitreous  substance  from  adult 367 

Portions  of  adult  vitreous  substance .    .  369 

Surface  view  of  fragment  of  hyaloid  membrane 370 

Portion  of  anterior  boundary  layer  of  vitreous  body  of  adult 371 

Diagram  of  anterior  segment  of  eye,  drawn  to  accurate  scale 372 

Meridional  section  through  the  ciliary  region  of  an  adult  human  eye 377 

The  base  of  the  brain 389 

Tracing  from  a  frozen  section  of  the  head  of  a  man 391 

Tracing  from  a  frozen  section  of  the  head  of  a  rabbit 391 

Diagram  of  field  of  vision 391 

Diagram  of  the  occipital  region  of  the  right  cerebral  hemisphere 405 

Diagram  of  a  microphthalmic  eye  with  a  cyst  attached 421 

Diagram  of  a  microphthalmic  eye  with  two  cysts  attached 421 

Double  congenital  coloboma  of  the  right  upper  lid 427 

Congenital  coloboma  of  the  left  upper  lid 428 

Small  congenital  coloboma  of  the  right  upper  lid 428 

Congenital  polycoria  and  anterior  synechia 436 

Congenital  microphthalmos   and    cataract,  with   persistence  of  numerous  tags  of 

pupillary  membrane 437 

Section  showing  rudimentary  iris  with  a  tag  of  pupillary  membrane 438 

The  front  of  an  eye  which  had  apparently  complete  congenital  absence  of  the  iris   .  438 
Congenital  adhesion  of  iris  and  of  a  persistent  pupillary  membrane  to  the  back  of 

the  cornea 439 

Congenital  coloboma  of  iris  and  lens  outward 442 

Microscopical  appearances  in  the  region  of  a  coloboma  of  the  iris 442 

Microscopical  appearance  of  the  front  part  of  a  microphthalmic  eye  with  coloboma 

of  the  iris 442 

Microscopical  appearances  of  the  posterior  half  of  a  microphthalmic  eye 444 

Semi-diagrammatic  sketch  of  a  microphthalmic  eye 450 

Diagram  of  the  section  of  an  eye  with  a  persistent  and  patent  hyaloid  artery    .    .    .  455 

Diagram  of  a  case  very  similar  to  the  preceding 465 

Shrunken  globe  in  which  a  tag  of  a  persistent  hyaloid  artery  was  found  adherent  to 

the  optic  nerve 456 

Diagram  illustrating  changes  in  wave-fronts 460 

Diagram  illustrating  regular  reflection      461 


LIST   OF   ILLUSTRATIONS   TO  VOLUME   I.  XV 

PAOB 

Reflection  of  parallel  rays  from  a  plane  mirror 461 

Reflection  of  divergent  rays  from  a  plane  mirror 461 

Reflection  of  parallel  rays  by  a  concave  spherical  mirror 462 

Reflection  of  divergent  rays  by  a  concave  spherical  mirror 462 

Reflection  of  parallel  rays  by  a  convex  spherical  mirror 462 

Diagram  illustrating  formation  of  an  image  by  a  plane  mirror 463 

Diagram  illustrating  formation  of  an  image  by  a  concave  mirror 463 

Diagram  illustrating  formation  of  an  image  by  a  concave  mirror,  the  object  being 

within  the  principal  focus 464 

Diagram  illustrating  formation  of  an  image  by  a  convex  mirror 464 

Diagram  illustrating  change  of  direction  in  refraction  of  light 465 

Diagram  illustrating  refraction  of  light  by  a  prism 466 

Scheme  to  show  the  effect  of  a  prism  upon  a  ray  of  light 467 

Diagram  representing  refraction  by  a  convex  spherical  lens 469 

Diagram  illustrating  spherical  aberration 470 

Diagram  illustrating  the  method  of  ascertaining  the  principal  focal  distance  of  a 

lens 470 

Diagram  illustrating  conjugate  foci 472 

Diagram  illustrating  refraction  by  concave  lenses 472 

Different  forms  of  lenses 473 

Diagram  illustrating  axes  and  nodal  points  of  convex  lenses 476 

Diagram  illustrating  axes  and  nodal  points  of  concave  lenses 476 

Diagram  illustrating  the  formation  of  a  real  image  by  a  convex  lens 477 

Diagram  illustrating  the  formation  of  a  virtual  image  by  a  convex  lens 477 

Diagram  illustrating  the  formation  of  an  image  by  a  concave  lens 477 

Diagram  illustrating  refraction  by  a  plano-convex  cylindrical  lens 478 

Diagram  illustrating  refraction  by  a  plano-concave  cylindrical  lens 478 

Diagram  showing  method  of  graduation  of  trial-frames 479 

Relative  indices  of  refraction  of  crystalline  lens 482 

The  aphakic  eye 483 

Principal  and  secondary  axes  in  the  aphakic  eye 484 

The  schematic  eye 485 

Diagram  illustrating  the  posterior  focal  distance  in  the  emmetropic  eye 489 

Circles  of  diffusion 489 

Diagram  illustrating  hyperopia 490 

Effect  of  position  of  correcting  lens  on  a  hyperopic  eye 491 

Diagram  illustrating  myopia 493 

Effect  of  position  of  correcting  lens  on  a  myopic  eye 493 

Diagram  illustrating  astigmatism 495 

Comparative  sizes  and  shapes  of  diffusion  areas 496 

Appearance  of  points  to  astigmatic  and  non-astigmatic  eye 497 

Lines,  angles,  and  centres  of  schematic  eye 501 

Changes  of  ciliary  muscle,  iris,  and  crystalline  lens  during  accommodation    ....  502 

Diagram  illustrating  the  region  of  accommodation 503 

Diagram  showing  the  relative  sensitiveness  of  eyes  for  color 607 

Diagram  illustrating  Bouguer's  method  of  determining  differences  of  light  and 

shadow 509 

Diagram  showing  appearance  of  Masson's  revolving  disks 509 

Diagrams  illustrating  appearance  of  v.  Helmholtz's  revolving  disks 610 

Diagram  illustrating  interference  of  idio-retinal  light 511 

Diagram  showing  form  of  apparatus  used  to  determine  small  differences  in  light- 
perception    512 

Diagram  illustrating  Fechner's  law 615 

Diagram  illustrating  the  phenomena  of  irradiation 522 

Diagram  showing  the  apparent  dipping  of  a  bright  light  into  a  black  screen     .    .    .  523 


XVI  LIST  OF   ILLUSTRATIONS   TO   VOLUME   I. 

PAGE 

Diagram  showing  the  relative  legibility  of  the  various  letters 625 

Diagram  illustrating  the  effects  of  visual  fatigue 626 

Diagram  exhibiting  the  relation  of  sensation  to  time  of  stimulation 627 

Diagram  illustrating  the  distortion  of  after-images    ..." 531 

Diagram  illustrating  fusion  of  sectors  and  uniformity  of  sensation «.  534 

Diagram  to  show  the  relationship  between  rate  of  stimulation  and  intensity  of  illu- 
mination    535 

Diagram  illustrating  relationship  between  vision  and  hand  movement 537 

Schroder's  flight  of  steps 640 

Monocular  and  binocular  fields  of  vision 642 

Diagram  to  explain  stereoscopic  vision 643 

Schematic  representation  of  the  mirror-stereoscope  of  Wheatstone 547 

Schematic  representation  of  the  lens-stereoscope  of  Brewster 548 

Schematic  representation  of  Helmholtz's  telestereoscope 548 

Helmholtz's  telestereoscope 649 

Schematic  representation  of  Zeiss's  relief  telescope 550 

Schematic  representation  of  binocular  ophthalmoscope 651 

Schematic  representation  of  Wheatstone 's  pseudoscope 651 

Diagram  illustrating  Nachet's  and  Wenham's  microscopes 552 

Diagram  illustrating  the  stereoscopic  effect  obtained  by  two  totally  reflecting  prisms  662 

Diagram  illustrating  stereoscopic  effect  produced  in  binocular  microscope 553 

Diagrams  illustrating  the  action  of  Hering's  haploscope 664 

Diagram  illustrating  the  method  for  determining  corresponding  points  on  the  mean 

cross  and  longitudinal  sections 555 

Diagram  illustrating  the  method  for  determining  corresponding  points  in  the  middle 

cross-sections 555 

Diagram  illustrating  the  method  for  determining  corresponding  points  within  any 

one  of  the  quadrants .....  556 

Diagram  illustrating  Hering's  mirror-haploscope 556 

Diagram  illustrating  construction  for  finding  corresponding  points  and  correspond- 
ing meridians 557 

Haploscopic  figure  of  Helmholtz 657 

Scheme  representing  a  section  passing  through  the  retinas  and  made  perpendicular 

to  the  visual  plane 560 

Diagram  illustrating  the  horopter  circle  of  Miiller 561 

Diagram  illustrating  section  through  the  visual  plane 561 

Haploscopic  drawing  illustrating  disparateness  and  fusion 565 

Wheatstone's  diagram  to  prove  double  vision  with  identical  points 566 

Helmholtz's  diagram  to  prove  double  vision  with  identical  points 567 

Chess-board-like  figure  of  Helmholtz 569 

Figure  showing  subdivided  line 569 

Figure  showing  apparent  increase  of  acute  angle 569 

Zollner's  lines 570 

Diagram  illustrating  actual  and  apparent  directions  of  objects 570 

Diagram  illustrating  heteronymous  and  homonymous  double  images 572 

Diagram  illustrating  Schon's  experiment 573 

Diagram  for  obtaining  union  of  two  haploscopic  fields     .    . 677 

Diagram  for  obtaining  union  of  two  haploscopic  fields 578 

Diagram  illustrating  the  effect  of  the  union  of  the  two  haploscopic  fields  in  diagram 

Fig.  34 578 

Diagram  illustrating  the  effect  of  the  union  of  the  two  haploscopic  fields  in  diagram 

Fig- 35 579 

Figure  for  obtaining  stereoscopic  lustre 580 

Diagram  illustrating  electric,  thermic,  photogenic,  and  actinic  rays  in  ether  ....  583 

Diagram  showing  relationship  between  end-organs  and  ganglion-cells 596 


LIST   OP    ILLUSTRATIONS   TO   VOLUME   I.  XV11 

PAGE 

Diagram  illustrating  action  of  spectral  colors  on  the  photo-chemical  substances  .  .  597 

Diagram  illustrating  anabolic  and  katabolic  changes 613 

Cell  of  the  retinal  epithelium  of  the  frog 620 

Microscopic  view  of  the  frog's  refina  from  behind 626 

Spectra  of  visual  purple,  visual  yellow,  and  green 627 

Diagram  of  the  spectrum  analyses  of  the  pigments  of  the  cones 633 

Rabbit's  retina  containing  visual  purple 634 

Shifting  of  the  fuscin  in  the  frog's  retina  ....  • -. 641 

Position  of  the  cones  and  of  the  pigment  in  frogs  kept  in  the  dark 643 

Position  of  the  cones  and  of  the  pigment  in  the  illuminated  frog's  eye  643 

Fluctuations  of  the  electrical  current  in  the  isolated  retina  of  a  frog  when  exposed 

to  light  , 647 

Fluctuations  of  the  electrical  current  in  a  frog's  eye  when  exposed  to  the  irritation 

of  light 648 

Variations  of  the  electrical  current  in  the  fresh  eyes  of  fish  when  exposed  to  the 

irritation  of  light • 648 

Variation  of  the  electrical  current,  under  the  influence  of  light,  in  the  optic  nerve 

of  the  frog 649 


SYSTEM 


OF 


DISEASES  OF  THE  EYE. 


EMBRYOLOGY,  ANATOMY,  AND  PHYSIOLOGY  OF 

THE   EYE. 


DEVELOPMENT  OF  THE  EYE. 

BY   JOHN   A.   EYDER,   PH.D., 

Professor  of  Comparative  Embryology,  University  of  Pennsylvania,  Philadelphia, 

Penna.,  U.S.A., 


DEVELOPMENT   OF   THE   EYE. 

THE  mammalian  embryo  is  developed  from  an  egg  which  at  first  repre- 
sents morphologically  a  single  cell.  In  the  course  of  its  development,  the 
substance  of  the  egg  divides  by  means  of  what  is  known  as  indirect  or 
karyokinetic  cell-division  into  two,  these  two  into  four,  these  four  into  eight 
cells,  and  so  on,  until  a  globular  cell-aggregate  is  formed.  Within  this 
cell-aggregate  a  cavity,  the  blastocoele,  is  soon  developed,  filled  with  fluid, 
surrounded  by  a  cellular  wall  composed  of  the  cells  resulting  from  the  re- 
peated division  of  the  original  germ-cell  as  mentioned  above.  This  spher- 
ical cellular  wall  is  essentially  an  epithelium.  The  germ  is  now  a  hollow 
globe,  known  in  mammalian  embryology  as  the  blastodermic  vesicle,  the 
walls  of  which  are  constituted  by  the  products  of  the  segmentation  of  the 
original  egg-cell.  At  one  side  of  this  hollow  germ  the  epithelial  wall  be- 
comes thicker,  owing  to  the  manner  in  which  certain  cells  of  its  wall  pro- 
liferate into  its  cavity.  The  thickening  or  area  thus  marked  out  soon 
becomes  oval  in  outline,  and  constitutes  the  germinal  area  from  which  the 

7 


8  DEVELOPMENT   OF   THE    EYE. 

embryo  is  differentiated,  together  with  its  enveloping  amnion.  This  epi- 
thelial area  of  the  globular  germ  of  mammals,  from  which  the  embryo  is 
developed,  is  sometimes  spoken  of  as  the  embryonic  area  of  the  blastoderm. 
It  very  early  becomes  split  up  into  the  three  so-called  primary  germ-layers. 
Of  these  the  outer,  or  ectoderm,  is  the  first  to  appear,  after  which  the  ento- 
derm,  or  innermost,  is  formed,  while  the  third  layer,  or  mesoderm,  appears 
last  of  all  between  the  two  first-named. 

All  the  structures  of  the  body  are  developed  from  the  three  primary 
germ-layers  of  the  embryo,  viz.,  ectoderm,  mesoderm,  and  entoderm,  of 
which  the  first  is  uppermost  and  outermost,  the  second  intermediate,  and 
the  last  lowermost  or  deepest  in  position.  These  three  layers  are  also  some- 
times spoken  of  as  epiblast,  mesoblast,  and  hypoblast. 

From  the  ectoderm  or  epiblast,  the  epidermis,  sensory  epithelia  of  the 
sense-organs,  brain,  cord,  nerves,  hair,  nails,  and  superficial  dermal  glan- 
dular structures,  the  enamel  of  the  teeth,  oral  epithelium  and  glands,  and 
epithelium  of  the  nasal  chamber,  are  formed.  From  the  mesoderm  or 
mesoblast,  the  muscles,  bones,  cartilages,  connective  and  adipose  tissuesr 
heart,  blood-  and  lymph-vessels,  blood-  and  lymph-corpuscles,  are  formed. 
The  entoderm  or  hypoblast  gives  rise  to  the  epithelium  of  the  alimentary 
canal  and  of  the  lungs,  to  the  secretory  cells,  ducts,  and  alveoli  of  the 
glandular  appendages  of  the  alimentary  canal,  such  as  the  liver,  pancreasr 
etc.,  while  the  smooth  muscular  fibres  of  the  walls  of  the  alimentary  canal 
and  the  vascular,  adenoid,  and  connective  tissues  generally,  of  its  append- 
ages, are  of  mesodermic  origin. 

All  the  parts  of  the  eye  are  developed  from  but  two  of  the  three 
primary  germ-layers.  Only  the  ectoderm  or  epiblast  and  the  mesoderm  or 
mesoblast  take  any  part  in  the  building  up  of  this  important  sense-organ  ; 
the  entoderm  or  hypoblast  is  entirely  excluded.  The  lens,  retina,  optic 
nerve,  pigmented  choroidal  epithelium ;  the  epithelia  of  the  conjunctiva, 
cornea,  third  eyelid ;  the  ocular  nerves,  the  blastema  of  the  nasal  duct,  the 
lacrymal  ducts  and  glands,  the  Meibomian  glands,  and  the  eyelashes,  arise 
from  the  ectoderm.  The  muscles,  vessels,  supra-choroid,  sclerotic,  the 
deeper  layer  or  corium  of  the  cornea,  the  anterior  layers,  vessels,  and 
muscles,  the  iris,  the  humors  of  the  eye,  and  the  bones  of  the  orbit,  arise 
from  the  mesoderm.  The  adjacent  and  associated  nareal  structures  arise 
partly  from  the  ectoderm  and  partly  from  the  mesoderm. 

From  a  narrow  strip  of  the  ectoderm  of  the  embryo,  the  foundation 
of  the  whole  of  the  cerebro-spinal  nervous  system  of  the  vertebrate  body 
is  evolved.  This  strip  of  the  ectoderm  constitutes  the  medullary  plate  and 
lies  along  and  marks  the  median  plane  of  the  future  body.  Its  edges  turn 
upward  to  form  the  so-called  medullary  groove.  Along  the  edges  of  this 
groove  are  also  developed,  very  early,  the  ectodermal  foundations  of  the 
sensory  ganglia  of  some  of  the  cranial  and  of  all  the  spinal  nerves.  The 
(•<l<r<'s  of  this  groove  are  gradually  lifted  upward  and  grow  or  bend  toward 
each  other ;  these  edges  also  finally  meet  and  coalesce'  so  as  to  enclose  a 


DEVELOPMENT   OF   THE    EYE.  9 

tubular  space,  the  so-called  medullary  canal  or  tube.  The  tube  thus  differen- 
tiated early  separates  from  the  remaining  ectoderm,  covering  the  rest  of  the 
bod  v  along  the  median  dorsal  line.  It  in  fact  sinks  inward  or  downward 
along  the  dorsal  line  and  begins  to  split  off  at  its  dorsal  side  from  the  rest 
of  the  ectoderm  at  its  anterior  end.  The  ectoderm  of  either  side  of  the 
body  closes  over  the  medullary  tube  completely  along  the  dorsal  or  median 
line  and  becomes  continuous  over  the  back  and  forms  the  foundation  of  the 
general  epidermis  of  the  head,  trunk,  and  limbs.  From  the  hinder  half 
of  the  medullary  tube  in  the  higher  vertebrates  the  spinal  cord  is  formed, 
and  from  its  anterior  half  are  developed  the  brain  and  sensory  epithelium, 
or  essential  visual  area,  and  the  optic  and  the  cranial  nerves.  From  the 
fact  that  the  brain  and  eyes  preponderate  in  importance  over  everything 
else  at  first  formed  from  the  anterior  end  of  the  medullary  canal,  the 
anterior  portion  of  the  latter  may  in  the  early  embryo  be  called  its  cerebro- 
ocular  portion.  The  eye  is  the  only  sense-organ  the  ectodermal  foundation 
of  the  sensory  epithelium  of  which  is  at  first  continuous  with  the  ecto- 
dermal epithelium  from  which  the  brain  is  formed. 

The  eye,  within  the  vertebrate  series,  has  unquestionably  arisen  primarily 
as  a  differentiation  of  a  part  of  the  wall  of  the  primitive  cerebro-spinal 
rudiment  or  medullary  plate  of  the  embryo.  Such  a  differentiation  seems 
from  the  first  to  have  been  anterior,  and  almost  median,  to  judge  from  the 
researches  of  H.  Ayers,  Kupffer,  and  Hatschek  upon  the  development  of 
Amphioxus.  This  conclusion  has  also  lately  been  fortified  by  the  researches 
of  Kupffer  upon  the  development  of  the  brain  in  Amphioxus,  the  sturgeon, 
and  the  lamprey.  In  these  forms  an  exceedingly  primitive  condition  of 
affairs  has  been  preserved.  The  later  studies  of  Hatschek  have  tended 
to  confirm  the  view  that  the  vertebrate  eye  is  a  structure  that  has  arisen 
immediately  from  part  of  the  lateral  cortex  of  the  embryonic  brain. 
Judging  from  the  condition  of  things  in  Amphioxus,  there  is  scarcely  any 
doubt  that  the  vertebrate  eye  was  functional  as  such  long  before  it  was 
pushed  out  from  the  brain-wall  as  a  lateral  diverticulum,  and  before  even 
the  development  of  an  optic  stalk.  This  view  seems  to  be  supported 
by  the  fact  that  the  pigmentary  screen  representing  the  first  trace  of  the 
pigmented  choroidal  epithelium  in  the  vertebrate  series  lines  the  inner  face 
of  the  anterior  portion  of  the  nervous  system  of  Amphioxus  at  the  points 
where  an  ocular  function  is  first  developed.  The  tissues  which  envelop 
the  brain  and  nervous  axis  in  Amphioxus  and  the  brain  itself  are  in  life 
quite  transparent,  so  that  there  was  no  necessity  for  the  evolution  of  the 
series  of  transparent  humors,  of  a  refracting  apparatus,  or  of  a  camera  in 
the  form  of  an  eyeball  with  its  automatic  shutter  or  iris,  and  muscular 
apparatus  of  accommodation,  and  adjustment  for  direction,  as  seen  in  higher 
types.  In  this  lowly  and  primitive  type  of  ocular  apparatus  of  Amphi- 
oxus, without  an  eyeball,  a  sharply-circumscribed  retinal  area,  an  optic 
nerve,  ocular  muscles  and  nerves,  lens,  or  any  of  the  usual  accessories 
of  the  vertebrate  eye,  we  very  probably  have  the  promise  and  possibility 


10 


DEVELOPMENT   OF   THE   EYE. 


FIG.  1. 


of  the  evolution  of  the  later  far  more  complex  visual  mechanism  of  higher 
vertebrates. 

Ayers  has  ingeniously  suggested  that  the  median  or  parietal  eye,  so 
fully  described  by  Spencer,  and  occurring  in  reptiles,  batrachians,  and 
fishes,  as  a  more  or  less  well  developed  organ  or  vestige  of  one,  was  also 
derived  ancestrally  from  the  extension  forward  and  upward  into  a  median 
dorsal  position  of  the  primitive  lateral  ocular  areas  of  the  brain-wall  of 
Amphioxus  or  of  some  similar  ancestral  form.  The  primitively  undivided 
median  position  of  the  olfactory  area  of  certain  low  forms,  as  the  sturgeon, 
where  it  also  at  first  forms  part  of  the  brain-wall,  as  shown  by  Kupifer, 
also  lends  support  to  the  belief  that  the  paired  eyes  of  the  higher  verte- 
brates were  primarily  nearly  median  in  their  origin. 

If  these  views  are  correct,  they  have  the  advantage  of  bringing  the 
ontogenetic  or  embryonic  history  of  the  eye  into  perfect  parallelism  with 
its  phylogenetic  history,  or  the  series  of  transformations  it  has  suffered 
in  the  course  of  the  evolution  of  vertebrates.  In  Fig.  1,  the  very  primi- 
tive relations  of  the  oval  ocular  patch  at  the  lower  anterior  part  of  the 
brain  of  a  very  young  Amphioxus  are  shown,  as  well  as  the  relations  of 

this  ocular  patch  in  the 
infero-lateral  brain-wall  to 
the  cavities  of  this  very 
primitive  condition  of  the 
vertebrate  brain.  The  dila- 
tation below  I,  lower  figure, 
is  clearly  the  homologue  of 
the  third  ventricle  of  the 
brains  of  other  vertebrates, 
in  which  the  space  within 
the  brain  under  III  is  as 
clearly  homologous  with  the 
fourth  ventricle,  with  its 
thin  roof,  as  seen  in  other 
vertebrata.  It  will  be  seen 
that  we  have  in  this  arrange- 
ment an  optic  organ  which 
has  not  yet  developed  to 
the  point  of  being  pushed  out  from  the  sides  of  the  brain,  though  Dr. 
Avers  has  informed  me  that  there  is  a  slight  thinning  and  outpushing  of 
the  lateral  walls  of  the  brain  of  the  young  of  this  creature  at  this  point. 
The  parts  are  in  such  relations  to  each  other  that  it  is  clearly  possible  to 
imagine  them  as  representing  a  stage  of  eye-development  still  more  primi- 
tive than  any  that  is  at  the  present  time  permanent  or  even  manifested 
temporarily  by  any  other  vertebrate. 

Between  this  very  primitive  state  of  the  vertebrate  eye  and  that  of 
the  fully-developed  one  of  the  higher  vertebrates,  a  great  many  additional 


Side  view  (lower  figure)  of  anterior  end  of  central  nervous 
system  of  a  young  Amphiwnis,  three  millimetres  long,  repre- 
senting the  brain,  cranial  nerves,  and  notochord,  Ch,  with  a 
dark  reniform  spot  at  the  side  below,  indicating  the  position 
of  the  ocular  area.  Cross-sections  of  the  brain,  I,  II,  III,  A,  at 
corresponding  points,  I,  II,  III,  in  lower  figure.  B,  Cross-section 
of  spinal  cord.  (After  Hatschek.) 


DEVELOPMENT   OF   THE   EYE.  11 

features  have  been  intercalated,  and  it  will  be  our  purpose  to  trace  the 
development  and  describe  the  method  of  accretion  of  these  new  features 
in  the  course  of  the  following  pages. 

In  the  very  earliest  form  of  the  vertebrate  eye,  therefore,  the  conclusion 
seems  to  be  established  that  the  ectoderm  alone  was  involved.  This  is  still 
further  established  by  the  recent  discovery  by  Eycleshymer  (Journal  of 
Morphology,  1893)  that  the  eyes  of  the  embryo  of  Neeturm  before  the 
closure  of  the  medullary  groove  are  already  denned  as  a  pair  of  pigmented 
and  thickened  areas  of  the  ectoderm  at  the  anterior  end  of  the  medullary 
plate.  The  pigment  here,  however,  is  developed  within  the  cells  of  the 
ectoderm  itself,  and  is  not  at  first  a  separate  layer,  as  it  becomes  at  a  later 
stage  in  the  development  of  the  same  animal.  It  is  also  to  be  borne  in 
mind  that  the  diffused  pigment-granules  seen  in  these  paired  ocular  patches 
at  the  anterior  end  of  the  medullary  plate  of  Necturus  lie  within  the  sub- 
stance of  the  ectoderm  cells  of  those  patches.  This  general  diffusion  of 
pigment-granules  through  the  cells  of  the  body  of  the  embryo  is  charac- 
teristic of  batrachians,  but  is  exceptional  among  other  vertebrate  forms. 

In  the  embryo  of  the  rabbit  of  the  ninth  day  the  anterior  end  of 
the  medullary  groove  begins  to  widen  at  the  point  where  the  eye  will  be 
developed  before  the  groove  itself  is  here  closed  and  separated  from  the 
ectoderm  covering  the  head.  In  fact,  in  an  embryo  of  the  rabbit  of  the 
age  mentioned,  the  first  traces  of  the  eyes  have  already  appeared  as  obvious 
lateral  expansions  of  the  anterior  end  of  the  medullary  plate  and  before 
the  latter  has  closed  dorsally  on  the  median  line.  The  development  of  the 
eyes  of  mammalia  is  therefore  seen  to  be  very  precocious, — more  so,  in  fact, 
than  in  birds,  as  will  appear  later. 

The  earliest  trace  of  the  optic  region  of  the  brain  or  first  embryonic 
cerebral  vesicle  of  vertebrates  above  Amphioxus  is  the  broadening  of  the  an- 
terior end  of  the  medullary  plate  or  tract  of  ectoderm  from  which  the  brain 
and  the  cord  are  formed.  As  soon  as  the  medullary  groove  closes,  a  process 
which  always  takes  place  first  of  all  at  the  anterior  end  of  the  medul- 
lary plate,  by  an  upfolding  of  its  opposite  halves  and  a  bending  of  those 
halves  toward  one  another  till  their  edges  meet  and  fuse  along  the  median 
dorsal  line,  there  is  at  once  developed  a  tendency  for  the  anterior  end  of 
the  cerebro-spinal  rudiment  to  enlarge.  This  enlargement  is  mostly  at  first 
in  the  direction  of  the  transverse  and  vertical  diameters  of  the  anterior  end 
of  the  medullary  canal  or  tube,  as  the  common,  but  now  involuted,  ecto- 
dermal  rudiment  of  the  brain  and  eyes  may  be  called.  The  detailed  history 
of  this  region  of  the  medullary  canal  differs  very  greatly  in  the  different 
vertebrates.  For  example,  there  is  at  first  a  complete  obliteration  of  the 
medullary  canal  in  this  region  in  the  embryos  of  bony  fishes,  lampreys 
and  Lepidosteus ;  in  consequence  of  which,  after  the  closure  and  detach- 
ment of  the  medullary  plate  from  the  rest  of  the  ectoderm  of  the  dorsal 
region,  this  important  organ  may  be  called  a  medullary  cord  more  properly 
than  a  canal.  In  these  lower  types  of  vertebrates — fishes  and  batrachia — 


12 


DEVELOPMENT  OF  THE   EYE. 


the  cerebro-ocular  portion  of  the  cerebro-spinal  rudiment  is  also  propor- 
tionally much  shorter  than  in  the  three  higher  series  of  vertebrates, — 
namely,  reptiles,  birds,  and  mammals, — in  which  this  region  extends  for 
almost  half  the  length  of  the  embryo  by  the  time  the  latter  has  been  first 
definitely  outlined.  In  that  we  are  more  especially  concerned  with  a  con- 
sideration of  the  development  of  the  eyes  of  the  higher  series,  especially 
the  mammalian,  to  which  man  belongs,  we  may  at  once  turn  to  a  more  de- 
tailed description  of  the  steps  by  means  of  which  the  anterior  or  cerebro- 
ocular  portion  of  the  medullary  canal  is  transformed  in  that  series  into 
the  essential  foundation  of  the  organ  of  vision.  Inasmuch  as  the  em- 
bryology of  the  bird  illustrates  this  part  of  the  subject  very  well,  we 
begin  with  it. 

In  Fig.  2,  representing  the  anterior  end  of  an  embryo  chick  of  about  the 


FIG.  2. 


FIG.  3. 


Head  of  embryo  chick  of  23  hours,  viewed 
from  above. — A,  anterior  end  of  head ;  1,  superior 
margin  of  medullary  fold  of  right  side  nearly  in 
contact  with  its  fellow  of  the  opposite  side;  2, 
lateral  limit  of  wall  of  medullary  or  neural  tube ; 
3,  point  posteriorly  where  the  edges  of  medul- 
lary groove  have  not  yet  united ;  4,  widely-open 
posterior  portion  of  same ;  5,  lateral  ectoderm  of 
head;  6,  lateral  limit  of  foregut.  Enlarged  30 
times.  (After  Duval.) 


Head  of  embryo  chick  of  25  hours,  viewed 
from  above. — A,  anterior  open  extremity  of  cere- 
bro-spinal canal;  ec,  lateral  limit  of  the  outer 
face  of  the  neural  canal ;  Vi,  first  cerebral  vesicle ; 
in,  lateral  limit  of  foregut;  2,  point  where  the 
edges  of  the  medullary  groove  have  not  yet  fused 
in  the  middle  line.  Enlarged  24  times.  (After 
Duval.) 


end  of  the  first  day,  the  edges  of  the  medullary  groove  are  just  about  closing 
and  coalescing  at  a  point  a  very  little  way  behind  the  extreme  anterior  end 
of  the  embryo  itself.  The  forward  end  of  the  medullary  canal  is  still 
open,  and  the  edges  of  the  medullary  plate  do  not  finally  close  at  this  point 
until  about  six  hours  later.  In  fact,  at  this  stage  the  cerebro-ocular  por- 
tion of  the  medullary  canal  is  still  open  anteriorly  and  posteriorly.  It  is 
not  until  about  two  hours  more  have  elapsed  that  the  development  of  the 
anterior  part  of  the  cerebro-ocular  portion  of  the  medullary  plate  has  pro- 
gressed so  far  as  nearly  to  close  off  this  portion  of  it  from  the  exterior,  as 
shown  in  Fig.  3.  This  stage,  however,  is  interesting  from  the  fact  that  we 
now  have  for  the  first  time  in  the  bird  distinct  traces  of  the  eyes.  At  the 
point  ec  the  medullary  canal  is  seen  to  be  distinctly  dilated  in  excess  of  the 


DEVELOPMENT   OF   THE    EYE. 


13 


regions  behind  it.  This  dilatation  is  the  first  indication  of  the  differentia- 
tion of  the  ocular  portion  of  the  ectodermal  cerebro-spinal  rudiment.  In 
another  representation  of  the  anterior  end  of  an  embryo  chick  another  hour 
older  than  the  last  (Fig.  4)  this  anterior  dilatation,  Vj,  is  still  more  marked, 
and  indications  of  a  second  less  marked  dilatation  of  the  medullary  canal, 
V2,  may  also  be  noted,  with  even  a  trace  of  a  third  dilatation  behind  the 
latter.  These  three  primary  dilatations  of  the  medullary  canal  constitute 
the  three  primary  or  embryonic  cerebral  vesicles.  Of  these,  the  first  is 
intimately  associated  with  the  future  development  of  the  retina,  the  eyeball, 
and  the  third  ventricle  of  the  brain ;  the  second,  with  the  development  of 
the  cerebral  peduncles,  optic  thalami,  optic  tract,  geniculate  bodies,  and 
aqueduct ;  while  the  fate  of  the  third  is  to  become  the  fourth  ventricle, 
cerebellum,  pons,  and  medulla  oblongata.  In  Figs.  5  and  6,  representing 


Fro.  4. 


FIG.  5. 


Head  of  embryo  chick  of  26  hours,  from 
above. — A,  anterior  end  of  head,  neural  tube  not 
yet  quite  closed  anteriorly ;  Vi,  first  cerebral 
vesicle ;  ec,  outer  lateral  surface  of  ectoderm  of 
head;  V%,  second  cerebral  vesicle:  in,  lateral 
limit  of  foregut.  Enlarged  26  times.  (After 
Duval.) 


Head  of  chick  embryo  of  27  hours,  viewed 
from  above.— A,  anterior  end  of  head;  in,  an- 
terior and  lateral  limit  of  foregut;  m,  mesoderm 
of  pericardiac  cavity ;  h,  paired  rudiment  of 
heart;  om,  omphalomeseraic  veins;  6,  margin 
of  opening  into  foregut.  Enlarged  28  times. 
(After  Duval.) 


still  more  advanced  stages  of  the  anterior  ends  of  chick  embryos,  the  fusion 
of  the  opposite  edges  of  the  medullary  plate  has  been  about  completed ; 
but  it  is  obvious  that  the  first  cerebral  vesicle,  op  (Fig.  6),  is  expanding  at 
a  relatively  much  more  rapid  rate  than  the  parts  of  the  medullary  canal  or 
cerebro-spinal  rudiment  behind  it.  In  fact,  there  is  now  in  progress  a  very 
rapid  lateral  dilatation  of  the  anterior  end  of  the  medullary  canal,  and  it 
may  be  said  that  the  foundations  of  the  primary  optic  vesicles  have  now 
been  established  as  a  pair  of  diverticula  from  the  sides  of  the  anterior  end 
of  the  hollow  cerebro-spinal  rudiment.  In  Fig.  7,  representing  the  anterior 
end  of  an  embryo  of  the  rabbit  of  a  stage  about  parallel  with  that  of  the 
chick  embryos  just  described,  the  same  relation  of  parts  is  seen  ;  the  primary 
optic  vesicles,  op,  are  conspicuous,  and  the  three  primary  cerebral  vesicles  are 
also  distinctly  evident,  though  the  medullary  groove  is  not  yet  closed.  There 
is  here  the  same  kind  of  epithelium  forming  the  walls  of  the  primary 


14 


DEVELOPMENT   OF   THE    EYE. 


optic  vesicles  and  of  the  primary  cerebral  vesicles  as  in  the  embryo  chick. 
From  the  cells  of  the  walls  of  the  optic  vesicles  the  ganglion-cells  of  the 
retina,  the  rods  and  cones,  and  the  axis-cylinders  of  nerve-fibres  of  the  first 
portion  of  the  optic  nerve  are  to  be  differentiated  as  the  result  of  a  com- 
plex series  of  histological  transformations.  From  parts  of  the  walls  of  the 
optic  vesicles  the  pigmented  epithelium  of  the  choroid,  part  of  the  iris,  and 
the  embryonic  optic  stalk  are  also  to  be  formed.  The  steps,  however,  by 
which  these  changes  are  brought  about  proceed  in  a  manner  analogous  to 
those  that  we  have  already  traced  in  the  progress  of  the  differentiation  of  the 
medullary  canal  itself, — namely,  by  the  dilatation  of  one  part  and  the  con- 
striction of  another,  or  the  fusion  of  two  adjacent  parts  of  the  epithelial 
rudiments  concerned  at  another  point.  Where  such  dilatation  occurs  locally 
and  a  vesicle  is  pushed  or  extended  outward,  the  process  is  spoken  of  as 


FIG.  6. 


FIG.  7. 


Anterior  end  of  rabbit  embryo  of  8  days  and 
14  hours,  viewed  from  above  and  enlarged  15 
Head  of  embryo  chick  of  29  hours,  viewed  times.— op,  optic  vesicles;  \\,  Vz,  V3,  first,  second, 
from  above. — Am,  head-fold  of  amnion ;  op,  optic  third  embryonic  cerebral  vesicles ;  in,  outer  limit 
vesicle;  Vj,  second  cerebral  vesicle;  V3,  third  of  foregut;  pp,  anterior  part  of  body  cavity, 
cerebral  vesicle.  Enlarged  22  times.  (After  lateral  pericardiac  spaces;  A,  left  half  of  heart ;  vo, 
Duval.)  left  ompbalomeseraic  vein;  a,  anterior  or  aortic 

end  of  right  half  of  embryonic  heart ;  TO,  somites, 
myotomes,    or   "  protovertebrse ;"    pm,    parietal 
mesoblast  of  sides  of  body.    (Reduced  from  K61- 
liker.) 
/ 

evagination ;  where  such  a  process  pushes  and  extends  a  membrane  inward 
into  the  form  of  a  cup  or  sac,  it  may  be  s.poken  of  as  invagination. 
Where  fusion  of  two  parts  takes  place  along  their  edges  and  along  a  line, 
as,  for  example,  along  the  coalescing  edges  of  the  upturned  borders  of  the 
medullary  plate,  such  a  process  may  be  spoken  of  as  concrescence.  These 
processes,  variously  modified  and  supplemented  by  cell-differentiation  and 
cell-proliferation,  constitute  the  principal  methods  by  which  the  complex 
transformations  of  the  mammalian  embryo  and  of  its  parts  are  effected. 

These  statements  are  especially  well  illustrated  by  the  events  of  the 
subsequent  history  of  the  development  of  the  eye.  In  Fig.  8,  for  example, 
the  optic  vesicles,  op,  are  beginning  to  show  evidences  of  constriction  at 
their  bases  so  as  to  form  a  hollow  stalk,  a  condition  which  becomes  still 
more  obvious  in  Fig.  9.  In  fact,  the  optic  vesicles  now  appear  smaller 
than  the  intervening  median  portion,  "Fj,  representing  the  growing  first  cere- 


DEVELOPMENT   OF   THE   EYE. 


15 


bral  embryonic  vesicle.  This  constriction  of  the  basal  part  of  the  optic 
vesicle  leads  to  the  differentiation  of  the  hollow  optic  stalk  as  distinguished 
from  the  more  dilated,  distal,  optic  vesicle  proper.  This  differentiation 
of  the  distal  and  basal  parts  of  the  optic  vesicles  is  the  first  indication  of 
a  growing  distinction  between  what  is  to  become  the  foundation  of  the 
eyeball  and  of  what  is  to  direct  the  course  of  the  development  of  the 
ingrowing  portion  of  the  optic  nerve,  since  the  optic  stalk  is  not  directly 
transformed  into  the  optic  nerve,  as  we  shall  learn  later.  The  tendency 
for  the  optic  vesicles  to  be  pushed  slightly  backward  distally  is  also  obvious 
now,  as  well  as  their  close  apposition  against  the  inner  face  of  the  super- 
ficial ectoderm  of  the  head,  as  shown  in  Figs.  6,  8,  and  9. 

While  the  last  two  figures  represent  the  three  primary  cerebral  vesicles 
very  strongly  accentuated  as  Vlf  V2,  V3,  it  is  obvious  that  there  are  evidences 


FIG.  8 


FIG.  9. 


Head  of  embryo  chick  of  32  hours,  viewed 
from  above. —  V\,  first  cerebral  vesicle;  op,  optic 
vesicle ;  F2,  second  cerebral  vesicle ;  g,  facial  and 
auditory  ganglion;  aw,  auditory  ganglion;  h, 
heart.  Enlarged  22  times.  (After  Duval.) 


Head  of  embryo  chick  of  38  hours,  viewed 
from  above.—  Fi,  first  cerebral  vesicle  =  third 
ventricle ;  op,  optic  vesicle ;  V->,  second  cerebral 
vesicle;  Fs,  third  cerebral  vesicle  =  fourth  ven- 
tricle ;  g,  facial  ganglion ;  au,  auditory  vesicle ; 
gl,  glosso-pharyngeal  ganglion.  Enlarged  about 
20  times.  (After  Duval.) 


of  a  subdivision  of  F3  into  a  series  of  segments  by  at  least  five  very  slight 
subordinate  constrictions.  These  subordinate  subdivisions  of  the  posterior 
portion  of  that  part  of  the  medullary  tube  which  is  to  form  the  brain  are 
indicative  of  the  segmental  nature  of  the  posterior  portion  of  that  organ. 
And,  since  it  is  possible  to  trace  a  tendency  toward  segmental  differentiation 
of  the  cerebro-spinal  embryonic  axis  into  the  trunk,  it  does  not  seem  improb- 
able that  the  indications  of  segmental  differentiation  in  the  posterior  part 
of  the  rudiments  of  the  brain  are,  like  those  of  the  trunk,  traceable  to  the 
originally  segmental  nature  of  this  part  of  the  head.  This  is  rendered 
quite  certain  by  the  fact  that  recent  research  upon  the  development  of  the 
head  of  the  shark-like  fishes  shows  a  segmented  condition  in  the  early 
stages  directly  comparable  to  that  which  is  so  very  obvious  in  the  trunks  of 
vertebrate  embryos  as  blocks  or  segments  of  mesoderm,  as  shown  in  Fig.  7 
at  m.  The  segmented  or  metameric  nature  of  the  head  of  all  vertebrate 
embryos  is  now  a  well-recognized  canon  of  morphology,  but  the  facts  upon 


16 


DEVELOPMENT   OF   THE    EYE. 


which  this  conclusion  is  based  have  been  almost  entirely  disclosed  by  em- 
bryological  research,  this  doctrine  no  longer  deriving  any  essential  support 
from  the  consideration  of  the  segmentation  of  the  hard  or  bony  part  of 
the  adult  cranium.  It  is  a  singular  fact,  however,  that  in  the  higher 
vertebrates,  birds  and  mammals,  in  which  the  early  steps  of  development 
have  been  much  condensed  or  abbreviated,  there  seems  to  be  no  very  clear 
record  left  of  the  exact  number  of  cranial  segments  when  the  embryonic 
history  of  the  soft  parts  is  examined.  In  the  shark-like  fishes  there  seems 
to  have  been  but  little  abbreviation  or  elision  of  primitive  embryonic 
features,  and  it  is  to  them  that  embryologists  have  been  compelled  to  appeal 
in  order  to  get  a  clear  notion  of  the  history  of  the  cranial  nerves,  and 
especially  of  the  relations  of  some  parts  of  these  primitive  cranial  segments 
to  the  development  of  the  muscles  that  move  the  eyeballs,  as  we  shall  learn 
later. 

The  three  primary  embryonic  vesicles  are  not  to  be  confounded  with 
the  single  segmental  or  metameric  elements  of  the  nervous  system,  of  which 
the  third  cerebral  vesicle  is  so  obviously  built  up.  It  is  far  more  probable, 
indeed,  that  the  two  anterior  cerebral  vesicles  will  ultimately  prove  to  be 
embryologically  composite  structures,  and  be  found  to  be  built  up  of  meta- 
meres  or  segments,  the  boundaries  between  which  have  become  obscured  by 
the  manifold  ways  in  which  their  development  has  been  abbreviated  and 
modified  in  higher  types. 


FIG.  11. 


FIG.  12. 


Side  view  of  the  head  of  a 
human  embryo  of  about  the 
same  age  as  the  preceding.  En- 
larged 20  times.  (After  His.) 


Oblique  view  of  the  head 
and  forepart  of  trunk  of  a 
human  embryo  of  12  to  15  days. 
The  almost  globose  forepart  of 
the  head  is  prominent  laterally 
at  o,  where  the  optic  vesicle  of 
the  right  side  rests  against  the 
cephalic  ectoderm.  The  form 
of  the  mouth  is  also  obvious, 
with  the  mandibles  not  yet  com- 
pletely joined  in  the  middle 
line.  Enlarged  20  times.  (After 
His.) 

The  embryonic  transformation  of  the  cerebro-ocular  region  of  the 
medullary  plate  in  the  human  embryo  is  very  similar  to  that  traced  and  il- 
lustrated above  in  the  case  of  the  embryonic  bird.  The  accompanying  Figs. 
10  to  14  show  the  heads  of  human  embryos  ranging  in  age  from  twelve 
to  twenty  days.  In  the  surface  views,  Figs.  10,  11,  and  13,  there  are  still 


Side  view  of  the  head  of  a 
human  embryo  of  the  same  age 
as  the  two  preceding,  but  with 
the  outer  ectoderm  and  meso- 
derm  of  the  head  removed  to 
show  the  form  and  volume  of 
the  brain  and  cord  and  the  very 
early  form  of  the  optic  vesicle 
or  rudiment  of  the  eye,  or>,  and 
its  stalk,  st.  Enlarged  20  times. 
(After  His.) 


DEVELOPMENT   OF   THE   EYE. 


17 


no  outward  indications  of  visual  organs  except  a  slight  prominence  at  o, 
which  indicates  the  position  externally  of  the  underlying  optic  vesicles, 
now  quite  developed  to,  or  slightly  beyond,  the  condition  shown  by  the 
oldest  bird  embryo  thus  far  described.  Upon  the  removal  of  the  external 
ectoderm  of  the  head,  as  has  been  done  in  Figs.  12  and  14,  .which  corre- 
spond respectively  to  Figs.  11  and  13,  the  optic  vesicles,  ov,  are  disclosed, 


FIG.  13. 


FIG.  14. 


Head  of  human  embryo  of  18  to  20  days, 
viewed  from  the  side.  Enlarged  20  times,  show- 
ing the  three  anterior  visceral  clefts  and  the 
lateral  thickening  at  o,  indicating  the  position  of 
the  primitive  optic  vesicles  or  rudiments  of  eyes. 
(After  His.) 


Side  view  of  the  head  of  a  human  embryo 
of  about  the  same  age  as  the  foregoing,  with  the 
ectoderm  and  mesoderm  of  the  head  removed  to 
display  the  form  and  great  relative  volume  of  the 
brain  and  cord,  as  well  as  the  optic  vesicle,  ov, 
with  its  stalk,  at.  The  primitive  aortic  arches  are 
shown  in  the  lower  part  of  the  figure.  (After 
His.) 


showing  their  attachment,  also,  to  the  sides  of  the  anterior  lower  border 
of  the  first  cerebral  vesicle  by  means  of  a  thick  stalk,  st.  These  stages 
of  the  development  of  man  correspond  pretty  closely  with  the  second  day 
of  the  chick,  and,  as  viewed  from  the  side,  they  show  very  clearly  the 
abrupt  flexure  or  bend  downward  of  the  anterior  end.  of  the  cerebro- 
spinal  rudiment,  which  has  carried  all  the  other  surrounding  organs  along 
with  it.  This  bend  is  known  as  the  cranial  flexure,  and  has  an  im- 
portant bearing  on  the  development  of  the  face.  This  cranial  flexure  is 
common  to  all  the  vertebrates  above  Amphioxus,1  but  is  most  marked  in 
the  higher  series.  It  is  noteworthy  that  the  eye  in  the  human  embryo  at 
this  stage  is  entirely  lateral  in  position.  It  is  only  in  consequence  of  the 
great  lengthening  of  the  optic  nerve  and  stalk,  and  the  correlated  shifting 
and  development  of  the  adjacent  organs,  that  the  eye  is  finally  brought 
round  into  an  anterior  position.  The  eye  at  this  stage  in  man  is,  in  fact, 
in  the  position  in  respect  to  the  axis  of  the  body  that  is  permanent  in 
the  great  series  of  lower  vertebrates,  or  the  fishes  and  batrachians.  This 
fish-like  character  of  the  early  human  embryo  is  also  very  clearly  shown 
by  the  obvious  visceral'  or  gill  clefts  visible  on  the  sides  of  the  future 
upper-neck-region  in  Figs.  10,  11,  and  13.  The  traces  of  the  first  pair 

1  Even  Amphioxus  shows  traces  of  the  development  of  the  cranial  flexure  in  the  embryo, 
according  to  Kupffer,  but  the  flexure  disappears  before  the  attainment  of  full  growth. 
VOL.  I.— 2 


18 


DEVELOPMENT   OF   THE   EYE. 


FIG.  15. 


of  sense-organs,  or  olfactories,  in  these  figures  are  still  less  obvious  ex- 
ternally than  the  development  of  the  foundations  of  the  eyes,  as  disclosed 
by  the  reconstructed  Figs.  12  and  14.  The  face,  as  a  whole,  is  still  with- 
out any  hint  of  resemblance  to  that  of  the  adult.  The  mouth  is  almost 
quadrangular  in  outline,  and  the  oral  and  nasal  epithelial  tracts  are  con- 
tinuous, and  in  no  way  differentiated  from  one  another,  judging  from 
external  appearances  alone.  The  foundations  of  the  brain  and  of  the 
eyes  lie  in  almost  immediate  contact  with  the  general  ectoderm  covering 
the  head,  and,  taken  together,  now  constitute  the  principal  part  of  the  sub- 
stance of  the  latter. 

A  cross-section  carried  through  the  point  o,  Fig.  13,  would  disclose  a 
relation  of  parts  somewhat  similar  to  that  shown  in  Fig.  15  through  the 
first  cerebral  vesicles  and  eyes  of  a  chick  embryo  of  the  early  part  of  the 
third  day,  though  the  latter  figure  really  represents  a  condition  slightly 
more  advanced.  In  Fig.  15  the  ectoderm  of  the  head  is  seen  to  lie  in 
close  contact  with  the  eyes  laterally  and  with  the  walls  of  the  first  cerebral 

vesicle,  Vly  dorsally  and  ventrally.  The 
optic  stalk  st  has  been  decidedly  nar- 
rowed, though  still  hollow,  as  indicated 
at  o.  The  optic  vesicles  are  in  fact  now 
pedunculate.  A  change  has  also  taken 
place  in  the  relations  and  form  of  the 
epithelium  of  the  distal  extremity  of  the 
optic  vesicle.  This  distal  extremity  is  no 
longer  convex  exteriorly,  as  shown  in 
Figs.  8  and  9,  but  concave.  The  distal 
extremity  of  the  primary  optic  vesicle  has 
08  in  fact  been  invaginated  so  as  partially  to 

Cross-section   through   the   forepart  of       rv,  ,,  ..  .  ,  . 

the  head  of  an  embryo  chick  of  52  hours,    Obliterate    the    cavity    Within,    as    IS   well 

showing  the  completion  of  the  involution   shown  in  Fig-.  9  at  an  earlier  stage.    The 

of  the  secondary  optic  cup.    (Reduced,  after  ,  . 

Duvai.)— os,  optic  stalk;  o,  canal  in  same   result  of  this  second  change  in  the  optic 

o7,rX™;S '™2  ***•  is  to  P™*™  o  «*<***  »  vesicle 
cerebral  vesicle,  F,;  op,  optic  cup;?,  lens;  R,   on  the  outer  face  of  its  distal  extremity 

retina.    None  of  the  mesoderm  is  shown  ex-    r  f  ,1  11   j         .• 

cept  the  walls  of  the  blood-vessels.  Enlarged    SO  as  to  *°rm  tne   SO-called   optic  cup,  or 
30  times.  secondary  optic  vesicle,  in  contradistinc- 

tion to  the  primary  one.    This  secondary 

optic  vesicle  or  optic  cup  now  becomes  the  foundation  of  the  eyeball,  or  globe. 
The  changes  which  are  a  prelude  to  the  condition  shown  in  Fig.  15  are 
well  illustrated  by  Fig.  16,  which  represents  the  relations  of  the  ectodermal 
layers  which  enter  into  the  formation  of  the  eye  of  a  chick  embryo  some- 
what younger  than  that  shown  in  the  preceding  figure.  The  ectoderm,  h,  is 
shown  as  lying  close  against  the  outer  extremity  of  the  optic  vesicle,  and 
where  it  comes  in  contact  with  the  latter  is  slightly  thickened  as  the  rudi- 
ment of  the  lens  I,  which  lies  in  close  contact  with  the  wall,  r,  of  the  outer 
end  of  the  optic  vesicle.  This  part,  r,  of  the  wall  of  the  primary  optic 


DEVELOPMENT   OF   THE   EYE. 


19 


FIG.  16. 


vesicle  is  destined  to  become  the  retina,  whilst  the  distal  part  of  p,  or  the 
wall  of  the  optic  stalk  nearest  the  retinal  rudiment,  is  destined  to  become 
the  pigmented  epithelium  next  the  supra-choroid.  The  proximal  part  of 
the  wall  of  the  stalk,  p,  becomes  the  definitive  hollow  optic  stalk,  with  a 
passage  leading  from  the  cavity  of  the  primitive  optic  vesicle  to  the  cavity 
within  the  brain-wall,  vh.  This  continuity  of  the  cavity  of  the  first  cere- 
bral vesicle  and  the  primary  optic  vesicle  will  be  self-evident  from  an 
inspection  of  Figs.  8  and  9.  As,  however,  the  retinal  wall  of  the  optic 
cup  is  pushed  inward,  as  in  Fig.  15,  the  cavity  of  the  primary  optic  vesicle 
is  obliterated,  since  this  secondary  retinal  vesicle  or  optic  cup,  as  we  may 
call  it,  is  expanding  inward,  outward,  upward,  forward,  backward,  and 
downward,  so  as  to  develop  a  spherical  enlargement  of  itself  as  the  founda- 
tion of  the  eyeball  and  its  included  vitreous  cavity 
within  the  cup.  While  this  process  is  in  progress, 
the  retinal  wall  of  the  optic  cup  is  thickening. 
The  retinal  wall  is  also  seen  to  be  continuous  with 
the  outer  wall  of  the  optic  cup  at  its  rim,  as  shown 
in  Fig.  15.  The  result  of  this  is  that  the  walls 
of  the  secondary  optic  vesicle  or  cup  are  double ; 
this  doubling  is  the  result  partly  of  a  thrusting 
of  the  wall  of  the  outer  extremity  of  the  primary 
optic  vesicle  into  the  hollow  proximal  or  stalk  por- 
tion of  the  vesicle,  accompanied  by  an  expansion 

*  Diagram  showing  relations 

of  the  stalk  portion,  next  the  retinal  vesicle,  over  the  of  the  ectodermai  rudiments  of 
latter.  The  outer  wall  of  the  double- walled  cup 
thus  formed  becomes  the  pigmented  or  choroidal 
epithelium.  The  relations  of  the  double  walls  of 
the  optic  cup  at  this  time  have  been  not  inaptly 
compared  to  the  relations  between  the  double  walls 
of  a  Tantalus's  cup  with  the  space  between  them. 
The  lens-rudiment  is  meanwhile  being  invagi- 
nated  from  the  ectodermai  area,  /,  Fig.  16.  In  Fig.  15  this  involution  has 
been  completed,  and  the  lens  now  lies  free  in  the  cavity  of  the  retinal  cup, 
the  latter  already  retreating  from  contact  with  the  lens,  thus  giving  rise,  in 
the  bird,  to  a  space  between  the  lens  and  the  retina,  that  represents  the  vitre- 
ous body.  The  vitreous  cavity  is  already  very  obvious  in  Fig.  15,  whereas 
in  the  younger  stage  represented  by  Fig.  16  the  rudiment  of  the  lens  I,  and 
the  retina  >•,  are  still  in  contact.  In  Fig.  15  the  origin  of  the  optic  stalk 
from  the  ventral  portion  of  the  wall  of  the  first  cerebral  vesicle  is  also  very 
obvious.  Upon  the  first  appearance  of  the  primary  optic  vesicles  the 
origin  of  the  optic  stalks  from  the  brain-wall  is  not  so  obviously  ven- 
tral, but  almost  lateral ;  in  fact,  there  is  very  strong  ground  for  the  belief 
that  the  ventral  position  of  the  origin  of  the  optic  stalk  in  the  vertebrates 
is  a  secondary  and  not  a  primary  characteristic.  The  optic  nerve,  if  it  is 
to  be  homologized  serially  with  the  other  cranial  and  spinal  nerves,  must, 


the  eye  in  an  embryo  chick  at 
the  end  of  the  second  day.  En- 
larged 75  times.  (Eeduced 
from  Kolliker.) — h,  superficial 
ectoderm;  I,  portion  of  the 
latter  which  will  become  the 
lens:  r,  retinal,  pa,  choroid 
epithelium;  p,  optic  stalk;  s, 
brain- wall;  vh,  anterior  cere- 
bral vesicle. 


20 


DEVELOPMENT  OF   THE   EYE. 


on  account  of  its  function,  be  regarded  as  sensory.  This  view  of  its 
nature  is  strongly  supported  by  the  early  history  of  its  ganglionic  rudi- 
ment or  retina,  which,  before  the  closure  of  the  medullary  plate,  is  now 
known  to  lie  quite  near  the  lateral  margin  of  the  latter,  or  in  the  analo- 
gous position  of  the  serially  homologous  rudiments  of  the  ganglia  of  the 
sensory  roots  of  the  spinal  nerves  along  the  course  of  the  spinal  cord. 
Since  these  arise  from  differentiations  developed  along  the  extreme  outer 
edges  of  the  medullary  plate,  the  neural  crest  of  Balfour  is  probably  made 
up  of  the  rudiments  of  these  ganglia  that  have  fused  along  the  line  of 
closure  of  the  medullary  canal,  as  contended  by  Beard.  If  the  retinal 
area  at  the  anterior  end  of  the  medullary  plate  is  serially  homologous 
with  the  rudiments  of  the  sensory  ganglia  of  the  spinal  nerves,  as  would 
seem  to  be  indicated  by  its  lateral  position  in  some  forms  (Necturus),  Miss 
Platt's  contention  that  the  optic  nerve  is  primarily  of  dorsal  origin  gains 
in  probability,  and  her  ingenious  explanation  of  the  change  in  the  ultimate 
position  of  its  origin  also  becomes  more  plausible.  Finally,  it  may  be 
added  that  the  primarily  centripetal  ingrowth  of  the  axis-cylinder  fibres 
of  the  ganglionic  cells  of  the  retina,  as  established  by  recent  investigators, 
is  further  proof  of  the  same  conclusion. 

If  the  whole  secondary  optic  cup  and  stalk  could  be  removed  and  iso- 
lated from  an  embryo  chick  somewhat  older  than  that  shown  in  Fig.  15, 
we  should  get  a  structure  which  in  perspective  when  viewed  from  in  front 

obliquely  would  appear  very  much  as  in  Fig.  17. 
In  this  condition,  however,  the  edges  of  the  cup 
would  begin  to  bend  inward  toward  the  lens  all 
round  except  below  at  ch,  the  position  of  the  so- 
called  choroid  fissure.  At  this  stage  the  retina, 
r,  is  already  markedly  thicker  than  the  pigmented 
layer,  p.  The  space,  s,  between  the  retina  and 
the  pigmented  layer  is  becoming  reduced.  The 
vitreous  cavity,  v,  is  spacious.  The  choroid 
fissure  leads  from  below  into  the  vitreous  space. 
This  choroid  fissure  is  also  extended  as  a  groove 
on  the  under  side  of  the  optic  stalk,  op,  for  some 
distance  inward  from  the  lower  margin  of  the 
optic  cup.  This  figure  must  be  supposed  to  be 
drawn  from  an  eye  in  the  stage  of  the  secondary 
optic  cup  with  all  the  investing  mesoderm  cleared 
away,  and  with  the  future  corneal  area  of  the 
ectoderm  on  its  outer  face  lifted  off,  together 
with  the  neighboring  ectoderm  of  the  head.  It 
represents  in  a  generalized  form  the  relations 
of  all  the  structures  of  ectodermal  origin  that  enter  into  the  formation 
of  the  vertebrate  eye  and  that  lie  within  the  sclerotic  and  the  cornea,  both 
of  which  latter  are  developed  considerably  later. 


FIG.  17. 


Enlarged  model  or  reconstruc- 
tion of  secondary  optic  cup  of  a 
vertebrate  embryo.  (Modified 
from  Hertwig.)— p,  outer  wall  of 
cup  destined  to  become  the  pig- 
mented choroidal  epithelium;  r, 
its  inner  wall,  or  retina;  «,  tem- 
porary space  between  the  two 
walls;  v,  vitreous  space;  I,  lens; 
ch,  choroid  fissure;  op,  hollow 
stalk  of  secondary  optic  vesicle, 
with  groove  on  the  under  side 
continuous  with  the  choroid  fis- 
sure, ch,  on  the  under,  outer  bor- 
der of  the  optic  cup. 


DEVELOPMENT   OF   THE   EYE.  21 

The  histological  changes  which  accompany  the  differentiation  of  the 
parts  of  the  eye,  as  these  are  laid  down,  are  very  remarkable.  For  example, 
it  may  be  said  that  the  whole  of  the  cerebro-ocular  portion  of  the  medul- 
lary plate  in  the  very  earliest  stages  of  its  differentiation  is  essentially  a 
continuous,  simple  epithelium  made  up  of  columnar  or  cubical  cells.  In 
the  course  of  the  foldings  and  transformations  which  have  attended  the 
evolution  of  the  eye  and  brain  up  to  the  point  illustrated  by  the  last 
few  of  our  figures,  histological  differentiation  has  been  going  on  at  a 
very  rapid  rate.  These  changes  are  more  profound  and  important  than 
one  is  at  first  disposed  to  think,  as  the  following  statement  of  fact  will 
disclose. 

The  retinal  area  begins  to  thicken  as  soon  as  the  secondary  optic  vesicle 
or  cup  begins  to  be  involuted.  This  thickening  of  the  retina  is  the  result 
mainly  of  the  rapid  multiplication  of  the  cells  on  what  is  now  its  convex 
and  future  outer  face.  While  the  constituent  cells  of  the  embryonic  retina 
still  show  a  tendency  to  remain  columnar,  they  soon  become  piled  upon 
one  another  in  layers  in  consequence  of  this  multiplication,  so  that  all  the 
cells  do  not  extend  entirely  through  its  thickness.  The  retina  thus  comes 
at  a  very  early  period  of  its  development  to  be  composed  of  a  number  of 
superposed  layers.  These  layers  very  soon  become  broken  up  into  groups, 
of  which  at  least  three  are  developed  quite  early.  Of  these  the  innermost, 
or  that  upon  its  concave  side,  is  destined  to  become  the  ganglionic  layer  of 
the  retina,  the  next  is  the  middle  cellular  layer,  and  the  outermost  is  the 
true  sensory  epithelial  layer,  from  some  of  the  cells  of  which  are  developed 
those  peculiar  sensory  extensions  known  as  the  rods  and  cones.  The  in- 
tervals of  separation  between  these  layers  soon  become  filled  up  with  finely 
reticular  or  granular  layers  of  substance,  forming  the  foundation  of  the  so- 
called  "  inner  and  outer  molecular  layers"  of  the  adult  retina.  The  first 
of  these  molecular  layers  to  appear  in  the  embryo  seems  to  be  the  "  inner" 
one.  From  what  is  now  known  of  the  history  of  the  inner  or  retinal  wall 
of  the  optic  cup,  there  can  scarcely  be  any  doubt  that  it  really  represents  a 
portion  of  the  primitive  brain  wall  or  cortex  precociously  projected  out- 
ward upon  the  optic  stalk,  where  its  histological  elements  undergo  a  most 
complex  series  of  formal  differentiations  that  are  analogous  to  some  of  those 
that  go  on  within  the  developing  brain.  If  it  is  admitted  that  the  retina 
is  a  precociously  separated  portion  of  the  brain  wall  or  cortex,  we  are  in 
some  measure  enabled  to  understand  the  peculiar  mode  of  development  of 
the  optic  nerve,  and  also  why  it  is  doubtful  whether  this  pair  is  a  true 
cranial  one.  This  difficulty  in  great  measure  disappears,  however,  if  it  is 
supposed  that  the  primary  retinal  area  is  primarily  lateral  or  at  the  edge 
of  the  medullary  plate,  and  therefore  ganglionic  in  position  when  its  first 
traces  appear  in  the  embryo. 

The  changes  which  go  on  in  the  shapes  and  relations  of  the  cells  forming 
the  outer  wall  of  the  secondary  optic  vesicle  are  much  simpler.  Here  there 
is  no  proliferation  of  cells  tending  to  thicken  this  wall ;  on  the  contrary, 


22 


DEVELOPMENT   OF   THE   EYE. 


FIG.  18. 


there  is  a  steady  diminution  of  thickness  of  the  outer  wall,  which  eventu- 
ally becomes  reduced  to  the  condition  of  a  simple  pavement  epithelium 
composed  of  a  single  layer  of  cells.  These  cells  are  so  arranged  in  rela- 
tion to  one  another  that  each  undergoes  extension  at  an  equal  rate  in  every 
direction  in  the  area  of  the  membrane  of  which  it  is  a  part,  so  that  the 
equal  pressures  it  exerts  in  every  direction  are  met  by  equal  resistances 
from  its  fellow-cells  that  surround  it.  Since  six  cells  is  the  usual  number 
that  surrounds  a  single  cell,  the  pressures  and  interactions  due  to  growth 
develop  a  tendency  in  each  of  these  cells  to  assume  a  six-sided  or  hex- 
agonal form.  Before  their  extreme  flattening,  however,  has  been  attained, 
these  cells  of  the  outer  wall  of  the  optic  cup  begin  to  develop  granules  of 
pigment  within  their  substance.  In  this  manner  the  cells  of  the  outer  wall 
become  transformed  into  the  pigmented  layer  or  choroidal  epithelium  of 
the  eye.  The  very  great  differences  already  presented  during  the  early 
stages  of  the  development  of  the  inner  and  outer  walls  of  the  optic  cup 
are  well  shown  in  Figs.  38  and  39  at  r  and  p. 

In  Fig.  18  we  have  represented  a  cross-section  of  the  optic  stalk  of  an 
embryo  mouse,  showing  the  generally  columnar  character  of  the  cells  of  the 

very  thick  inner  layer  continuous  with 
the  retinal  or  inner  layer  of  the  second- 
ary optic  cup.  The  outer  layer,  espe- 
cially on  the  upper  side  of  the  figure,  is 
seen  to  be  composed  of  columnar  or 
cubical  cells  in  a  single  layer,  and  this 
is  continuous  with  the  outer  wall  or  pig- 
mented layer  of  the  optic  cup.  On  the 
under  side  of  the  optic  stalk  there  is  a 
deep  fissure  caused  by  the  inflection  or 
infolding  of  the  ventral  portion  of  the 
originally  tubular  optic  stalk,  into  which 
mesoderm  has  been  intruded  along  with 
a  blood-vessel,  and  in  which  a  group  of 
blood-corpuscles  are  lying  free.  The 
endothelium  of  this  vessel  is  also  well 
marked  as  a  single  layer.  This  fissure 
pigmented  choroidal  epithelium  of  the  second-  js  the  extension  inward  upon  the  under 

ary  optic  cup.    Enlarged  200  times.    (After       .  ,          _    J          ...  ..        .  11        r   ^ 

Kessier.)  side  of   the   hollow  optic  stalk  of   the 

choroid  fissure  of  the  optic  cup.     Into 

it  have  been  intruded  a  vessel,  the  arteria  retinae  centralis,  already  noted, 
and  a  strip  of  mesodermal  tissue  continuous  with  that  which  immediately 
invests  the  stalk  externally.  The  great  differences  in  the  thicknesses  of 
the  inner  and  outer  layers  of  the  optic  stalk  are  here  as  noteworthy  as  the 
differences  between  the  thicknesses  of  the  inner  and  outer  walls  of  the 
optic  cup  itself.  It  is  also  worthy  of  note  that  the  originally  tubular  canal 
of  the  optic  stalk,  as  shown  in  Figs.  8  and  9,  has  now  been  almost  com- 


Optic  stalk  of  eye  of  mouse  embryo  in 
cross-section,  showing  lumen  of  central  artery 
Intruded  into  the  continuation  of  the  choroid 
fissure  ventrally ;  thick  inner  wall  continuous 
with  retina ;  thin  outer  wall  continuous  with 


DEVELOPMENT   OF   THE   EYE. 


23 


pletely  obliterated  by  the  inflection  of  its  inferior  wall  within  the  superior 
outer  wall,  so  that  the  two  are  in  contact  almost  the  whole  way  round. 
The  continuity  of  the  inner  and  outer  layers  of  the  stalk  along  the  edges 
of  the  extension  of  the  choroid  fissure  upon  the  optic  stalk  is  also  very 
evident.  The  edges  of  this  groove  ultimately  close,  so  that  little  or  no 
trace  of  it  is  left,  a  condition  of  affairs  that  is  still  further  modified  upon 
the  ingrowth  of  the  first-developed  fibres  of  the  optic  nerve  toward  the 
brain,  as  we  shall  learn  later. 

A  very  vivid  conception  of  the  position  and  relation  of  the  choroid  fissure 
can  be  got  from  an  inspection  of  Figs.  19  and  20,  representing  the  heads 
of  chick  embryos  of  the  third  day  of  incubation  as  viewed  from  the  side. 


FIG.  20. 


FIG.  19. 


Side  view  of  head  of  an  embryo  chick 
of  52  hours.  Enlarged  14  times.  (Reduced 
from  Duval.)—  op,  optic  cup  from  exterior, 
showing  choroid  fissure ;  ol,  olfactory  pit ; 
Vz,  second  cerebral  vesicle :  heart,  auditory 
vesicle,  and  three  visceral  clefts  are  also 
conspicuous. 


r, 
— —  \ 

ol  h 

Side  view  of  head  of  an  embryo  chick  of  68  hours. 
Enlarged  14  times,  to  show  the  relations  of  the  eye  to 
the  other  cerebral  structures  and  the  open  choroid  fis- 
sure. (Reduced  from  Duval.)— aw,  ear;  3,  third  visceral 
cleft,  1  and  2  in  front  of,  and  4  behind  it ;  h,  cerebral 
hemisphere ;  V\,  V2,  and  F3,  first,  second,  and  third  cere- 
bral vesicles ;  ol,  olfactory  pit.  The  somites  of  the  neck, 
head  under  V3,  and  trunk  are  also  shown,  and  the 
heart  is  the  large  organ  shown  in  outline  below  the 
visceral  clefts. 


The  double  wall  of  the  optic  cup  is  shown  in  optic  section  as  seen  by  trans- 
mitted light.  The  inner  retinal  layer  is  seen  to  be  already  thicker  than  the 
outer  layer,  which  is  destined  to  become  the  choroidal  epithelium.  The 
embryonic  eyeball,  or  optic  cup,  is  viewed  in  both  cases  from  its  outer  or 
external  aspect,  and  the  choroid  fissure  is  seen  to  interrupt  the  continuity 
of  its  ventral  margin.  Both  layers  of  the  cup  are,  however,  seen  to  pass 
into  or  be  continuous  with  each  other  on  either  side  of  the  fissure.  The 
eye,  as  a  whole,  is  seen  to  lie  in  a  rather  ventral  position  with  respect  to  the 
brain,  so  that  that  portion  of  the  first  cerebral  vesicle,  "Pi  (Fig.  20),  which  is 
to  form  the  future  optic  lobes — thalami — now  projects  high  above  the  level 
of  the  top  of  the  rudimentary  eyeball.  The  olfactory  pits,  ol,  have  also  been 
distinctly  differentiated,  as  well  as  the  rudiments,  in  Fig.  20,  of  the  cerebral 
hemispheres,  which  are  here  just  beginning  to  bud  outward  and  upward. 
The  vitreous  humor  is  at  this  stage  of  considerable  volume,  as  shown  in 


24  DEVELOPMENT   OF   THE    EYE. 

the  last  figure  by  the  space  between  the  retinal  wall  of  the  optic  cup  and 
the  lens  in  the  centre.  The  vitreous  humor  of  the  eye,  it  may  be  re- 
marked, soon  becomes  very  much  more  voluminous  in  embryo  birds  than 
in  embryo  mammals  of  the  same  relative  stage  of  development. 

It  is  during  the  stages  just  referred  to  that  the  differentiation  of  the 
epithelial  walls  of  the  brain  at  different  points  begins  to  be  very  distinctly 
manifest.  The  sides  of  the  walls  of  the  future  third  ventricle  (first  em- 
bryonic vesicle)  begin  to  thicken  where  the  optic  thalami  are  to  be  developed. 
A  little  farther  backward  the  corpora  bigemina  (developed  from  the  region 
marked  F2,  Fig.  20)  also  begin  to  undergo  a  slight  thickening  ventrally 
and  laterally.  This  region  of  the  corpora  bigemina  (corpora  quadrigemina 
of  mammals)  is  very  important  in  its  relation  to  the  development  of  the 
geniculate  bodies  and  optic  tract.  While  it  cannot  be  said  that  the  optic  tract, 
the  geniculate  bodies,  or  the  thalamus  are  yet  definitely  marked  out,  there  is 
now  an  obvious  tendency  for  the  internal  ganglionic  layers  in  these  regions 
to  begin  to  be  marked  off  from  the  pale  external  layer  or  mantle  of  axis- 
cylinder  fibres.  Farther  back,  in  the  region  of  the  future  fourth  ventricle, 
Vfr  there  is  a  tendency  for  the  dorsal  epithelial  wall  of  the  neural  tube  to 
become  quite  thin,  except  at  its  extreme  anterior  portion,  where  the  cere- 
bellum is  developed.  Ventrally  and  laterally  the  walls  of  the  third  cerebral 
vesicle,  F3,  are  now  rapidly  thickening.  This  thickening  is  here  associated, 
as  in  the  more  anterior  parts  of  the  brain,  with  the  development  of  the 
ganglionic  tracts  internally,  while  the  axis-cylinder  fibres  are  developing 
externally.  This  gives  to  cross-sections  through  the  walls  of  the  nervous 
tube  at  this  time  the  appearance  of  a  more  deeply  stainable  and  opaque 
inner  portion  corresponding  to  the  rudiments  of  the  ganglionic  tracts,  while 
externally  there  is  the  appearance  of  a  clear  belt  or  zone  that  does  not 
readily  stain,  that  represents  a  delicate  outer  layer  of  neuroglia  and  the 
beginnings  of  the  nerve-fibres,  which  have  already  grown  outward  from 
the  more  deeply  embedded  ganglionic  or  nerve  cells,  and  have  made 
their  way  longitudinally  for  some  distance  along  the  outer  faces  of  the 
thalamus,  pons,  and  medulla.  Of  the  cord  it  may  be  said  that  in  general 
the  differentiation  of  the  more  deeply  situated  ganglionic  tracts  begins  at 
its  cephalic  end  and  at  the  anterior  or  motor  side.  The  white  columns 
also  begin  to  develop  their  anterior  and  lateral  portions  first,  the  posterior 
columns  of  white  matter,  or  axis-cylinder  fibres,  appearing  last  of  all. 
The  relations  of  the  pale  outer  and  more  deeply  stainable  inner  layers  of 
the  brain  are  well  shown  in  the  walls  of  Vl  and  F2  of  Fig.  46,  representing 
a  section  through  the  head  of  an  advanced  embryo  bird. 

Some  idea  of  the  relations  of  the  optic  cups  to  the  central  nervous 
system  in  an  embryo  bird  of  the  fourth  day  may  be  formed  by  reference  to 
Fig.  21.  Here  the  optic  cups  have  been  fully  developed  as  such,  and  the 
whole  head  is  seen  from  above,  viewed  as  a  transparent  object.  It  is  ob- 
vious from  this  figure  that  the  changes  in  the  form  of  the  primary  cavities 
of  the  brain  since  the  end  of  the  first  day  have  been  many  and  profound. 


DEVELOPMENT   OF   THE   EYE. 


25 


FIG.  21. 


Head  of  an  embryo 
chick  82  hours  old,  viewed 
from  above. — km,  cerebral 
vesicles  or  hemispheres ; 
V\,  first  cerebral  vesicle  (= 
third  ventricle) ;  F2,  second 
cerebral  vesicle:  F8,  third 
cerebral  vesicle  (=  fourth 
ventricle).  Enlarged  about 
12  times.  (After  Duval.) 


The  optic  stalks  have  become  quite  narrow  and  slender.  The  lens  has  been 
separated  from  the  overlying  ectoderm  for  some  time.  The  first  primary 
cerebral  vesicle,  Vly  or  what  is  left  of  it,  may  now  be  said  to  represent 
distinctly  the  rudiment  of  the  third  ventricle  of 
human  anatomy.  It  has  also  been  much  narrowed 
laterally,  as  the  last  figure  shows.  The  hemispheres 
are  now  very  distinct  dorso-lateral  outgrowths  from 
the  first  embryonic  cerebral  vesicle,  and  both  still 
have  their  simple  epithelial  walls.  The  second  em- 
bryonic cerebral  vesicle,  F2, — the  future  corpora  bi- 
gemina, — has  grown  even  more  transversely  tHan  it 
has  vertically,  and  there  is  now  very  obviously  a 
spacious  cavity  within  it,  communicating  anteriorly, 
through  a  narrow  cleft,  with  F1?  and  posteriorly, 
through  an  equally  constricted  passage, — the  future 
aqueduct  in  part, — with  Vy  The  vesicle  V3  is  now 
constricted  anteriorly,  but  widens  again  abruptly  and 
then  slowly  narrows,  as  the  rudiment  of  the  medulla, 
as  it  is  continued  backward  into  the  spinal  cord.  The 
eyes,  however,  are  still  very  prominent  organs,  and, 
while  the  adjacent  parts  of  the  brain  have  been  ex- 
panding at  a  very  rapid  rate,  the  optic  cups  will  soon 
begin  to  grow  in  the  embryo  bird  at  a  disproportionately  quick  rate,  so 
that  they  will  soon  become  the  most  conspicuous  and  bulky  organ  of  the 
head.  The  maximum  volume  of  the  eyes  of  embryo  birds  is  reached 
about  the  ninth  day  of  incubation,  when  their  bulk  almost  or  quite  equals 
that  of  all  the  other  organs  of  the  head  taken  together.  In  this  regard 
the  embryos  of  birds  differ  very  greatly  from  those  of  mammals.  In 
mammals  the  eye  of  the  embryo  never  reaches  so  disproportionately  great 
a  size,  but  remains  small  in  comparison  with  the  other  parts  of  the  head, 
especially  as  compared  with  the  cerebral  hemispheres.  These  latter  soon 
overtop  all  the  other  organs  of  the  head  of  the  mammalian  embryo  in 
their  rate  of  growth,  since  they  very  early  become  extremely  massive,  pro- 
ject upward  and  forward,  and  cause  the  peculiarly  prominent  and  bullate 
form  of  the  forehead  of  young  foetuses  of  the  higher  mammalia  and  the 
human  species.  In  the  fishes  the  maximum  dimension  of  the  eyes  of  the 
embryo  is  reached  much  earlier,  proportionately,  than  in  birds. 

Up  to  the  end  of  the  fourth  week  the  eye  in  the  human  embryo  still  occu- 
pies almost  a  lateral  position,  as  seen  in  Fig.  22.  The  eye,  in  other  words, 
is  still  looking  laterally,  as  in  fishes,  and  is  not  prospective  or  forward- 
looking,  as  in  the  adult.  A  blunt  process,  or  downgrowth,  from  above  the 
external  nasal  process  separates  the  eye  externally  from  the  nasal  fissure  or 
pit,  the  rudiment  of  the  nostril  of  the  same  side.  Another  process,  or  up- 
growth, the  maxillary  process  of  the  soft  tissues  below  and  in  front  of  the 
eye,  meets,  but  does  not  yet  coalesce  with,  the  external  nasal  process.  The 


26 


DEVELOPMENT  OF   THE   EYE. 


FlG.  22. 


fissure  thus  left  between  the  eye  and  the  nasal  opening  of  the  embryo  marks 
the  course  of  the  lacrymo-nasal  groove,  which  remains  apparent  externally 
for  a  relatively  long  time.  It  is  along  the  floor  of  this  groove  during 
the  coalescence  of  its  edges  that  the  tear-duct  is  marked  off  and  separated 
from  the  epiblast  covering  the  groove.  During  the  later  transformations 
of  the  face  the  proximal  or  median  end  of  the  duct  becomes  more  and 
more  deeply  involved  by  the  surrounding  tissues 
and  down-growing  margins  of  the  external  nostrils. 
This  will  be  rendered  obvious  by  an  inspection  of 
the  figure  just  referred  to,  as  well  as  of  those  that 
follow.  The  mouth  as  such  is,  relatively  speaking, 
an  enormous  transverse  cleft  on  the  anterior  side  of 
the  head  during  the  fourth  week.  At  two  points 
on  either  side  of  the  median  line  it  communicates 
with  the  nasal  pits  or  fossae  above,  as  shown  in 
the  figure.  Should  there  be  a  failure  to  close  of 
either  or  both  of  these  clefts  that  connect  the  mouth 
and  nose  of  the  embryo  at  this  stage,  the  deformity 

Front  view  of  head  of  a    known  M  ^re-lip  would  be  the  result 
human  embryo  of  4  weeks.          The  cerebral  hemispheres  of  the  human  embryo 

Enlarged    10   times.     (After        n  ,1        /»        ,v  i  i  • 

Hi8.)-Showing  the  oral  open-     °f  the   f°Urth  Week   are   Seen  to  be  VeiT  massive   in 

ing,  joined  on  either  side  of   proportion  to  the  size  of  the  eyes  ;  much  more  so, 

the  middle  line  by  the  nasal     '  ,,  .,      ,        j      /.  ,1       i  •    i 

clefts  above,  and  these  later-    m  *ac*>  than  in  the  head  of  the  bird  embryo  repre- 
aiiy  by  the  lacrymo-nasal    sented  in  Fig.  21.    The  forehead  is  already  bullate 

grooves  from  the  eyes,  now 

lateral  in  position.  and  prominent,  and  a  slight  median  depression  indi- 


cates the  position  in  which  the  future  falx 
will  be  developed  between  the  hemispheres.  There  is  still  no  transverse,  hori- 
zontal separation  between  the  nasal  and  oral  chambers.  This  is  effected  later 
by  a  median  ingrowth  from  the  roof  and  a  pair  of  lateral  ingrowths  from  the 
sides  of  the  mouth  at  this  stage,  which  finally  meet  in  the  middle  line,  thus 
not  only  separating  the  internal  nasal  from  the  permanent  oral  cavity,  but 
also  dividing  the  nasal  chamber  itself  medially  into  its  right  and  left  halves. 
The  face  when  viewed  in  profile  is  concave,  with  scarcely  more  than  a  begin- 
ning of  an  indication  of  the  "  nose,"  in  the  form  of  the  fronto-nasal  pro- 
cess lying  between  the  two  nasal  pits  and  above  the  oral  opening  in  Fig.  22. 
As  we  shall  see  in  the  succeeding  figures,  the  "  nose,"  so  conspicuous  a 
feature  of  the  adult  human  profile,  develops  very  gradually  and  very  tardily 
by  the  growth  forward  and  downward  of  the  margins  of  the  nasal  pits,  the 
tip  of  the  nose  being  ultimately  formed  by  the  advancing  apex  of  the  fronto- 
nasal  process,  while  the  upper  lip  is  formed  partially  by  the  downgrowth 
of  the  fronto-nasal  process  and  partially  by  the  coalescence  of  that  process 
with  the  maxillary  processes  of  either  side. 

The  foregoing  account  of  the  early  development  of  the  eyes,  head, 
and  face  of  the  human  embryo  may  appropriately  be  supplemented  with 
an  account  of  the  development  of  the  corresponding  parts  of  the  em- 


DEVELOPMENT  OF   THE   EYE. 


27 


bryo  of  the  bird.  The  parts  corresponding  in  the  bird  to  the  nasal  pits 
or  fossae  are  represented  in  Figs.  23,  24,  25,  and  26.  The  very  gradual 
steps  by  which  the  external  nasal  and  maxillary  processes  fuse  with  the 


Front  of  head  of  chick  embryo  of  6  days. 
Enlarged  6  times.  (Reduced  from  Duval.)— h, 
cerebral  hemisphere;  F,  fronto-nasal  process: 
N,  internal  nasal  process ;  n,  external  nasal  pro- 
cess; o,  olfactory,  external  nasal,  opening;  I, 
fissure  along  which  the  nasal  duct  develops ;  m, 
maxillary  process;  mb,  mandibular  process  or 
arch ;  1,  auditory  meatus ;  hy,  hyoid  arch. 


FIG.  24. 


Front  of  head  of  chick  embryo  of  7  days. 
Enlarged  4%  times.  (Reduced  from  Duval.)— F, 
fronto-nasal  process;  n.  cerebral  hemisphere; 
o,  nasal  opening;  m,  maxillary  process ;  mb,  man- 
dibular process  or  arch ;  1,  auditory  meatus. 


fronto-nasal  process  to  form  the  upper  beak  of  the  bird,  with  the  ex- 
tension of  all  these  and  their  convergence  toward  the  median  line,  will  be 
obvious  from  the  mere  inspection  of  the  figures.  These  figures  will  serve  to 
render  still  more  impressive  some  of  the  other  changes  which  have  gone 
on  in  the  development  of  the  external  eye  and  eyeball.  The  great  and 


FIG.  25. 


FIG.  26. 


Front  view  of  head  of  8-day  chick  embryo. 
Enlarged  4  times.  (Reduced  from  Duval.)—  F, 
fronto-nasal  process,  with  egg-tooth  rudiment 
appearing  on  its  tip ;  n,  external  nasal  process ; 
o,  external  nasal  opening ;  m,  maxillary  process, 
lacrymo-nasal  groove  still  obvious,  but  eyelids 
more  pronounced;  1,  auditory  meatus. 


Front  view,  slightly  oblique,  of  head  of  an 
embryo  chick  of  9  days.  (Reduced  from  Duval.) 
— F,  fronto-nasal  process  becoming  pointed,  with 
a  suggestion  of  its  future  form,  as  the  upper  beak ; 
t,  ridge  marking  the  position  of  line  along  which 
the  nasal  duct  is  formed ;  margins  of  eyelids  and 
margin  of  third  eyelid  now  very  well  differen- 
tiated ;  1,  auditory  meatus. 


rapid  increase  in  the  size  of  the  eyeball,  already  noted,  is  one  of  the  most 
striking  changes  which  these  figures  illustrate.  They  also  illustrate  very 
clearly  the  fact  that  the  optic  cup  of  Figs.  17  and  21  is  losing  its  cup- 
like  character  and  may  now  be  more  properly  spoken  of  as  the  optic  globe 


28  DEVELOPMENT   OF   THE   EYE. 

or  definitive  eyeball ;  the  edges  of  the  optic  cup  have,  in  fact,  been  now 
deflected  inward,  while  the  cup  itself  has  grown  so  greatly  in  size  that  the 
primitively  wide  opening  into  it  on  its  outer  aspect  is  reduced  to  a  small 
round  hole,  the  pupil.  The  edges  of  the  optic  cup  now  represent  also 
the  margin  of  the  iris,  of  which  we  shall  have  more  to  say  later. 

The  relatively  great  size  of  the  eyes  in  the  embryo  bird  during  the 
stages  represented  by  the  last  four  figures  gives  to  the  head  an  appearance 
of  inordinate  width.  The  eyes  are,  in  fact,  now  the  most  conspicuous 
organs  in  the  whole  head.  They  constitute  the  most  conspicuous  dilatation 
of  the  head  in  front  of  the  neck.  Just  a  little  behind  and  below  the  eye, 
on  each  side  of  the  head,  the  upper  portion  of  the  first  visceral  cleft, 
1,  remains  open,  as  the  external  auditory  meatus  or  external  ear,  but 
there  is  no  considerable  development  of  the  margin  of  this  opening,  as 
in  mammals,  to  form  the  pinna,  or  external  ear. 

The  eyelids  are  already  developing  as  slight  folds  around  the  equator 
of  the  optic  globe,  as  shown  in  Figs.  23  and  24,  but  the  third  eyelid,  or 
membrana  nictitans,  does  not  become  a  very  conspicuous  object  until  some 
time  later,  or  about  the  ninth  day,  when,  as  in  Fig.  26,  it  is  conspicuous  as  a 
wide  semilunar  fold  (plica  semilunaris)  within  the  outer  eyelid  proper,  next 
the  inner  canthus.  The  eyelids  proper,  however,  are  not  yet  extended  over 
the  whole  external  face  of  the  eyeball,  and  this  does  not  happen  in  the 
embryo  bird  until  near  the  middle  of  the  third  week  of  incubation.  In 
Figs.  25  and  26  the  course  of  the  nasal  or  tear  duct  from  the  eye  toward 
the  nasal  cavity  can  be  distinctly  traced  from  the  inner  canthus.  The 
features  which  are  distinctly  diagnostic  of  the  class  of  birds,  so  far  as  the 
appearance  of  the  head  and  body  is  a  guide,  at  the  stage  represented  by 
Fig.  26,  are  the  general  form  of  the  head,  the  obvious  beak,  the  large  eyes, 
and  the  incipient  germs  of  feathers  over  some  parts  of  the  body.  The 
nostrils  already  occupy  their  lateral  positions  near  the  base  of  the  upper 
beak,  and  the  position  of  the  egg-tooth  is  indicated ;  the  oral  and  nasal 
cavities  are  diiferentiated  from  each  other.  Figs.  23  and  24  also  illustrate 
quite  as  clearly  as  Fig.  22  the  relations  of  the  first  visceral  arches,  mb,  to 
the  development  of  the  mandible,  and  of  the  maxillary  processes,  m,  to  the 
development  of  the  maxillae  or  upper  maxillary  region. 

The  preceding  sketch  of  the  earlier  stages  of  the  development  of  the 
face  may  be  completed  by  a  description  of  the  appearances  in  profile  of  a 
series  of  human  embryos  ranging  in  age  from  the  sixth  to  the  eleventh 
week  of  intra-uterine  development.  Figs.  27  and  28  show  the  lacrymo- 
nasal  groove  extending  from  the  eye  to  the  outer  border  of  the  nasal 
opening  along  a  line  marked  in  the  adult  approximately  by  a  line  drawn 
from  the  inner  canthus  of  the  eye  to  a  point  on  the  outer  posterior  margin 
of  the  nostril  next  the  upper  lip  and  just  within  the  ala,  or  thickened 
margin  of  the  edge  of  the  nostril  where  it  joins  the  cheek.  This  line  also 
in  the  adult  lies  approximately  parallel  with  the  tear  or  nasal  duct,  thus 
showing  how  nearly  the  involution  of  the  ectoderm  which  gives  rise  in  the 


DEVELOPMENT   OF    THE    EYE. 


29 


embryo  to  the  nasal  or  tear  duct  lies  parallel  or  coincident  with  the  line 
along  which  the  lacrymo-nasal  groove  also  closes  in  the  embryo. 

The  nasal  duct  arises  as  a  thickening  of  the  under  side  of  the  epidermis 
along  the  line  of  the  lacrymo-nasal  groove.  In  man  this  differentiation 
begins  about  the  end  of  the  fifth  or  the  beginning  of  the  sixth  week.  The 
thickening  forms  a  solid  ridge,  which  then  separates,  except  at  each  end,  as 
a  solid  cord.  This  cord  then  acquires  a  lumen,  or  passage,  within,  so  as  to 
become  a  canal  of  epidermal  (ectodermal)  origin.  The  upper  end  of  the 
originally  solid  cord  expands  at  the  inner  canthus,  preliminary  to  dividing 
into  two  branches  which  are  to  become  the  lacrymal  canaliculi,  that  end  at 
two  small  openings,  the  puncta  lacrymalia,  near  the  inner  borders  of  the 
upper  and  lower  lids  respectively,  just  external  to  the  lacus. 

The  external  lacrymo-nasal  groove  itself  ultimately  disappears  upon 
the  approximation  and  coalescence  of  the  juxtaposed  edges  of  the  external 
nasal,  fronto-nasal,  and  maxillary  processes.  At  a  late  stage  no  external 


FIG.  27. 


FIG.  28. 


Side  view  of  head  of  human  embryo  of  37  to 
38  days  and  14.5  millimetres  long,  to  show  the 
relations  of  the  eye  to  the  lacrymo-nasal  groove. 
(Reduced  from  His's  Atlas.) 


Side  view  of  head  of  human  embryo  of  39  to 
40  days  and  15.5  millimetres  long,  showing  in- 
cipient eyelids  appearing  as  faint  folds  above 
and  below  the  eye.  (Reduced  from  His's  Atlas.) 


traces  of  this  are  apparent  in  the  Batrachia,  Reptilia,  Birds,  and  Mammals, 
but  in  Selachians  part  of  what  represents  the  continuation  of  this  groove 
to  the  upper  border  of  the  mouth  in  mammalian  embryos  persists  as  a  per- 
manent fissure  connecting  the  olfactory  pit  on  each  side  with  the  upper 
border  of  the  mouth. 

The  upper  part  of  the  face  is  bullate  in  all  the  remaining  figures  of  the 
human  embryo  here  given,  owing  to  the  great  size  of  the  overlying  cerebral 
hemispheres.  In  Fig.  27,  traces  of  the  connection  of  the  external  auditory 
meatus  with  the  remainder  of  the  first  visceral  cleft  are  still  visible.  The 
head  is  strongly  flexed  upon  the  thorax,  and  the  first  indications  of  the 
digits  and  elbow  are  apparent.  The  eyes  are  still  lateral  in  position.  The 
nose  is  not  yet  prominent.  The  fourth  ventricle  is  still  a  spacious  and 
massive  part  of  the  central  nervous  system  in  proportion  to  the  other  por- 
tions, and  there  is  a  sharp  bend  at  the  point  where  in  future  the  upper  part 
of  the  neck  will  be  situated.  The  profile  is  almost  plane.  The  optic  cup 


30 


DEVELOPMENT   OF   THE    EYE. 


is  in  an  advanced  stage  of  development,  so  that  the  pupil  is  outlined.  The 
inner  and  outer  canthi  are  not  yet  marked  out,  however.  In  the  next 
phase  (Fig.  28),  the  first  well-marked  traces  of  the  folds  above  and  below 
the  eye  that  are  appearing  are  the  rudiments  of  the  future  eyelids.  The 
nose  is  here  becoming  just  a  little  more  prominent  than  in  the  foregoing 
figure,  the  forehead  still  more  bullate,  the  head  not  so  strongly  flexed  on 
the  trunk,  and  the  bend  in  the  elbow  still  more  marked.  The  eyes  are  also 
beginning  to  be  turned  so  as  to  look  obliquely  forward ;  that  is,  the  growth 
of  the  parts  adjacent  to  the  eyes  is  taking  place  in  such  a  way  that  the  eyes 
are  being  swung  forward.  The  optic  axes  of  both  eyes  would  now  con- 
verge if  produced  backward  into  the  head. 

The  next  three  figures  illustrate  the  further  external  changes  that  take 
place  in  the  head  of  the  human  embryo  up  to  the  time  of  the  closure  of  the 
eyelids  and  the  consequent  completion  of  the  formation  of  the  conjunctival 
sac.  The  method  of  the  development  of  the  lids  has  been  already  indicated 
in  the  account  of  the  development  of  the  bird.  Here  we  shall  have  to 
describe  the  changes  in  the  development  of  the  eyelids  more  in  detail.  The 

plica  semilunaris,  or  the  feature  representing  the 
well-developed  third  eyelid  of  the  advanced  em- 
bryo of  the  bird,  is  very  inconspicuous  from  an 
external  inspection  of  the  human  embryo  of  the 
stages  represented  in  Figs.  29,  30,  and  31.  The 
strong  flexure  of  the  head  forward  is  disappearing, 
and  there  is  only  a  strong  convexity  at  the  back 
of  the  neck  where  there  was  formerly  a  sharp 
bend.  The  brain,  especially  the  cerebrum,  has 
grown  very  greatly  in  volume,  so  that  the  face  is 
becoming  overhung  by  the  forehead.  The  head, 
as  a  whole,  is  becoming  globular,  and  is  growing 
faster  proportionally  than  the  body.  The  edges 
of  the  external  meatus  of  the  ear  are  growing 
upward  so  as  to  form  the  beginnings  of  the  pinna, 


FIG.  29. 


29,  while  in  Fig.  31  the  form  of  the  external  ear 
is  already  fully  outlined.  The  eyes  do  not  yet 
look  forward  in  Figs.  29  and  30,  but  obliquely 
outward  and  forward.  The  form  of  the  eves 


Side  view  of  head  of  human 
embryo  of  40-45  days  and  16 
millimetres  long,  showing  the 

further  differentiation  of  the    or  external  ear  in  the  embryo  represented  in  Fig. 

pupil  and  eyelids  beyond  the  * 

condition  shown  in  the  pre- 
ceding figure,  and  the  appear- 
ance of  the  inner  and  outer 
canthi  and  closure  externally  of 
the  lacrymo-nasal  groove.  (Re- 
duced from  His's  Atlas.) 

of  the  human  ftetus,  as  viewed  in  profile  at  this 

time,  reminds  one  very  forcibly  of  the  form  of  the  eye  as  modelled  by  the 
ancient  Egyptian  sculptors  in  profile  representations  of  the  adult  human 
figure  on  the  walls  of  their  temples.  The  course  of  the  line  of  closure  of 
the  lacrymo-nasal  groove  is  traceable  in  Fig.  29  when  compared  with  Fig. 
28.  It  appears  that  the  inner  canthus  is  slightly  deflected  toward  the  ala 
of  the  nose,  and  that  the  line  from  the  apex  of  the  inner  canthus  to  the  pos- 
terior part  of  the  nasal  opening  marks  the  course  of  the  line  of  closure  of  the 


DEVELOPMENT    OF   THE    EYE. 


31 


lacrymo-uasal  groove,  along  which  the  nasal  duct  is  included.  The  nose 
now  becomes  decidedly  more  prominent,  and  in  Fig.  29  we  begin  to  have 
an  obviously  human  expression  of  countenance.  While  the  upper  and 
lower  integumentary  folds  which  are  the  first  indications  of  the  develop- 
ment of  the  eyelids  have  not  yet  converged  internally  or  externally  to  form 
the  canthi,  such  a  convergence  has  occurred  in  the  stage  represented  by  Fig. 
29.  The  inner  canthus  is  still  more  definitely  established  in  the  stage 
represented  in  Fig.  30,  and  the  upper  end  of  the  nasal  duct  has  divided 


FIG.  30. 


FIG.  31. 


Side  view  of  head  of  human  embryo  of  58-62 
days  and  23  millimetres  long,  showing  lids  and 
canthi  still  more  developed  than  in  the  preceding 
figure.  (Reduced  from  His's  Atlas.) 


Profile  view  of  head  of  human  embryo  55 
millimetres  in  length  from  cranial  vertex  to  end 
of  coccyx ;  age  about  75  days.  Enlarged  about 
twice,  to  show  the  now  closed  lids  and  the  thick, 
whitish  superficial  layer  of  epithelium,  or  epi- 
trichium,  between  Itheir  edges,  by  means  of 
which  they  adhere  together.  (Original.) 


into  the  canaliculi  of  the  upper  and  lower  lids,  which  end  in  the  adult  at 
the  puncta  lacrymalia.  The  conjunctival  sac  has,  however,  not  yet  been 
completed,  and  this  is  not  accomplished  until  the.  folds  that  are  to  form  the 
upper  and  lower  eyelids  meet  in  front  of  the  eyeball,  as  shown  in  Fig.  31. 
From  the  manner  in  which  these  folds  grow  over  the  eyeball,  it  is  evident 
that  the  conjunctival  sac  is  lined  by  the  ectoderm.  After  the  eyelids 
meet,  as  shown  in  the  last  figure,  their  edges  coalesce,  and  remain  so  until 
shortly  before  birth.  The  epidermis  of  the  inside  of  the  lids,  as  well  as 
that  of  their  outer  faces,  is  ectodermal,  and  the  connective  and  muscular 
tissue  interposed  between  the  inner  and  outer  ectodermal  investment  is 
mesodermal  in  origin.  The  orbicularis  palpebrse  muscle,  in  common  with 
the  superficial  facial  muscles  generally,  is  derived  from  a  thin  sheet  of 
superficial  muscles  which  are  represented  in  the  neck  by  the  platysma 
myoides,  and  on  the  trunk  by  the  panniculus,  or  "  fly-shakers,"  of  many 
mammalia.  The  origin  of  the  orbicularis  palpebrae  is  probably  to  be  traced 


32 


DEVELOPMENT  OF   THE   EYE. 


to  the  blastema  of  the  primitive  myotomes  or  embryonic  muscular  segments 
of  the  upper  part  of  the  neck  of  the  embryo. 

The  edges  of  the  eyelids  are  seen  to  be  fused  together  in  Fig.  31,  and  a 
narrow  light  band  of  tissue  seems  to  separate  the  edges  of  the  upper  and 
lower  lids.  This  thin  film  of  superficial  ectoderm  that  cements  the  cd^-cs 
of  the  lids  together  is  probably  cast  off  when  the  lids  separate,  as  a  part 
of  the  general  epitrichium  of  the  body,  at  the  time  of  birth.  From  the 
point  of  junction  of  the  lids  the  eyelashes  arise.  These  develop  in  the  same 

way  as  hairs  over  other  portions  of  skin,  and 
they  are  entirely  ectodermal  in  origin.  Along 
the  edges  of  the  lids  the  Meibomian  or  seba- 
ceous glands  of  the  eye  also  arise  during  the 
fourth  month  as  simple  involutions  of  the 
ectoderm.  The  at  first  indifferent  mesoderm 
of  the  eyelids  becomes  subdivided  during  the 
later  stages  into  three  structures :  an  outer  or 
dermal  layer,  continuous  with  the  dermis  of 
the  adjacent  skin ;  an  inner  layer,  continuous 
with  the  dermis,  or  connective  tissue,  of  the 
conjunctiva  proper,  and  perhaps  with  that  of 
the  cornea ;  and  a  middle  layer,  in  which  the 
muscular  fibres  of  the  palpebral  muscle  are 
developed. 
Lacrymai  gland  of  4  months,  The  lacrymal  gland  in  man  arises  during 

human  embryo.    Enlarged  60  times.       ,       ,,.    ,  .          „       ,.  , 

— i.  young  branch  in  the  form  of  a    tne  third  month  as  a  series  of  solid  upgrowths 
solid  cord  of  ceils;  2  and  3,  more    of  the  ectodeVm  of  the  coniunctival  sac  into 

advanced  portions,  with  alveoli  and       .  -  31*' 

canals  appearing  at  centre ;  e,  epi-    the  mesoderm  underlying  the  basal  part  of  the 

thelium-./.connectiYe-tissuesheaths     upper  eyelid 


of  gland; 
alveoli. 


a,  still  solid   budding 


thus,  under  the  upper  lid.     These  outgrowths 
are  at  first  solid,  but  soon  acquire  a  lumen,  and 

also  send  out  diverticula,  or  branches,  which  are  likewise  at  first  solid.  The 
tear-ducts  of  the  lacrymal  gland  and  their  branches  and  the  alveoli  of  the 
gland  are  therefore  lined  by  cells  that  are  derived  from  the  ectoderm.  The 
connective-tissue  investment  of  the  lacrymal  gland,  its  ducts  and  follicles, 
and  its  blood-supply,  are,  of  course,  of  mesodermal  origin.  Fig.  32  rep- 
resents the  lacrymal  gland  as  figured  by  Kolliker  from  a  four  months' 
human  foetus,  and  illustrates  very  strikingly  the  manner  in  which  racemose 
glands  generally  grow  into  and  displace  the  surrounding  mesoderm  before 
them. 

The  lacus  lacrymalis  at  the  inner  canthus  of  the  eye  is  developed  by 
the  ninth  week,  but  the  caruncula  lacrymalis  develops  considerably  later. 
The  glands  of  the  caruncula  are  also  of  ectodermal  origin,  and  are  con- 
cerned in  the  production  of  the  whitish  secretion  that  accumulates  at  the 
inner  angle  of  the  eyelids.  The  diminutive  glands  of  the  caruncula  are 
supposed  to  belong  to  the  same  series  as  the  Meibomian,  but  it  has  been 


DEVELOPMENT   OF   THE   EYE. 


33 


FIG.  33. 


ha   r 


suggested  by  Giacomini  that  the  caruncular  glands  in  man  represent  the 
Harderian  gland  which  opens  at  the  inner  canthus  of  the  eye  in  other 
vertebrates.  The  development  of  the  glands  of  the  conjunctival  sac 
varies  greatly  in  different  mammals  ;  for  example,  the  lacrymal  gland  of  the 
common  shrew-mole  (Blarina)  invests  not  only  the  whole  embedded  part  of 
the  diminutive  and  degenerate  eyeball,  but  also  its  muscles,  leaving  only 
the  corneal  face  exposed.  The  phylogenetic  development  of  the  glands 
that  open  into  the  conjunctival  sac  is  not  improbably  to  be  associated  with 
glandular  structures  that  primarily  opened  upon  the  free  surface  of  the 
ectoderm  before  the  conjunctival  sac  was  involuted  in  front  of  the  eye- 
ball in  the  course  of  the  evolution  of  the  eyelids.  The  eyelids,  like  other 
structures,  have  been  gradually  evolved  in  the  higher  types,  since  they  are 
imperfectly  developed  in  fishes. 

The  relations  of  the  optic  cup  to  the  mesodermal  envelope  which  covers 
it  are  well  shown  in  Fig.  33,  in  a  horizontal  section  of  the  eye  of  a  mam- 
malian embryo  about  one  and 
one-fifth  inches  in  length. 
This  figure,  in  fact,  illus- 
trates the  earlier  relations 
of  nearly  all  the  structures 
that  are  found  in  the  eye 
of  the  adult.  The  prin- 
cipal differences  which  it  pre- 
sents as  compared  with  the 
fully-developed  eye  are  the 
following :  the  imperfectly- 
developed  lids,  with  their 
thickened  margins  of  pro- 
liferated epidermis  at  ac  and 
pc ;  the  greatly-developed 
system  of  hyaloid  vessels  in 
the  vitreous  that  arise  from 

the   arteria    retinae    Centralis  ;  Horizontal  section  of  the  eye  of  an  embryo  of  the  do- 

thp  nxiil  VPSSP!   that   pxtpnds  mestic  ox>  3'5  centimetres  lonS-     Enlarged  20  times.-ac, 

'  anterior  commissure  of  eyelid ;  pc,  posterior  commissure  of 

temporarily  through   the  Vlt-  same;  c7,  epithelium  and  superficial  layers  of  corium  of 

,  ,,       f  .  cornea;  c,  deeper  connective  tissue  (corium)  of  cornea;  ha, 

reOUS   along  tne   lUtUre    OptlC  anterior  piexus  of  hyaloid  vessels  in  tunica  vasculosa  lentis ; 


hp,  posterior  retinal  hyaloid  plexus;  i,  iris;  Ig,  lacrymal 
gland ;  m,  recti  muscles ;  mp,  membraua  pupillaris ;  o,  optic 
nerve,  through  which  passes  the  arteria  retinae  centralis, 
which  sends  a  branch  forward  in  the  optic  axis  to  join  the 
anterior  hyaloid  plexus  in  the  tunica  vasculosa  lentis;  p, 
pigmented  choroidal  epithelium ;  r,  retina :  s,  sclerotic ;  t, 
tear-duct;  just  beneath  ac,  in  which  t  lies, may  be  noted  a 
small  fold,  which  is  the  third  eyelid. 


axis  to  the  anterior  hyaloid 

plexus ;  the  disproportionate 

size  of  the  lens,  which  still 

lies  in  contact  with  the  cornea, 

under  which  there  is  still  but 

little  indication  of  the  aqueous 

chamber ;  the  thin  and  imperfectly-developed  sclerotic  and  its  vessels,  as 

well  as  the  still  poorly  developed  lamina  suprachoroidea. 

The  retina  is  already  beginning  to  differentiate  into  layers,  while  the 

VOL.  1—3 


34  DEVELOPMENT   OF  THE   EYE. 

cells  of  the  outer  or  pigmented  layer  of  the  optic  cup  are  now  laden  with 
pigmented  granules.  The  retinal  wall  of  the  front  portion  of  the  optic 
cup  is  seen  to  be  thinner  than  the  posterior  wall ;  the  cup  is,  in  fact,  already 
beginning  to  show  signs  of  differentiating  into  an  anterior  thin-walled 
uveal  tract  and  a  posterior  thick-walled  true  retinal  portion.  The  rudi- 
ments of  the  lacrymal  gland  have  been  laid  down  as  a  simple  cylindrical 
proliferation  from  the  conjunctival  epithelium  at  Ig.  At  the  inner  canthus 
the  lacrymal  canal  at  t,  leading  to  the  nasal  duct,  has  already  been  formed, 
while  just  within  the  anterior  commissure  (canihus)  at  ac  is  seen  a  distinct 
fold  completely  covered  by  that  portion  of  the  eyelid  represented  by  the 
commissure.  This  concealed  fold  represents  the  third  eyelid  or  plica  semi- 
lunaiis  of  the  adult.  While  the  sclerotic  is  still  thin,  the  muscles  of  the 
eyeball,  the  internal  and  external  recti,  are  seen  to  be  already  attached  to 
it  by  their  distal  tendinous  portions. 

The  great  proportional  volume  of  the  lens  at  this  stage  in  comparison 
with  that  of  the  whole  of  the  rest  of  the  ocular  globe  is  a  striking  feature 
of  the  development  of  the  mammalian  eye.  The  convexity  of  the  cornea 
would  appear  to  be  at  first  directly  determined  by  the  convexity  of  the 
lens,  since  the  cornea  is  now  directly  and  closely  superimposed  upon  the 
anterior  surface  of  the  latter.  The  subsequent  and  relatively  more  rapid 
distention  of  the  portion  of  the  optic  globe  enveloped  by  the  sclerotic  would 
seem  to  co-operate  with  the  earlier  mechanical  relations  above  described 
that  subsist  between  the  lens  and  the  cornea  in  giving  to  the  eyeball 
its  definitive  and  characteristic  form,  with  its  anterior  corneal  convexity 
greater  than  the  convexity  of  any  other  part  of  its  surface. 

The  vitreous  space  at  this  stage  is  still  relatively  inconsiderable,  but, 
as  Fig.  33  shows,  it  is  traversed  by  two  plexuses  of  vessels, — namely,  a 
retinal  net-work  and  a  net-work  over  the  posterior  face  of  the  lens.  A 
vascular  stem  is  continued  straight  forward  from  the  central  retinal 
artery,  which  traverses  the  distal  part  of  the  optic  nerve  or  stalk,  until  it 
reaches  the  posterior  face  of  the  lens,  where  it  branches  into  a  fine  capillary 
plexus.  This  artery  is  the  capsular  or  anterior  hyaloid  artery,  which  passes 
forward  through  the  hyaloid  canal  in  the  vitreous  to\vard  the  lens. 

Before  proceeding  further,  however,  with  our  detailed  description,  we 
must  recur  to  a  consideration  of  the  manner  in  which  the  vitreous  is  devel- 
oped. We  have  already  pointed  out  that  the  arteria  centralis  was  an  in- 
growth into  the  continuation  of  the  choroid  fissure  upon  the  under  side  of 
the  optic  stalk,  as  shown  in  Fig.  18.  As  a  matter  of  fact,  the  whole  blas- 
tema of  the  vitreous — namely,  the  delicate  retiform  tissue  of  which  it  is 
composed — is  a  product  of  the  mesoderm,  and  is  involuted  into  the  optic 
cup  at  its  inferior  side  through  the  choroid  fissure.  This  involution  begins 
during  the  fourth  week  of  intra-uterine  development,  but  the  growth  of  the 
vitreous  is  not  completed  until  much  later.  The  first  steps  of  the  involution 
of  the  vitreous  are  represented  in  Figs.  34  and  35.  The  lens,  I,  in  Fig.  34  is 
seen  to  be  already  invested  by  a  distinct  layer  of  tissue  between  its  margin 


DEVELOPMENT   OF   THE   EYE. 


35 


and  the  retina,  r.  This  layer,  which  gives  rise  to  the  vascular  tunic  of  the 
lens,  or  tunica  vasculosa  lentis,  is  a  structure  which  provides  in  mammals 
for  the  embryonic  growth  of  the  lens,  but  undergoes  complete  atrophy  long 
before  adult  life  is  reached.  There  exists  no  special  means  in  the  adult  for 
the  nutrition  of  the  lens,  for  after  attaining  its  full  size  the  substance  of  the 
lens  seems  to  manifest  only  a  very  feeble  metabolism. 

The  tunica  vasculosa  lentis  of  the  mammalian  embryo  is  a  highly  vas- 
cular membrane  which  in  its  completed  form  envelops  the  lens  on  all  sides, 
and  in  the  human  embryo  is  distinctly  developed  by  the  second  month. 
Its  vessels  are  derived,  as  already  stated,  from  the  capsular  or  anterior  hyaloid 

FIG.  35. 


The  hinder  half  of  the  same  eye  as  the  pre- 
ceding, viewed  with  reflected  light.  Enlarged 
42  times.— c,  cavity  between  pigmented  layer,  p, 
and  retina,  r ;  v,  vitreous,  and  t/,  points  where  the 
vitreous  tissue  is  continuous  through  the  choroid 
fissure  with  the  mesoblastic  tissue  that  invests 
the  eye.  (Reduced  from  Kolliker.) 


6  vf 

Anterior  half  of  the  eyeball  of  a  human 
embryo  of  4  weeks,  in  vertical  section  through 
its  equator.  Enlarged  66  times.— I,  lens  with 
central  cavity;  v,  vitreous  with  its  stalk,  i/, 
passing  through  the  choroid  fissure ;  b,  vascular 
loop  that  has  been  intruded  into  the  fissure 
within  the  tissue  of  the  vitreous ;  r,  the  retinal 
lamella  of  the  secondary  optic  cup ;  p,  p,  outer 
lamella  or  choroidal  epithelium  of  same ;  c,  space 
between  the  two  latter,  representing  the  remains 
of  the  cavity  of  the  primitive  optic  vesicle. 
(Reduced  from  Kolliker.) 

artery.  This  vessel,  after  reaching  the  lens,  breaks  up  into  radiating  branches 
that  spread  out  over  the  posterior  face  of  the  lens,  ramifying  through  the 
mesodermal  tissue  of  the  tunic,  as  shown  in  Figs.  36  and  37.  The  mode 
of  radiation  and  branching  of  these  vessels  on  the  posterior  face  of  the 
lens  is  shown  in  Fig.  37.  The  branches  upon  reaching  the  equator  or 
margin  of  the  lens  bend  round  over  its  front  face  and  converge  into  a 
system  of  vascular  loops,  as  shown  in  Fig.  36,  traversing  the  anterior 
part  of  the  tunica  vasculosa  lentis,  or  lens-capsule.  These  anterior  loops 
of  the  vessels  of  the  tunica  vasculosa  lentis  also  unite  about  this  time 
around  the  margin  of  the  lens  with  the  vessels  of  the  choroid  which  are 
now  appearing.  The  tunica  vasculosa  lentis  reaches  its  greatest  develop- 
ment during  the  seventh  month,  after  which  it  begins  to  degenerate,  together 
with  the  capsular  artery. 

From  the  circumstance  that  the  several  parts  of  the  nutritive  membrane 
of  the  lens  were  discovered  by  different  investigators  at  different  times, 
these  parts  have  received  distinct  names,  such  as  membrana  pupillaris,  m. 
capsulo-pupillaris,  and  m.  capsularis.  The  membrana  pupillaris  was  first 


36 


DEVELOPMENT   OF   THE   EYE. 


observed,  perhaps,  because  it  was  situated  under  the  pupil  on  the  anterior 
face  of  the  lens,  and  was  therefore  most  easily  found.  Since  it  occasionally 
persists  even  after  birth  as  a  thin  membrane  closing  the  pupil,  it  may  thus 
cause  a  congenital  defect  of  the  eye  known  as  atresia  pupillce  congenita.  It 
has,  however,  been  found  that  the  membrana  pupillaris  is  continued  later- 
ally from  the  pupil  over  the  anterior  face  of  the  lens,  this  part  receiving  the 
name  of  membrana  capsulo-pupillaris.  Last  of  all,  it  was  discovered  that 


FIG.  36. 


FIG.  37. 


Distribution  of  arteria  hyaloidea  on  anterior 
wall  of  the  capsule  of  the  lens  of  a  newly-born 
kitten.  From  an  injection  by  Thiersch.  (After 
Kolliker.) 


Distribution  of  hyaloid  artery  on  posterior 
wall  of  the  capsule  of  the  lens  of  a  newly-born 
kitten.  From  an  injection  by  Thiersch.  (After 
Kolliker.) 


this  membrane  was  continuous,  as  illustrated  above,  Figs.  36  and  37,  with 
the  membrana  capsularis  on  the  posterior  face  of  the  lens.  It  is  therefore 
needless  to  retain  all  these  names,  since  they  simply  apply  to  parts  of  a 
single  structure, — the  membrana  vasculosa  lentis. 

The  other  vessels  of  the  eyeball  which  are  external  to  the  vitreous  and 
retina  are  fully  developed  at  a  relatively  later  period.  The  ciliary  arteries 
which  penetrate  the  sclerotic  a  little  distance  from  the  entrance  of  the  optic 
nerve  are  derived  partly  as  an  outgrowth  from  the  same  branch  of  the 
ophthalmic  artery  that  gives  rise  to  the  arteria  retinaB  centralis.  The 
vorticose  veins  do  not  reach  their  completed  development  till  much  later. 
The  principal  ramifications  of  the  ciliary  arteries  and  vasa  vorticosa  are 
within  the  lamina  suprachoroidea,  the  first  traces  of  which  are  laid  down 
about  the  same  time  as  the  membrana  vasculosa  lentis. 

The  ciliary  nerves  which  innervate  the  eyeball  penetrate  the  sclerotic 
at  first  at  about  the  same  points  posteriorly  as  the  posterior  ciliary  arteries. 
They  are  centrifugal  outgrowths  from  the  ciliary  ganglion,  and  their  points 
of  entrance  through  the  sclerotic  are  arranged  in  a  circle  around  and  a 
little  distance  from  the  point  of  entrance  of  the  optic  nerve. 

The  nerve  supplying  the  lacrymal  gland  is  a  centrifugal  outgrowth  of 
the  ophthalmic  branch  of  the  fifth,  or  trigeminus.  The  blood-vessel,  the 
lacrymal  artery,  supplying  the  lacrymal  gland,  is  a  comparatively  late 


DEVELOPMENT   OF   THE   EYE.  37 

outgrowth  of  the  ophthalmic  artery,  the  capillary  branches  of  which  grow 
in  between  and  around  the  developing  follicles  of  the  gland. 

The  vessels  of  the  iris  are  outgrowths  in  part  from  the  hyaloid  plexus 
supplied  by  the  arteria  retinae  centralis  and  in  part  from  the  anterior  ciliary 
arteries  which  pierce  the  sclerotic  near  the  outer  margin  of  the  cornea, 
where  they  divide  into  two  branches  which  grow  toward  each  other  and 
fuse  along  a  circular  course  to  form  the  great  arterial  circle  of  the  iris. 

The  other  surrounding  mesodermal  structures  that  may  be  further  con- 
sidered are  especially  the  orbit,  and  the  orbital  muscles  that  control  the 
movements  of  the  ball.  The  latter,  however,  will  require  special  considera- 
tion, but  of  the  orbit  it  may  be  said  that  its  conformation  is  largely  due  to 
the  globular  form  of  the  optic  cup  itself.  This  is  owing  to  the  fact  that 
the  optic  cup  or  foundation  of  the  eyeball  is  formed  before  even  the  cranial 
cartilages  and  membranous  matrices  of  the  membrane-bones  of  the  skull 
have  been  outlined.  The  result  is  that  the  surrounding  hard  parts  must 
conform  in  a  measure  during  their  growth  to  the  shape  assumed  by  the  pre- 
viously-developed .  ocular  cup  or  globe,  as  they  develop  partly  in  cartilage 
and  partly  in  membrane. 

DEVELOPMENT  OF  THE  LENS. 

The  lens  is  developed,  as  we  have  already  learned,  from  a  circular  patch 
of  the  epidermis  (ectoderm)  of  the  sides  of  the  head  of  the  embryo.  As 
the  primitive  optic  vesicles  grow  out  from  the  anterior  end  of  the  medul- 
lary tube,  their  distal  ends  are  pushed  out  against  the  ectoderm  at  the  sides 
of  the  head.  The  circular  area  of  the  external  ectoderm  thus  touched 
internally  by  the  outer  face  of  the  optic  vesicles  begins  to  thicken  and  push 
itself  inward.  The  outer  or  distal  faces  of  the  primitive  optic  vesicles  at 
this  time  also  become  correspondingly  depressed  and  concave  externally. 
The  appearance  of  the  first  traces  of  the  secondary  optic  cup  is  therefore 
intimately  associated  with  the  growth  and  evolution  of  the  lens.  These 
changes  occur  quite  early  in  the  process  of  development :  in  the  rabbit,  for 
example,  the  involution  of  the  lens  begins  on  the  eleventh  day ;  in  birds, 
still  earlier,  or  by  the  end  of  the  second  day  of  incubation ;  in  the  human 
embryo,  during  the  fourth  week  of  intra-uterine  life. 

The  thickening  of  the  lens-rudiment  is  due  to  the  growth  or  multipli- 
cation of  the  cells  in  the  area  of  ectoderm  from  which  the  lens  develops 
(see  Fig.  16,  /).  The  karyokinetic  figures  or  nuclear  spindles  are  said  by 
Minot  to  be  found  in  active  division  in  the  cells  forming  the  outer  face  of 
the  ectodennal  rudiment  of  the  lens  during  the  eleventh  day  of  its  develop- 
ment. Nuclear  spindles  or  cleavage-figures  are  also  abundant,  according 
to  the  same  authority,  on  the  inner  face  of  the  outer  wall  of  the  optic  cup, 
which  is  also  now  rapidly  thickening  to  form  the  future  retina.  This 
multiplication  of  the  nuclei  therefore  proceeds,  as  pointed  out  by  the  same 
author,  in  homologous  situations  in  both  the  ectodermal  rudiments  from 
which  the  lens  and  the  retina  respectively  are  destined  to  be  developed. 


38  DEVELOPMENT  OF  THE   EYE. 

The  pit-like  involution  of  the  ectoderm  from  which  the  lens  is  formed, 
as  already  stated,  is  intimately  associated  with  a  corresponding  involution 
of  the  outer  or  distal  and  retinal  wall  of  the  primitive  optic  vesicle.  In 
fact,  this  involution  proceeds  until  the  cavity  between  the  outer  or  retinal 
wall  and  the  inner  or  proximal  wall — the  future  pigmented  layer — of  the 
optic  vesicle  ig  obliterated,  and  the  latter  is  converted  into  a  double-walled 
cup, — the  secondary  optic  vesicle  or  cup.  As  the  ectodermal  involution 
destined  to  form  the  lens  becomes  deeper  and  more  pit-like,  the  rim  bound- 
ing the  opening  into  the  pit  begins  to  constrict,  and  eventually  closes.  This 
pit-like  involution  is  thus  converted  into  a  completely-closed  sac  or 
vesicle  with  cellular  walls.  When  this  closure  has  been  completed,  the 
ectodermal  lens-vesicle,  as  we  may  now  term  it,  detaches  itself  entirely 
from  the  immediately  overlying  epidermis  at  the  point  where  the  pit  closed. 
At  the  same  time  the  ectoderm  (epidermis)  immediately  overlying  the  lens- 
vesicle  also  closes,  so  as  to  leave  no  trace  of  the  opening  that  originally 
led  from  the  exterior  surface  into  the  ectodermal  lens-pit.  In  this  way  the 
ectoderm  again  becomes  quite  continuous  over  the  underlying  vesicular 
lens-rudiment.  The  ectoderm  overlying  the  lens  at  this  stage  will  eventu- 
ally become  the  epidermis  of  the  cornea. 

It  is  clear  from  what  has  preceded  that  the  future  essentially  refractive 
elements  of  the  lens — that  is,  the  cells  of  its  body — are  derived  from  the 
ectoderm.  The  lens-vesicle  now  occupies  for  a  considerable  time,  in  mam- 
mals, at  least,  the  whole  of  the  cavity  of  the  secondary  optic  vesicle ;  that 
is,  the  lens  now  lies  close  against  the  future  retina,  as  shown  in  Fig.  38,  in 
which  the  rudiments  of  the  membrana  vasculosa  lentis  are  also  already 
visible.  In  the  developing  chick  embryo  (Fig.  15)  a  concavo-convex  space 
begins  to  appear  almost  immediately  between  the  proximal  or  retinal  aspect 
of  the  lens-vesicle  and  the  future  retina.  The  space  thus  formed  by  the 
retreat  of  the  retina  from  contact  with  the  retinal  aspect  of  the  lens-vesicle 
in  the  eye  of  embryo  birds  is  the  vitreous  chamber,  and  in  them  there  is 
also  for  a  long  time  no  intrusion  of  blood-vessels  into  this  space  through 
the  choroid  fissure.  In  the  embryos  of  mammals  and  man  the  lens- vesicle 
of  this  stage  also  at  once  begins  to  undergo  histological  differentiation,  so 
that  considerable  progress  is  made  toward  a  realization  of  its  adult  state 
before  the  retina  begins  to  retreat  to  any  marked  extent  from  its  retinal 
aspect.  By  the  time  the  retreat  of  the  retina  from  the  posterior  face  of 
the  lens-vesicle  in  mammals  has  begun,  there  has  been  pushed  into  the 
still  very  narrow  concavo-convex  space  between  the  lens-vesicle  and  the 
retina  a  delicate,  vascular,  mesodermal  membrane,  the  rudiment  of  the 
lens-capsule. 

The  lens-vesicle  immediately  after  its  involution  is  thick-walled,  and 
may  be  said  to  close  up  the  mouth  of  the  optic  cup,  the  rim  of  which  is 
now  bent  inward  all  round  toward  the  equator  of  the  lens.  The  posterior 
wall  is  from  the  first  slightly  thicker  at  its  middle  than  any  other  parts  of 
the  walls  of  the  vesicle,  and  there  is  still  a  quite  spacious  cavity  within  it. 


DEVELOPMENT   OF   THE   EYE. 


39 


In  form,  the  lens-vesicle  is  now  slightly  flattened,  and  its  exterior  or  corneal 
aspect  is  somewhat  less  convex  than  the  posterior  or  retinal. 

The  subsequent  changes 

in  the  anterior  and  posterior  Fl°-  38. 

walls  of  the  lens- vesicle  are 
very  different.  The  an- 
terior wall  (Fig.  38)  be- 
comes much  thinner,  and 
remains  concavo-convex, 
while  the  posterior  wall 
becomes  very  thick  and 
bi-convex,  so  that  the  two 
walls  are  eventually  per- 
fectly coadapted  and 
brought  into  contact  with 
each  other  over  their  entire 
inner  faces.  From  this 
cause  it  results  that  the 
original  cavity  within  the 
lens-vesicle  is  at  last  com- 
pletely obliterated.  The 
obliteration  of  the  lens- 
cavity  is  nearly  complete 
in  the  stage  represented  in 
Fig.  38.  This  cavity  is 
filled  with  fluid  in  the  em- 
bryo of  birds,  but  in  mam- 
mals at  the  time  of  invo- 
lution of  the  lens  there  is 
left  a  little  superficial  mass 
of  loose  cells  (except  in  the 
mouse)  at  the  bottom  of 
the  lens-pit.  These  cells 
seem  to  have  been  derived 

from  the  outer  surface  of  the  ectodermal  area,  which  is  involuted  to  form 
the  lens.  They  are  scattered  in  the  cavity  of  the  lens-vesicle  of  mam- 
malian embryos,  but  are  said  to  break  down  and  disappear  as  the  cavity 
of  the  lens  vanishes,  due  to  the  thickening  of  its  posterior  wall,  as  described 
above.  These  free  cells  in  the  cavity  of  the  lens-vesicle  are  suspected  by 
Mi  not  to  be  a  part  of  the  epitrichial  layer  of  the  ectodermal  area  which 
was  involuted  to  form  the  lens. 

The  histological  structure  of  the  walls  of  the  lens-vesicle  is  that  of  a 
columnar  epithelium  with  the  nuclei  of  adjacent  cells  lying  at  different 
levels  in  vertical  sections.  The  posterior  part  of  the  wall  of  the  lens-vesicle 
rapidly  thickens ;  in  fact,  at  the  time  of  the  closure  of  the  vesicle  this  part 


Vertical  section  through  the  eye,  at  an  early  stage,  of  an 
embryo  mouse.  Enlarged  130  times.  (After  Kessler,  from  Hert- 
wig's  Lehrtmch.)— p,  pigmented  epithelium,  forming  the  outer 
layer  of  the  secondary  optic  cup ;  r,  thickened  inner  layer,  the 
future  retina ;  m,  marginal  zone,  or  border  of  secondary  optic 
vesicle,  which  develops  later  into  the  non-sensory,  ciliary,  and 
iritic  portions  of  the  retina  (uveal  tract) ;  v,  vitreous  body,  with 
blood-vessels ;  tv,  tunica  vasculosa  lentis ;  be.  blood-corpuscles ; 
eft,  choroid;  If,  lens-fibres ;  le,  anterior  epithelium  of  lens ;  I,  nu- 
clear zone  of  lens-fibres ;  c,  connective-tissue  layer  (corium)  of 
cornea;  ce,  outer  corneal  epithelium. 


40 


DEVELOPMENT   OF  THE   EYE. 


FIG.  39. 


of  its  wall  is  perceptibly  thicker  than  the  anterior  portion.     This  is  due 
to  changes  which  go  on  simultaneously  in  both  walls :  while  the  cells  of 

the  posterior  wall  are  be- 
coming more  elongated 
and  columnar,  the  cells  of 
the  anterior  wall  are  be- 
coming converted  into  a 
thin  epithelium  composed 
of  a  single  layer  of  cubical 
cells,  as  seen  in  Figs.  39 
and  41.  The  cells  of  the 
posterior  wall  meanwhile 
become  greatly  elongated 
at  the  expense  of  their 
thickness,  and  are  thus 
converted  into  the  fibres 
of  the  later  and  more 
developed  condition  of 
the  lens.  During  this 
process  of  elongation  of 
the  cells  of  the  posterior 
wall  of  the  lens,  their 
nuclei,  in  a  vertical  sec- 
tion, as  in  Fig.  38,  may 
be  seen  to  extend  as  a 
scattered  band  across  the 
middle  of  this  wall  from 
one  edge  of  the  organ  to 
the  other.  A  gradual 
transition  of  the  elon- 
gated cells  of  the  poste- 
rior wall  of  the  lens  to 
the  epithelial  cells  of  the 
anterior  wall  is  effected 
all  round  the  margin  or  equator  of  the  organ,  as  shown  at  I  in  Fig.  39. 
During  this  elongation  of  the  cells  of  the  posterior  wall  they  gradually 
undergo  certain  internal  changes  of  constitution,  as  a  consequence  of  which 
they  acquire,  as  a  whole,  the  transparency  and  refractive  powers  that  char- 
acterize the  fully-developed  organ.  The  refractive  properties  of  the  lens 
are  wholly  due  to  properties  acquired  by  the  cells  of  the  posterior  Avail 
of  the  lens-vesicle  by  the  time  its  metamorphosis  is  completed.  All  the 
fibres  extend  from  the  front  to  the  posterior  face  of  the  lens ;  those  of  the 
centre  have  a  nearly  straight  course  through  its  axis,  while  those  outside 
the  axis  are  more  and  more  curved  as  the  periphery  of  the  lens  is  more 
nearly  approached.  Since  all  the  fibres  of  the  lens  have  blunt  instead  of 


v  I  m         x          letv  a  dc   ce 

Section  through  a  part  of  the  lens  and  rim  of  the  optic  cup 
of  a  mouse  embryo  somewhat  more  advanced  than  that  shown 
in  Fig.  38.  Enlarged  130  times.  (From  Hertwig's  Lehrbuch,  after 
Kessler.) 

The  section  passes  through  the  front  of  the  lens,  the  margin 
of  the  optic  cup,  cornea,  parts  of  aqueous  and  vitreous  chambers, 
choroid  and  adjacent  structures.— p,  pigmented  choroidal  epi- 
thelium or  outer  layer  of  cup;  v,  blood-vessels  of  the  vitreous 
body  in  the  vascular  capsule  of  the  lens  (the  vessels  cut  through 
next  the  retina,  r,  represent  the  retinal  plexus  of  the  hyaloid 
artery);  m,  marginal  zone  of  optic  cup;  tv,  tunica  vasculosa 
lentis ;  x,  point  of  connection  of  the  latter  with  the  choroid ;  I, 
point  of  transition  of  anterior  epithelial  layer  of  cells  of  the 
lens  into  lens-fibres ;  If,  anterior  epithelium  of  lens ;  a,  aqueous 
chamber  of  eye;  d,  Descemet's  membrane;  c,  embryonic  blas- 
tema of  the  clear  corium  of  the  cornea;  ce,  corneal  epithelium. 


DEVELOPMENT   OF   THE    EYE. 


41 


FIG.  40. 


acute  ends,  they  cannot  converge  at  opposite  poles  of  the  optic  axis  of 
the  organ.  There  is  consequently  developed  a  system  of  radiating  lines 
of  junction  along  which  the  ends  of  the  successive  layers  of  fibres  are  ap- 
posed.  These  so-called  lens  "  stars"  in  the  lens  of  the  human  adult  have 
as  many  as  nine  rays,  representing  the  lines  of  apposition  of  the  ends  of 
the  lens-fibres  of  one  face  of  the  organ. 

The  growth  of  the  lens  is  due  to  the  apposition  of  new  fibres,  resulting 
from  the  division  of  the  cells  around  the  equator  of  the  organ,  just  at  the 
point  where  the  anterior  and  posterior 
walls  are  continuous  with  each  other,  as 
at  I,  Fig.  39.  Around  the  embryonic 
core  represented  by  the  original  posterior 
embryonic  wall  of  the  lens-vesicle  new 
lens-fibres  thus  arise.  These  new  fibres 
are  developed  and  arranged  conformably 
with  the  curved  surface  of  the  lens,  and 
at  first  extend  from  pole  to  pole  of  its 
axis,  but  later  extend  only  from  one  of 
the  radial  lines  of  the  posterior  face  to 
another  nearly  opposite  radial  line  of 
the  anterior  face.  These  lines  in  newly- 
born  mammals  are  triradiate,  as  repre- 
sented in  the  annexed  Fig.  40.  These 
triradiate  lines  or  "stars"  of  the  em- 
bryonic lens  alternate  with  one  another, 
so  that  if  the  "  star"  of  the  anterior  face 
were  projected  upon  that  of  the  posterior 
face  the  radii  of  the  anterior  "star"  would 
exactly  halve  the  angle  between  the  rays 
of  the  posterior  "star."  In  the  adult, 
as  already  stated,  the  "  stars"  become  more  complicated,  since  "  rays"  or 
lines  of  juncture  of  lens-fibres  are  developed  and  intercalated  in  addition 
to  those  seen  in  the  lens  of  the  newly-born  animal.  This  is  due  to  the 
appositional  mode  of  growth  of  the  new  fibres,  which  are  laid  down  in 
layers  over  those  that  have  been  previously  developed.  It  results  from 
this  fact  that  when  the  lens  of  an  adult  is  macerated  the  fibres  peel  off  in 
layers  like  the  coats  of  an  onion.  The  site  of  the  production  of  the  lens- 
fibres  is  around  the  margin  of  the  lens,  where  nuclei  in  a  condition  of 
division  are  frequently  seen  during  a  late  stage  of  the  growth  of  the  organ. 

On  account  of  the  manner  in  which  the  lens-fibres  are  produced,  it  also 
results  that  an  involution  or  pit  is  developed  on  the  posterior  or  retinal 
aspect  of  the  lens.  The  reason  for  this  is  to  be  sought  in  the  mode  in 
which  the  new  fibres  are  produced  around  the  equator  of  the  organ.  The 
ends  of  the  newly-produced  fibres  extend  backward  and  their  posterior 
ends  meet  those  of  their  fellows  of  the  opposite  side  along  a  line  which  lies 


Diagram  showing  the  arrangement  and 
mode  of  convergence  on  the  anterior  and 
posterior  aspects  of  the  lens  of  a  very  young 
mammal  of  the  fibres  developed  from  the 
cells  of  its  posterior  wall.  (Modified  from 
Hertwig.)— at  and  pt,  anterior  and  posterior 
triradiate  figures  or  lines  formed  by  the 
points  of  apposition  of  the  anterior  and  pos- 
terior ends  of  the  lens-fibres ;  I,  I,  lines  in- 
dicating the  direction  of  curvature  of  the 
fibres  of  the  lens  on  its  anterior  face  and 
their  terminations  along  the  triradiate  line, 
at;  V,  V,  continuation  of  the  same  fibres  to 
the  posterior  triradiate  line,  pt.  (Slightly 
modified  from  Hertwig's  Lehrbuch.) 


42  DEVELOPMENT  OF  THE   EYE. 

exactly  in  the  optic  axis  of  the  organ.  This  pit  or  depression  is  very 
narrow,  almost  linear,  in  fact,  as  may  be  seen  in  the  longitudinal  section 
of  the  lens  of  a  larval  salamander,  represented  in  Fig.  41,  at  d'.  In  the 
same  figure  the  arched  form  of  the  zone  of  nuclei  of  the  lens-fibres  at  / 
is  also  clearly  indicated,  so  that  the  lens  actually  seems  to  have  suffered 
involution  of  its  posterior  wall  in  the  course  of  its  development.  This 
linear  axial  posterior  involution  is  also  apparent  in  the  adult  human  lens, 
according  to  Babuchin,  and  is  visible  in  sections  through  its  axis. 

The  growth  of  the  lens  appears  almost  to  cease  toward  the  end  of 
embryonic  life.  Huschke  has  shown  that  the  lens  has  a  weight  of  one 
hundred  and  twenty-three  milligrammes  in  the  newly-born  child  and  of  one 
hundred  and  ninety  milligrammes  in  the  adult.  This  shows  that  the  total 
increase  in  the  weight  of  the  organ  during  adolescence  is  only  sixty-seven 
milligrammes. 

DEVELOPMENT  OF  THE   RETINA   AND  OPTIC  NERVE. 

The  retina,  as  stated  in  the  introductory  part  of  this  article,  may  be 
regarded  as  a  portion  of  the  margin  of  the  medullary  plate  which  has  been 
precociously  separated  from  the  latter  and  projected  outward  upon  the 
hollow  optic  stalk.  A  study  of  the  histological  changes  which  it  under- 
goes in  the  course  of  its  development  proves  that  this  proceeds  in  a  manner 
which  is  closely  parallel  to  that  of  the  development  of  the  central  nervous 
system,  with  the  exception  of  the  peculiar  differentiation  of  its  exterior 
layer  of  sensory  cells  that  terminate  in  the  so-called  rods  and  cones. 

It  appears  that  in  the  embryos  of  mammals,  as  in  the  cord  and  the  brain, 
there  is  an  early  differentiation  of  the  tissue  of  the  retina  into  two  zones, — 
an  inner,  non-cellular  zone,  and  an  outer,  much  thicker,  nucleated  cellular 
zone.  This  differentiation  may  be  seen  in  the  retina  of  an  embryo  rabbit 
from  four  to  five  millimetres  in  length,  before  there  is  any  sign  of  the  de- 
velopment of  the  rods  and  cones,  or  any  indication  of  the  splitting  of  the 
cells  of  the  embryonic  retina  into  strata.  Minot  regards  the  non-nucleated 
inner  stratum  of  this  stage  as  homologous  with  the  non-nucleated  stratum 
on  the  inner  side  of  the  developing  spinal  cord,  and  the  thick  outer  nucleated 
layer  as  homologous  with  the  internal  neuroglia  and  ganglionic  stratum  of 
the  developing  wall  of  the  spinal  cord.  The  next  step  in  the  differentia- 
tion of  the  retina  appears  to  be  the  subdivision  of  the  wide  outer  nucleated 
zone  into  two  layers  of  nearly  equal  thickness,  distinguished  by  the  appear- 
ance of  their  nuclei.  The  nuclei  of  the  outer  zone  are  smaller  and  tend 
to  stain  more  deeply  than  those  of  the  inner  layer.  This  stage  is  ob- 
servable in  a  rabbit  embryo  of  twenty  millimetres,  or  in  a  human  embryo 
of  thirty-eight  millimetres.  The  outer  layer  is  to  be  regarded  as  the  rudi- 
ment of  the  true  sensory  epithelium  of  the  retina,  and  gives  rise  to  the  rods 
and  cones. 

The  inner  layer  of  the  embryonic  retina,  as  described  above,  now  under- 
goes differentiation  into  the  inner  reticular  or  molecular  layer  (Fig.  41,  M) 


DEVELOPMENT   OF   THE   EYE. 


43 


and  the  inner  layer  of  ganglion-cells,  g.  The  outer  layer  also  undergoes 
differentiation  into  no  fewer  than  three  strata ;  the  outermost  of  these  is 
nucleated,  gives  rise  on  its  surface  to  the  rods  and  cones,  and  is  separated 
by  a  thin  reticular  layer  (M,  Fig.  41)  from  a  thick,  inner  nucleated  layer. 


mb  ~"~~ 


op 


Vertical  section  through  the  optic  axis  of  the  eye  of  a  recently-hatched  larva  of  a  salamander 
(Amblystoma),  11  millimetres  long.— a,  space  for  aqueous  humor ;  c,  retinal  rods  and  cones ;  op,  optic 
nerve;  cc,  intruding  corneal  connective-tissue  cells ;  cv,  choroidal  vessels ;  d,  scattered  cells  of  the  dis- 
integrating walls  of  the  optic  stalk ;  d',  pit  in  the  posterior  wall  of  the  lens ;  e,  epithelium  of  comea ;  /, 
nuclei  of  fibres  of  posterior  wall  of  lens ;  g,  ganglionic  or  inner  layer  of  retina ;  i,  i,  connective  tissue 
of  front  of  iris,  with  its  vessels,  vi;  I,  epithelium  of  anterior  wall  of  lens;  M,  outer  molecular  layer; 
m',  inner  molecular  layer ;  mb,  membrane  of  Bowman,  with  traces  underneath  it  of  endotheliurn  of 
aqueous  chamber ;  p,  pigment-cells  that  have  been  developed  from  outer  layer  of  optic  stalk  (the  deeply- 
pigmented,  black,  and  thick  choroidal  epithelium  shows  digitations  on  its  inner  face  from  which  the 
cones  of  the  retina  have  been  withdrawn) ;  s,  space  caused  by  shrinkage  of  retina  away  from  choroid ; 
si  points  to  a  row  of  cells  which  mark  the  first  traces  of  the  sclerotic ;  v,  vessel  in  the  connective  tissue 
investing  the  eye;  vt,  vitreous  chamber;  *  *  indicate  the  point  external  to  which  the  uveal  tract 
begins.  (Original.) 

This  differentiation  of  the  retina,  however,  does  not  extend  over  the  whole 
of  its  area.  As  shown  in  Fig.  41,  the  true  sensory  area  of  the  retina 
extends  only  between  the  two  points  marked  *  in  the  figure.  Anterior 
to  these  the  embryonic  retina  undergoes  degenerative  changes  and  becomes 
the  foundation  of  the  uveal  tract,  or  pars  iridis  retinse.  The  true  retinal 
epithelium,  in  front  of  the  points  *  *,  finally  becomes  reduced  to  a  single 
layer  of  cells  closely  adherent  to  the  pigmented  choroidal  epithelium.  It 
is  also  finally  raised  into  longitudinal  folds  at  short  intervals  about  the 


44  DEVELOPMENT   OF   THE    EYE. 

points  *  *,  and  over  that  portion  of  its  surface  next  the  margin  of  the  lens 
also  is  raised  into  annular  ridges,  or  folds,  that  project  all  round  toward 
the  lens.  This  is  the  ciliary  ridge,  with  its  processes.  The  portion  of  the 
retina  next  the  margin  of  the  optic  cup  enters  into,  and  forms  part  of,  the 
posterior  wall  of  the  iris. 

The  true  sensory  part  of  the  retina  forming  the  posterior  lining  of  the 
eyeball,  and  lying  between  the  points  *  *,  in  Fig.  41,  throws  out  from  each 
of  the  cells  of  its  outer  layer  a  process  which  in  batrachian  embryos 
appeal's  very  early ;  the  processes  of  the  cells  thus  developed  represent  the 
rods  and  cones.  These,  as  Babuchin  has  pointed  out  in  the  frog,  at  first 
differ  but  little  from  one  another  in  form.  The  rods  and  cones  (c,  Fig.  41) 
are  longest  near  the  point  of  exit  of  the  optic  nerve,  and  gradually  dimin- 
ish in  length  toward  the  ciliary  region,  where  they  present  the  form  of 
very  slight  papilliform  elevations  of  the  free  ends  of  the  cells.  The  black 
pigmented  layer  of  the  retina  (choroidal  epithelium)  shows  projections  of 
its  substance  which  were  interposed  between  the  rods  and  cones  before  the 
reagents  used  in  hardening  the  embryo  had  pulled  these  two  layers  apart. 
It  is  evident,  therefore,  that  in  life  at  this  early  stage  the  rods  and  cones 
were  already  embedded  in  the  pigmented  layer. 

According  to  Max  Schultze,  there  is  from  the  first,  in  the  retina  of  the 
embryo  chick,  an  appreciable  difference  in  the  form  of  the  rods  and  cones. 
Over  the  membrana  limitans  externa  there  appear  closely  crowded  small 
hemispherical  projections  of  two  sizes.  Of  these  the  larger  ones  are  the 
beginnings  of  the  cones,  and  the  much  smaller  and  more  numerous  ones, 
scattered  evenly  between  the  larger  ones,  are  the  developing  rods.  Each  of 
these  is  an  outgrowth  from  a  single  sense-cell  of  the  outer  sensory  layer  of 
cells  of  the  retina.  The  rods  and  cones,  as  we  have  already  noted,  push 
their  way  into  the  pigmented  layer  or  choroidal  epithelium  overlying  them. 
The  inner  portion  of  the  rods  and  cones  develops  first,  and  as  the  tips  grow 
longer  they  form  the  external  portions  or  outer  segments.  In  the  chick  the 
development  of  the  rods  and  cones  begins  on  the  seventh  to  the  tenth  day. 
In  man  and  ruminants  they  are  present  at  birth,  though  smaller  than  in 
the  adult.  The  rods  have  been  observed  to  begin  their  development  in  a 
human  embryo  of  two  hundred  and  fifteen  millimetres. 

There  are  two  sets  of  cells  developed  in  the  retina,  namely,  nerve-  or 
ganglion-cells  and  neuroglia-cells.  The  fibres  of  Miiller  belong  to  the 
latter  class  and  develop  very  early,  Herrick  having  found  them  in  the  eye 
of  the  salamander  of  about  the  same  stage  as  that  represented  in  Fig.  41. 
They  extend  from  the  internal  to  the  external  limiting  membrane,  these 
being  clearly  homologous  in  the  retina  with  the  same  membranes  developed 
on  the  inner  and  outer  faces  of  the  walls  of  the  medullary  canal  or  tube 
of  the  embryo.  The  ganglion-cells  of  the  retina  seem  to  be  of  two  sorts, — 
namely,  those  which  send  processes  from  themselves  into  the  molecular  or 
reticular  layers,  and  those  which  also  send  an  axis-cylinder  process  into  the 
optic  nerve.  Those  which  send  axis-cylinder  processes  into  the  optic  nerve 


DEVELOPMENT   OF   THE   EYE.  45 

compose  the  inner  layer,  g,  Fig.  41.  These,  however,  also  send  branched 
processes  into  the  inner  reticular  layer,  according  to  Dogiel  and  Cajal.  The 
details  regarding  the  development  of  the  amacrinal  cells  of  Ramon  y  Cajal, 
found  in  the  middle  layer,  are  not  fully  known. 

The  other  layer  which  is  concerned  in  sending  branched  processes  into 
and  over  the  reticular  layers,  thus  forming  a  reticulum  over  the  surfaces  of 
these  layers,  is  principally  the  middle  layer  of  cells.  The  complexity  of 
these  nervous  reticuli,  formed  by  processes  of  the  three  layers  of  cells  of 
the  retina,  is  very  great,  as  shown  by  Dogiel  with  the  aid  of  the  picrate  of 
ammonia — methylene-blue — method.  Herrick  has  shown  that  the  middle 
layer  sends  processes  into  the  inner  reticular  layer  in  the  larval  salamander. 
He  has  also  shown  that  the  ganglion-cells  of  the  retina  in  all  probability 
also  originally  arise  upon  what  is  morphologically  the  outer  face  of  the 
retina ;  that  is,  the  ganglionic  cells  originally  proliferate  from  that  face  of 
the  retina  which  ultimately  bears  the  rods  and  cones.  In  this  way  it  has 
been  possible  to  co-ordinate  completely  the  development  of  the  ganglion- 
cells  of  the  retina  with  that  of  the  brain  and  cord,  in  both  of  which  the 
ganglion-cells  originally  proliferate  from  near  the  surface  of  the  inner  wall 
of  the  medullary  tube,  or  from  that  part  of  its  surface  which  was  exterior 
before  the  closure  of  the  medullary  groove. 

The  retina  grows  more  rapidly  in  birds  and  mammals  than  its  exterior 
envelope  the  sclerotic.  From  this  cause  it  is  thrown  into  folds,  which  pro- 
ject into  the  vitreous  chamber.  Kolliker  states  that  the  first  fold  arises 
below  the  point  of  exit  of  the  optic  nerve,  and  that  numerous  other  radi- 
ating folds  are  added  later.  These  folds  gradually  disappear  toward  the 
end  of  foetal  life,  when  the  retina  is  smooth  and  lies  for  its  whole  extent 
closely  in  contact  with  the  pigmented  layer.  Compare  Figs.  19,  20,  and  41. 

The  development  of  the  special  regions  of  the  retina  proper  has  been 
the  subject  of  special  investigation  at  the  hands  of  Chiewitz,  who  has  shown 
that  the  fovea  or  macula  lutea  is  marked  out  very  early  as  a  region  slightly 
thinner  than  the  adjacent  parts  of  the  embryonic  retina. 

The  blood-vessels  of  the  retina,  according  to  O.  Schultze,  are  developed 
quite  late.  They  appear  in  the  pig  of  ninety  millimetres,  and  in  man  after 
the  third  month.  Over  the  surface  of  the  retina,  next  the  vitreous,  a  layer 
of  cells  is  developed  at  this  time.  These  cells  arrange  themselves  into  a 
net-work,  from  which  blood-vessels  are  formed.  The  formation  of  vessels 
begins  next  the  optic  nerve,  and  radiates  over  the  retina  toward  the  margin 
of  the  lens.  A  layer  of  vessels  is  thus  formed, — the  membrana  vasculosa 
retina?.  Red  blood- corpuscles  develop  in  this  net-work,  and  in  a  foetal  pig 
of  one  hundred  and  seventy-five  millimetres  fine  vessels  were  seen  to  have 
grown  into  the  retina  from  this  vascular  membrane.  This  net-work  is  not 
supplied  by  the  central  artery  of  the  retina,  but  probably  by  the  short, 
posterior  ciliary  arteries.  There  are  no  blood-vessels  developed  in  the  layer 
of  rods  and  cones,  or  in  the  outermost  layer  of  the  retina. 

The  lymphatics  of  the  retina  appear  to  develop  and  accompany  the 


46 


DEVELOPMENT   OF   THE   EYE. 


vessels  as  circumvascular  spaces,  as  pointed  out  by  His  in  respect  to  other 
parts  of  the  lymphatic  system  of  the  eye  and  the  vessels  that  traverse  the 
nervous  system. 

The  optic  nerve  seems  at  first  to  be  formed  solely  of  axis-cylinder  pro- 
cesses from  the  ganglionic  layer  of  the  retina,  which  converge  from  every 
part  of  its  internal  functional  surface  toward  the  extreme  median  extrem- 
ity of  the  choroid  fissure  of  the  eyeball,  where  they  make  their  exit  as  a 
bundle  of  naked  nerve-fibres.  The  mode  in  which  these  nerve-fibres  arise 


FIG.  42. 


o 


Series  of  figures  from  successive  sections  of  the  retina  of  an  embryo  torpedo  near  the  point  where 
the  retina  joins  the  optic  stalk,  o.  The  upper  figures  represent  sections  through  the  lower  border  of  the 
retina  and  the  proximal  part  of  the  choroid  fissure.  The  axis-cylinder  fibres,  or  nerve-fibres,  of  the 
retina  are  seen  to  be  directed  toward  the  roof  of  this  fissure,  s,  and,  as  one  passes  to  the  sections  in  the 
lower  part  of  the  figure  that  cut  through  the  optic  stalk,  it  is  evident  that  the  nerve-fibres  grow  inward 
toward  the  brain  between  the  cells  of  the  lower  wall  of  the  stalk,  as  shown  by  the  minute  dots  or 
circles.  That  this  growth  is  at  first  entirely  from  the  retina  inward  is  proved  by  the  fact  that  in  the 
lowermost  and  last  section  of  the  optic  stalk  there  are  no  fibres  present  in  the  lower  wall.  (After 
Froriep.) 

is  shown  in  Fig.  41.  They  pass  through  the  retina  and  through  the  pig- 
mented  layer,  a  few  cells  of  which  are  seen  scattered  along  the  course  of 
the  optic  nerve  at  p.  The  optic  nerve  does  not  yet  seem  to  have  reached 
the  brain  ;  at  least  in  my  series  of  sections  the  connection  could  not  be  traced. 
A  few  scattered  cells  of  the  outer  wall  of  the  optic  stalk  are  seen  at  d. 
From  all  this  it  is  obvious  that  the  optic  stalk  itself  is  not  converted  into 
the  optic  nerve. 

The  manner  in  which  the  optic-nerve  fibres  grow  toward  the  brain 
from  the  retina  is  well  illustrated  by  the  accompanying  series  of  sections 


DEVELOPMENT   OF   THE   EYE.  47 

copied  from  Froriep  (Fig.  42),  the  significance  of  which  can  be  fully  ap- 
preciated from  what  has  preceded,  as  well  as  from  the  subjoined  summary 
of  the  investigations  of  Assheton. 

In  relation  to  the  development  of  the  optic  nerve  much  difference  of 
opinion  formerly  existed.  Recent  investigations,  however,  of  which  those 
of  Miiller,  His,  and  Froriep  were  among  the  first,  seem  to  have  pretty 
firmly  established  the  conclusion  that  the  optic  stalk  takes  a  subordinate 
part  in  the  development  of  the  optic  nerve,  only,  perhaps,  directing  the 
course  of  the  growth  of  the  axis-cylinder  fibre  of  the  optic  nerve  from  the 
neuroblasts  of  the  retina  centrad  and  distad,  or  to  and  from  the  optic  tract. 
Assheton  has  traced  the  development  of  the  optic  nerve  in  the  frog,  and 
reaches  the  following  conclusions  : 

The  optic  stalk  does  not  share  in  the  formation  of  the  nervous  parts  of 
the  eye. 

The  optic  stalk  is  broken  down  in  the  course  of  development,  and  the 
cells  forming  it  are  separated  from  one  another,  in  part  by  the  mechanical 
stretching  due  to  the  growth  in  thickness  of  the  optic  nerve,  and  in  part 
by  the  growth  of  nerve-fibres  between  its  component  cells. 

The  optic  nerve  is  developed  independently  of  the  optic  stalk ;  its  com- 
ponent nerve-fibres  lie  along  the  posterior  border  of  the  stalk,  and  at  first 
entirely  outside  it,  but  on  the  breaking  down  of  the  stalk  some  of  the 
nerve-fibres  grow  in  between  the  cells  of  the  latter. 

The  great  majority  of  fibres  forming  the  optic  nerve  arise  as  outgrowths 
from  nerve-cells  in  the  retina,  and  grow  toward  and  into  the  brain. 

According  to  Cajal's  researches,  certain  fibres  also  exist  which  would 
seem  to  grow  from  the  central  nervous  system  to  the  retina,  but  these 
Assheton  has  not  been  able  to  find. 

The  nerve-fibres  of  the  optic  nerve  pass  over  the  ventral  edge  of  the 
optic  cup,  and  thereby  cause  the  formation  of  the  choroidal  fissure. 

The  choroidal  fissure  of  the  embryo  represents  a  condition  in  the  evolu- 
tion of  the  eye  which  was  persistent  in  the  adult  prior  to  the  evolution  of  the 
lens.  Cajal  also  regards  it  as  quite  inaccurate  to  consider  the  fibres  of 
the  optic  nerve  as  becoming  connected  with  the  elements  of  the  inner  or 
retinal  wall  of  the  cup  after  piercing  the  outer  wall  of  pigmented  choroidal 
epithelium,  since  the  development  shows  that  the  fibres  never  really  pierce 
either  wall,  but  from  the  moment  of  their  formation  are  on  the  outside 
of  both.  He  also  asserts  that  it  is  only  by  the  subsequent  growth  of  the 
rim  of  the  optic  cup  that  the  bundle  of  nerve-fibres  becomes  surrounded  by 
the  walls  of  the  cup,  and  so  apparently  pierces  it.  The  choroidal  fissure 
is,  according  to  this  author,  due  to  an  interference  of  the  optic  nerve-fibres 
with  the  growth  of  the  ventral  margin  of  the  optic  cup,  as  a  result  of 
which  both  its  internal  and  its  external  wall  are  interrupted  at  this  point. 
He  also  suggests  that  the  choroidal  fissure  represents  a  stage  in  the  evolu- 
tion of  the  optic  cup,  that  it  was  due  to  the  eye  having  primitively  a  deep- 
seated  origin  from  the  cerebral  wall,  and  that  it  subsequently  grew  toward 


48  DEVELOPMENT  OF  THE  EYE. 

the  surface,  as  seems  also  to  be  indicated  by  what  has  been  said  in  the 
first  part  of  this  article  in  reference  to  Amphioxus. 

In  chicks  of  four  days,  thick  nerve-fibres  may  be  found  in  the  retina 
radiating  toward  and  into  the  just-forming  choroidal  fissure,  but  can  be 
traced  no  farther.  In  five-day  chicks,  the  fibres  are  thinner,  and  can  be 
easily  traced  into  the  choroidal  fissure,  but  along  the  optic  stalk  near  the 
brain  there  is  no  trace  of  nerve-fibres.  In  embryo  chicks  of  six  days, 
nerve-fibres  can  be  traced  all  the  way  to  the  brain  (Assheton). 

Mihalkovics  and  Kolliker  declare  that  fibres  arise  in  the  wall  of  the 
thalamencephalon  and  grow  as  a  pair  of  bundles,  following  the  optic  tract 
toward  the  median  ventral  line  from  each  side.  Continuing  to  grow  in 
their  original  directions,  they  at  last  cross  each  other,  and  each  sends  fibres 
into  the  optic  nerve  of  the  opposite  side.  Nerve-fibres  thus  also  find  their 
way  centrifugally  from  the  brain  toward  the  retina,  and  at  the  point  where 
these  fibres  cross  the  optic  chiasma  is  developed.  The  complicated  relations 
of  the  fibres  within  the  chiasma  remain  to  be  more  fully  investigated.  It 
appears  to  be  rendered  certain,  by  the  investigations  of  Froriep  and  Asshe- 
ton and  myself,  that  the  centripetal  fibres  are  the  first  to  be  developed, 
at  least  in  the  lower  vertebrates.  That  there  are  also  centrifugal  fibres 
developed  at  a  later  period  seems  equally  certain.  It  is  therefore  exceed- 
ingly probable  that  the  optic  nerve  in  its  fully-developed  condition  is 
formed  of  two  sets  of  fibres,  one  of  which  is  of  centripetal  or  retinal  origin, 
and  the  other  of  centrifugal  or  thalamic  origin.  But  of  these  two  the  cen- 
trifugal seems  to  be  developed  at  a  later  period  than  the  centripetal  or 
retinal  set.  The  discussion  of  the  development  of  the  optic  tract  will  be 
taken  up  later. 

DEVELOPMENT  OF  THE   SCLEKOTIC   AND  CORNEA. 

The  first  traces  of  the  sclerotic  in  a  just  hatched  salamander  larva  (Fig. 
41,  sZ)  are  represented  by  a  single  layer  of  cells  which  can  be  traced  from 
near  the  front  of  the  optic  cup,  at  this  stage,  to  the  point  of  exit  of  the 
optic  nerve.  The  vessels  of  the  choroid  are  beginning  to  appear  at  cv, 
Fig.  41 ;  in  fact,  the  mesoblastic  rudiment,  i,  of  the  anterior  stratum  of  the 
iris  is  beginning  to  become  vascular  at  vi,  in  continuity  with  the  vascular 
membrane  associated  with  the  development  of  the  choroid.  In  front  of 
the  optic  cup  the  cornea  is  represented  externally  by  its  epithelium,  e,  and 
internally  by  the  basement-membrane  of  Bowman,  ra&,  with  traces  of  the 
epithelium  of  the  anterior  chamber.  There  is  no  canal  of  Schlemm  yet 
developed,  and  the  connective  tissue  of  the  cornea  has  but  just  begun  to 
proliferate  inward  around  the  edge  of  the  cornea,  as  at  cc.  There  is  no 
tunica  vasculosa  lentis  developed.  The  sclerotic  and  cornea  are  present 
in  their  simplest  forms,  but  the  aqueous  chamber,  a,  is  already  spacious, 
although  not  yet  divided  by  the  iris  into  a  posterior  and  an  anterior 
chamber. 

While  the  sclerotic  develops  its  numerous  and  interlaced  fibres  in  the 


DEVELOPMENT   OF   THE    EYE.  49 

batrachian,  with  included  connective-tissue  cells,  more  tardily  than  in 
mammalia,  some  of  the  other  parts  are  more  precociously  developed,  as  the 
aqueous  chamber  and  the  iris,  for  example,  the  foundations  of  which  have 
been  already  in  part  laid  down  at  the  stage  represented  in  Fig.  41.  In 
a  mammalian  embryo  proportionally  but  little  more  advanced  than  that 
from  which  the  section  of  the  eye  just  referred  to  was  obtained,  the  sclerotic 
and  cornea  are  far  better  developed.  Comparing  Fig.  33,  the  fibrous 
connective-tissue  layer  forming  the  sclerotic  is  already  of  considerable 
thickness,  and  serves  for  the  attachment  of  the  ocular  muscles.  The 
connective-tissue  stratum,  c,  of  the  cornea  that  is  continuous  with  the 
sclerotic  has  also  grown  under  the  epithelium,  c',  and  already  forms  a  uni- 
formly thick  layer  over  the  whole  extent  of  the  internal  face  of  the  corneal 
epithelium.  In  the  chick,  as  early  as  the  fourth  day,  the  mesoderm  grows 
inward  around  the  edge  of  the  corneal  epithelium  to  form  the  connective- 
tissue  layer  of  the  cornea.  In  mammalia,  the  optic  cup  of  the  embryo 
(Fig.  38)  is  surrounded  at  a  very  early  stage  by  mesoderm,  which  furnishes 
the  connective-tissue  cells  between  which  the  fibres  of  the  sclerotic  appear, 
as  well  as  the  basal  substance  and  blastema  of  the  corium  of  the  cornea.  In 
mammals,  however,  this  mesodermal  blastema  in  front  of  the  optic  cup 
soon  divides  into  two  layers,  as  shown  in  Fig.  39.  One  of  these,  tv,  be- 
comes the  membrana  pupillaris,  and  the  other,  c,  the  corium  of  the  cornea, 
while  a  thin,  adherent  epithelial  layer,  d,  becomes  the  endothelium  of  the 
aqueous  chamber.  In  the  cleft  thus  left  between  these  layers  of  the  meso- 
dermal rudiments  of  the  deeper  layers  of  the  cornea  and  the  pupillary 
membrane,  the  aqueous  humor  appears. 

The  vascular  layer  on  the  external  surface  of  the  optic  cup  of  mam- 
malia develops  at  a  very  early  stage,  as  shown  in  Fig.  38,  ch.  From  it  the 
choroid  vessels  and  the  lamina  suprachoroidea  are  formed.  In  the  lamina 
suprachoroidea  scattered  pigment-cells  also  appear  external  to  the  primary 
external  pigmented  or  choroidal  epithelium,  p. 

Underneath  the  sclerotic,  in  association  with  the  choroid  and  the  lamina 
suprachoroidea,  an  important  lymph-space  is  developed.  This  space  is,  in 
fact,  interposed  between  the  lamina  suprachoroidea  and  the  inner  face  of  the 
sclerotic.  It  is  developed  at  first  along  the  courses  of  the  vessels,  and  com- 
municates, by  way  of  the  passages  of  exit  of  the  vasa  vorticosa,  with  the 
lymphatic  space  of  Tenon,  which  surrounds  the  eyeball  as  far  forward  as 
the  edge  of  the  cornea.  The  space  of  Tenon  is  bounded  externally  by  the 
adipose  and  connective  tissue,  of  mesodermal  origin,  of  the  orbit,  in  which 
the  eye,  optic  nerve,  muscles,  nerves,  and  vessels  are  embedded.  A  lymph- 
space  is  also  developed  about  the  optic  nerve  within  the  space  of  Tenon 
and  is  separated  from  the  latter  by  a  delicate  sheath  surrounding  the  nerve. 
This  space  communicates  with  the  arachnoid  of  the  brain.  These  spaces 
complete  their  development  comparatively  late  in  the  course  of  the  growth 
of  the  embryo,  though  it  is  doubtless  true  that  here,  as  elsewhere,  the 
lymph-spaces  appear  very  early  around  blood-vessels.  Budge  has  shown 

VOL.  I.— 4 


50  DEVELOPMENT   OF   THE   EYE. 

that  such  lymph-spaces  and  vessels  exist  in  the  blastoderm  of  the  chick  of 
the  second  day  of  incubation.  The  canal  of  Schlemm,  for  example,  has 
already  appeared  in  the  embryo  pig  of  twenty-three  millimetres  as  a 
passage  that  persists  through  adult  life  in  the  marginal  mesoderm  of  the 
cornea. 

VITREOUS  HUMOR 

This,  as  already  noted,  is  a  mass  of  tissue  of  a  peculiar,  loose,  trans- 
parent texture,  which  is  intruded  through  the  choroid  fissure  at  the  under 
side  of  the  optic  cup.  In  mammalian  embryos  blood-vessels  accompany 
this  intrusion  to  form  the  tunica  vasculosa  of  the  retina  and  lens.  It  is 
stated  that  mesodermal  cells,  in  addition  to  those  forming  the  walls  of  the 
vessels  and  their  contained  corpuscles,  are  found  in  the  eyes  of  rabbit 
embryos  of  thirteen  days.  In  the  eyes  of  embryo  birds  there  appears 
to  be  no  intrusion  of  either  cells  or  blood-vessels  to  form  the  vitreous 
humor,  though  in  the  eyes  of  embryo  birds  of  the  fourth  day  a  non- 
nucleated  reticulum  may  be  seen  in  sections  that  represent  a  well-developed 
vitreous  body.  This  vitreous  reticulum  seems  to  be  entirely  absent  in  the 
eye  of  the  embryo  salamander,  Fig.  41. 

The  growth  of  the  vitreous  in  the  mammalian  embryo  probably  takes 
place  by  the  rapid  development  of  a  great  quantity  of  its  basal  substance, 
or  glassy  matrix,  with  an  absorption  later  of  the  mesodermal  cells  that  were 
at  first  intruded.  The  hyaloid  canal  through  its  centre  persists  after  the 
atrophy  of  the  anterior  hyaloid  artery  which  supplies  the  vascular  tunic  of 
the  lens.  It  is  generally  believed  that  this  canal  becomes  a  lymph-space. 

A  homogeneous  layer — the  hyaloid  membrane — covers  the  surface  of 
the  vitreous  at  an  early  stage  (on  the  fourth  day  in  the  chick).  It  lies  in 
contact  with  the  retina,  the  lens,  and  the  ciliary  processes.  In  the  region 
of  the  ciliary  body  it  becomes  thickened  and  covers  the  ciliary  processes. 
It  here  constitutes  the  suspensory  ligament  of  the  lens,  or  zonula  Zinni,  in 
which  radiating  fibres  are  developed  that  are  attached  to  the  capsule  of  the 
lens  at  its  equator.  Angelucci  has  found  these  fibres  in  chicks  of  the  ninth 
day,  and  in  embryos  of  the  ox  ninety  millimetres  in  length.  The  capsule 
of  the  lens  is  a  product  of  the  membrana  vasculosa  lentis  after  the  degener- 
ation and  disappearance  of  the  vessels  from  the  latter. 

The  imperfect  closure  of  the  choroid  fissure  during  development  results 
in  a  defective  inferior  border  of  the  iris.  Two  pathological  conditions  re- 
sult from  an  imperfect  closure  of  the  choroid  fissure  after  the  intrusion  of 
the  vitreous  humor.  If  this  closure  is  defective  in  the  region  of  the  choroid, 
coloboma  choroidea  results.  If  the  defective  closure  extends  to  the  edge 
of  the  optic  cup  where  the  iris  is  developed,  coloboma  iridis  results.  The 
lack  of  a  uniform  development  of  pigment-cells  in  the  anterior  mesodermal 
layer  of  the  iris  gives  rise  to  a  party-colored  appearance  of  this  organ, 
as  a  result  of  which  some  of  the  radii  of  the  iris  may  appear  much 
lighter  in  color  than  others.  A  change  thus  produced  by  defective  develop- 
ment of  pigment  may  affect  any  radius  of  the  iris,  whereas  a  congenital 


DEVELOPMENT   OF   THE   EYE.  51 

coloboma,  due  to  the  defective  closure  of  the  choroid  fissure,  can  affect  only 
some  of  its  inferior  radii. 

THE  IRIS,  CILIARY  PROCESSES,  AND  CILIARY  MUSCLE. 

These  structures  may  very  properly  be  considered  together,  since  the 
courses  of  development  of  all  these  parts  are  very  intimately  associated. 
After  the  nearly  complete  investment  of  the  optic  cup  by  the  sclerotic,  and 
the  coalescence  of  the  fibrous  mesodermal  tissue  of  the  latter  (co,  Fig.  42) 
with  the  corium  (c)  of  the  cornea,  a  complete  connective-tissue  envelope  is 
formed,  covering  the  essential  structures  of  the  eye  that  were  of  ectoderm  al 
origin.  This  process  leads  to  the  completion  of  the  optic  globe,  or  eyeball. 

Coincidently  with  this  process  of  investment  of  the  optic  cup  and  lens 
by  a  connective-tissue  envelope,  a  series  of  very  important  changes  in  the 
relative  thickness  of  the  outer  or  marginal  portion  of  the  optic  cup  begin, 
as  a  result  of  which  its  edge  becomes  much  thinner  than  the  inner  or  true 
retinal  portion.  This  thinning  begins  at  the  points  marked  *  *  in  Fig. 
41,  where  the  edge  of  the  optic  cup  makes  an  abrupt  bend  toward  the 
lens.  This  attenuation  of  the  margin  of  the  optic  cup  is  also  well  shown 
in  Fig.  39  at  m,  and  again,  still  more  obviously,  from  the  point  marked  os  in 
Fig.  42.  These  points  mark  the  boundary  between  the  anterior  or  marginal 
zone  of  the  optic  cup  and  its  true  posterior  retinal  portion.  This  boundary 
is  known  as  the  ora  serrata  of  anatomists,  and  in  the  embryo  it  marks  the 
point  from  which  the  inner  wall  of  the  optic  cup  undergoes  degenerative 
changes  of  such  a  character  that  its  cells  become  transformed  into  a  one- 
layered  epithelium  composed  of  cubical  cells.  This  layer  also  lies  closely 
in  contact  with  the  outer  or  pigmented  layer  of  the  cup.  Meanwhile,  the 
portion  of  the  inner  wall  of  the  optic  cup  within  the  ora  serrata  (r,  Fig. 
42)  remains  thick  and  composed  of  several  layers  of  cells,  and  becomes 
the  retina.  That  is,  the  bottom  of  the  cup  remains  as  the  retina,  whereas 
its  rim,  or  marginal  portion,  is  associated  with  the  development  of  the  iris 
and  ciliary  body. 

As  the  anterior  marginal  walls  of  the  cup  become  attenuated  they  also 
extend  more  and  more  into  the  aqueous  chamber  between  the  lens  and  the 
cornea.  This  growth  of  the  thin  edges  of  the  optic  cup  over  the  lens  pro- 
ceeds until  it  leaves  only  a  small  opening,  the  pupil,  which  leads  into  the 
optic  cup.  This  thin  marginal  portion  of  the  optic  cup,  overlying  the  lens, 
subsequently  becomes  incorporated  into  the  iris.  In  newly-born  mammals 
the  eyes  are  blue ;  this  is  due  to  the  reflection  through  the  translucent  outer 
mesodernml  layer  of  the  iris,  3,  Fig.  43,  of  the  color  of  the  dark  bluish- 
black  pigment  of  the  outer  layer  of  the  edge  of  the  optic  cup.  The 
outer  layer  of  the  iris,  3,  is  split  off  from  the  mesoderm  which  forms  the 
corium  of  the  cornea  at  the  time  of  the  formation  of  the  aqueous  cham- 
ber, Fig.  39,  a.  Into  this  outer  mesodermal  layer  of  the  iris  amoeboid  pig- 
ment-cells proliferate  after  birth  which  are  often  of  a  different  color  from 
those  that  form  the  black  epithelium  of  the  outer  cup  or  layer,  2,  Fig.  43. 


62 


DEVELOPMENT   OF   THE   EYE. 


In  fact,  these  cells  may  be  brown,  yellow,  greenish,  or  even  brilliant  red,  as 
in  the  irides  of  some  birds  and  reptiles.  In  man,  these  originally  amoeboid 
pigment-cells  develop  after  birth,  and  probably  migrate  into  the  outer 
stratum  of  tissue  of  the  iris,  which  is  of  mesodermal  origin,  from  the  choroid 
around  the  edge  of  the  iris,  or  become  pigmented  in  situ  as  cells  of  meso- 
dermal origin.  These  cells  of  various  tints  of  brown  and  black  give  the 
various  tints  of  brown,  hazel,  and  black  that  are  seen  in  the  iris  of  the 
human  eye. 


FIG.  43. 


eb 


Section  through  the  margin  of  the  optic  cup  of  an  advanced  embryo  of  a  thrush  of  the  second 
week  of  incubation  (Turdus  musicus).  Enlarged  60  times.  (After  Kessler,  from  Hertwig's  Lehrbuch.)— 
r,  retina ;  p,  pigmented  outer  layer  or  wall  of  cup ;  co,  connective-tissue  envelope  of  the  optic  cup,  the 
thick  sclera  externally  and  choroid  internally  continued  partly  over  the  front  of  the  iris  and  partly 
into  the  cornea,  c,  at  the  lower  part  of  the  figure ;  os  indicates  the  position  of  the  ora  serrata  or  point 
where  the  retina  is  differentiated  around  the  circumference  of  the  globe  of  the  eye  into  a  posterior 
sensory  and  an  anterior  non-sensory  epithelium ;  c6,  ciliary  body ;  1,  inner  (retinal)  and,  2,  outer  pig- 
mented lamella  of  the  pars  iridis  retinae ;  3,  connective-tissue  lamella  of  iris  in  which  smooth  muscular 
fibres,  vessels,  nerves,  and  pigment  are  developed ;  Ip,  pectinate  ligament  of  iris ;  s,  canal  of  Schlemm ; 
ce,  corneal  epithelium. 

This  outer  layer,  3,  Fig.  43,  of  mesodermal  origin,  also  gives  rise  to  the 
stroma  of  the  iris,  as  well  as  to  its  abundant  non-striated  muscles  and  blood- 
vessels. In  mammals,  Fig.  39,  at  x,  this  outer  layer  in  the  embryo  is  for 
a  time  continuous  with  the  tunica  vasculosa  lentis  anteriorly  and  the  choroid 
posteriorly.  This  fact  explains  how  the  pupillary  part  of  the  vascular 
tunic  of  the  lens  for  a  time  closes  the  pupil,  and  also  why  a  failure  to 
absorb  this  pupillary  part  of  the  vascular  membrane  leads  to  congenital 
atresia  pupillaris. 

The  marginal  portion  of  the  optic  cup  external  to  the  ora  serrata  begins 
to  become  attenuated  in  the  eyes  of  the  embryos  of  the  domestic  ox  when 
they  have  reached  a  length  of  about  one  and  one-fifth  inch.  In  rabbit 
embryos  this  marginal  thinning  begins  on  the  sixteenth  day. 

The  portion  of  the  optic  cup  which  is  immediately  adjacent  to  the  inner 
primary  pigmented  layer  of  the  iris,  and  which  surrounds  the  equator  or 
margin  of  the  lens,  also  belongs  to  the  so-called  marginal  zone,  and  gradu- 
ally undergoes  elevation  and  plication,  Fig.  43,  cb.  In  connection  with  the 
adjacent  layer  of  connective  tissue  outside  this  region,  it  is  gradually  trans- 
formed into  the  ciliary  body.  The  inner  aspect  of  the  ciliary  body  is  that 
of  a  cycle  of  meridional  folds,  which  in  man  reach  the  number  of  seventy 
to  eighty,  the  corona  tiliaris,  the  free  borders  of  which  project  toward  the 


DEVELOPMENT  OF   THE   EYE.  53 

optic  axis  of  the  eye.  There  are  also  two  or  more  annular  folds  apparent 
in  the  ciliary  body  in  vertical  sections  of  the  eye,  as  shown  in  Fig.  42. 
The  cycle  of  processes  that  arise  together  with  the  annular  folds  constitute 
the  ciliary  body  of  the  adult.  In  a  cat  embryo  ten  millimetres  in  length 
these  folds  or  ciliary  processes  are  already  well  developed.  Cross-sections 
of  the  folds  in  the  eye  of  such  an  embryo  show  that  but  little  connective 
tissue  of  mesodermal  origin  is  carried  into  the  spaces  enclosed  between  the 
walls  of  the  folds.  The  cause  of  the  production  of  these  folds  is  evidently 
to  be  ascribed  to  the  more  rapid  growth  of  the  area  of  the  walls  of  the 
optic  cup  in  the  ciliary  region  than  in  the  regions  immediately  behind  and 
before  it.  The  inner  or  retinal  layer  of  these  folds  or  ciliary  processes 
remains  unpigmented,  whereas  the  outer  layer  is  very  strongly  pigmented. 

The  ciliary  processes  gradually  become  very  much  thickened,  and  finally 
project  inward  and  forward  as  rather  blunt  processes,  owing  to  the  increase 
in  amount  of  the  tissue  of  mesodermal  origin  within  them  which  is  pro- 
liferated from  the  outer  rudiment  of  the  choroid  overlying  them  externally. 
The  ciliary  processes  gradually  acquire  a  firm  union  with  the  lens  through 
the  formation  of  the  zonula  Zinni,  which  in  man  is  said  to  be  formed 
during  the  fourth  month.  Some  authors  believe  the  zonula  to  arise  from 
the  vitreous  body,  and  state  that  when  the  iris  and  the  ciliary  body  are  de- 
veloped, the  vitreous  is  traversed  by  fine  fibres  that  extend  from  the  ora 
serrata  to  the  margin  of  the  lens.  It  is  asserted  by  Lieberkiihn  that  the 
zonula  is  distinctly  developed  in  eyes  that  have  attained  to  half  their  full 
size. 

The  ciliary  processes  in  man  begin  to  appear  in  the  human  embryo 
about  the  end  of  the  second  or  early  in  the  third  month.  In  the  fifth 
month,  Kolliker  states,  the  processes  are  from  0.12  to  0.18  millimetre  high 
and  0.10  to  0.12  millimetre  wide.  The  mesoderm  which  grows  into  these 
processes  from  the  outside  becomes  converted  partly  into  the  ciliary  mus- 
cles and  partly  into  the  pectinate  ligament,  Ip,  Fig.  43.  The  proximal 
portion  of  the  latter  next  to  the  ciliary  body,  c6,  becomes  the  ligament,  and 
the  distal  part,  the  muscle.  The  mesodermal  rudiment  which  gives  rise  to 
the  ciliary  muscles  and  pectinate  ligaments  is  originally  continuous  pos- 
teriorly with  the  choroidal  layer,  ch,  Fig.  38,  and  anteriorly  \vith  the  meso- 
derm of  the  iris,  3,  Fig.  43.  The  rudiments  of  the  ciliary  muscles,  both  radi- 
ating and  circular,  seem  to  split  off  from  the  outer  side  of  the  embryonic 
rudiment  of  the  choroid.  It  is  asserted  that  in  young  birds  of  the  last 
day  of  incubation  the  fibres  of  the  ciliary  muscle  are  transversely  striated. 

DEVELOPMENT  OF  THE  THIRD,  FOUKTH,  FIFTH,  AND  SIXTH  NERVES. 

The  third,  fourth,  fifth,  and  sixth  nerves  and  their  associated  ganglia 
comprise  all  the  cranial  pairs  that  concern  the  student  of  the  evolution  of 
the  eye.  In  the  lowest  vertebrate  known,  Amphioxus,  even  when  any  of 
these  nerves  are  developed,  as  the  fifth,  for  example,  they  do  not  innervate 
any  part  of  the  eye,  the  latter  having  not  yet  been  separated  from  the 


54  DEVELOPMENT   OF   THE   EYE. 

brain.  In  the  lampreys  and  hags  we  first  meet  with  these  nerves  developed 
in  relation  to  the  eyes. 

Before  proceeding  further,  however,  it  may  be  well  to  state  that  the 
cranial  nerves  that  are  related  to  the  eye  arise  from  nests  or  groups  of 
ganglion-cells  which  bear  a  segmental  relation  to  one  another,  especially  in 
the  region  of  the  medulla  oblongata.  That  is,  there  is  embryological  evi- 
dence which  tends  to  show  that  some  or  all  of  these  nests  of  ganglion- cells 
in  the  cord  and  brain  are  genetically  related  to  the  so-called  somites  or 
primitive  segments  of  the  head  and  upper  cervical  regions  of  the  embryo 
long  before  the  nerves  themselves  have  developed.  There  are,  in  fact, 
serially  recurring  constrictions  of  the  medullary  tube  in  the  head  and  body 
which  correspond  in  number  to  that  of  the  segmental  rudiments  of  the 
muscles,  or  myotomes,  which  are,  as  is  well  known,  parts  of  the  so-called 
protovertebrae  or  embryonic  segments  of  the  embryo.  These  segmental 
differentiations  of  the  medullary  tube  are  visible  even  in  the  cephalic  part 
of  the  embryonic  neural  system,  especially  in  the  medulla.  From  the  fact 
that  this  segmental  differentiation  of  the  medullary  tube  divides  the  latter, 
in  the  embryo,  into  short,  serially  recurrent  segments  or  regions  trans- 
versely, the  latter  have  been  called  neuromeres. 

Along  the  dorsal  median  line  where  the  medullary  tube  closes,  the 
spinal  ganglia,  and  certainly  some  of  the  cranial  ganglia,  are  developed. 
This  dorsal  keel  of  the  medullary  tube,  from  which  the  ganglia  of  sensory 
roots  of  the  spinal  and  some  of  the  cranial  nerves  are  developed,  has  been 
called  the  neural  crest.  This  crest-like  outgrowth  of  the  medullary  tube 
itself  early  shows  a  tendency  toward  segmental  differentiation,  as  slight, 
serially  recurrent  thickenings,  or  knots  of  cells,  which  correspond  to  the 
neuromeres  in  number,  and  eventually  give  rise  to  the  spinal  ganglia  and 
some  of  the  cranial  ganglia  of  the  sensory  roots  of  the  nerve-pairs.  These 
knots  of  cells  grow  outward  and  downward  in  pairs  from  the  neural  crest, 
so  as  eventually  to  push  themselves  into  a  lateral  position  alongside  the 
medullary  tube  into  the  position  of  the  ganglia  of  the  sensory  roots  of  the 
spinal  and  cranial  nerves. 

While  the  foregoing  is  in  progress,  the  walls  of  the  medullary  tube, 
from  which  the  brain  and  the  spinal  cord  are  formed,  are  also  undergoing 
differentiation  into  regions  lengthwise,  marked  off  from  one  another  by 
differences  in  the  thickness  of  the  wall  and  by  four  internal  grooves, — a 
lateral  one  on  each  side,  and  a  ventral  and  a  dorsal  one, — which  traverse 
the  inner  surface  of  the  medullary  tube  longitudinally.  As  a  result  of  this 
differentiation,  no  fewer  than  six  longitudinal  bands  or  zones  may  be  dis- 
tinguished in  a  cross-section  of  the  medullary  tube  or  rudimentary  cerebro- 
spinal  axis  of  the  vertebrate  embryo.  These  longitudinal  bands  have  been 
called  by  Mi  not  the  zones  and  plates  of  His,  in  honor  of  the  distinguished 
German  embryologist  who  first  drew  attention  to  them,  and  who  has  con- 
tributed so  much  to  a  thorough  knowledge  of  the  early  development  of  the 
nervous  system.  There  are  six  of  these  zones  of  His :  two  thin  zones 


DEVELOPMENT   OF   THE   EYE.  55 

occupy  a  median  position,  one  forming  the  dorsal  and  the  other  the  ventral 
wall  of  the  medullary  tube ;  the  four  others  form  the  most  important  por- 
tion of  the  lateral  walls  of  the  medullary  tube,  two  on  each  side.  The  lateral 
zones  are  separated  from  each  other  by  a  groove  or  stilcus  which  extends 
from  the  foramina  of  Monro,  at  the  anterior  end  of  the  medullary  tube,  back- 
ward over  the  lateral  inner  surface  of  the  latter  through  the  brain,  as  well 
as  through  the  portion  that  is  to  be  the  spinal  cord.  The  entire  extent  of 
the  lateral  walls  of  the  medullary  tube  is  thus  divided  by  a  pair  of  lateral 
grooves — the  sulci  of  Monro — into  a  pair  of  dorsal  and  a  pair  of  ventral 
longitudinal  bands  or  zones.  This  demarcation  of  the  lateral  walls  of  the 
medullary  tube  into  two  zones  may  therefore  be  traced  through  the  brain  and 
the  entire  length  of  the  spinal  cord.  The  two  lateral  zones  project  some- 
what into  the  cavity  of  the  medullary  tube  on  each  side,  so  that  the  cavity 
in  the  tube — the  future  neural  canal — is  now  somewhat  lozenge-shaped,  as 
seen  in  cross-section.  These  zones  of  the  medullary  tube  are  of  such  im- 
portance in  leading  to  a  correct  understanding  of  the  development  of  the 
central  nervous  system  that  the  preceding  account  of  their  relations  will 
not  be  considered  superfluous. 

The  embryonic  brain  and  cord  being  thus  mapped  out  transversely  into 
serially  recurrent  segments  or  neuromeres,  and  these  again  divided  longi- 
tudinally by  the  sulci  of  Monro  laterally  and  the  thin  dorsal  and  ventral' 
plates  dorsally  and  ventrally,  six  zones  may  be  distinguished  in  each 
neuromere.  The  zones  differ,  however,  in  the  extent  of  their  development 
anteriorly  and  posteriorly.  At  the  anterior  part  of  the  cord,  or  region  of 
the  medulla  oblongata,  the  median  dorsal  plate  becomes  very  wide  and  also 
very  thin  where  it  forms  the  roof  of  the  fourth  ventricle ;  for  the  remain- 
ing extent  of  the  cord  the  dorsal  plate  remains  narrow.  The  ventral 
plate  on  the  under  side  of  the  cord  remains  narrow  through  the  entire 
length  of  the  embryonic  nervous  system,  and  becomes  important  as  the 
portion  through  which  the  ventral  or  anterior  commissural  fibres  pass  from 
one  side  to  the  other  through  the  entire  length  of  the  brain  and  the  cord. 

The  lateral  zones  are  the  only  ones  that  develop  ganglion-cells.  The 
dorsal  zone,  or  that  lying  above  the  sulcus  of  Monro,  is  associated  with  the 
development  of  the  dorsal  or  sensory  roots  of  the  spinal  nerves,  while  the 
ventral  zone,  below  the  sulcus,  is  associated  with  the  development  of  the 
motor  roots  of  the  spinal  and  cranial  nerves.  It  may  be  added  that  the 
lateral  or  dorsal  zones  of  His  lie  between  the  sulci  olf  Monro,  one  on  each 
side  of  the  dorsal  plate,  while  the  ventral  zones  of  His  lie  below  the  sulci 
of  Monro,  one  on  each  side  of  the  ventral  plate.  These  preliminaries  will 
enable  us  to  follow  intelligently  the  development  of  those  cranial  nerves, 
besides  the  optics,  which  are  concerned  in  the  innervation  of  the  eye  and 
its  accessories. 

In  the  wall  of  the  medullary  tube,  along  the  zones  of  His,  we  finally 
have  an  epithelium  of  ectodermal  origin,  several  cells  deep.  In  the  region 
of  the  dorsal  and  ventral  plates  and  on  the  sides  of  the  tube  along  the 


56  DEVELOPMENT  OF  THE  EYE. 

course  of  the  sulci  of  Monro  it  is  thinner  than  elsewhere.  The  first  step 
in  the  differentiation  of  the  cells  composing  the  walls  of  the  medullary 
tube  is  their  separation  into  two  sorts.  One  of  these  comprises  the  spongio- 
blasts  or  young  neuroglia-cells,  called  spongioblasts  from  the  fact  of  their 
producing  a  reticular,  fibrous,  or  spongy-looking  supporting  tissue  (myelo- 
spongium,  neuro-spongium)  for  the  second  sort, — the  so-called  germinating 
cells.  The  germinating  cells  undergo  rapid  division,  and  thus  become  the 
parents  of  the  nerve-cells,  or  neuroblasts,  which  give  rise  to  the  axis-cylinder 
processes  or  nerve-fibres. 

This  multiplication  of  nerve-cells  begins  in  the  inner  stratum,  next  the 
limitans  interna,  and  between  or  among  the  embryonic  neuroglia-cells,  or 
spongioblasts.  As  the  nerve-cells  here  multiply,  the  lateral  ,zones  of  His 
increase  in  thickness.  The  neuroglia-cells  extend  from  the  inner  side  of 
the  wall  of  the  medullary  tube  to  its  outer  side ;  their  outer  ends  branch 
and  anastomose  with  one  another.  The  outer  ends  of  the  spongioblasts 
also  soon  form  a  spongy,  non-nucleated,  superficial  reticulum  which  invests 
as  a  thin  mantle  (the  so-called  mantle-layer  of  neuroglia,  or  Randschleier 
of  His)  the  rest  of  the  tissues  of  the  medullary  wall.  It  is  through  and 
along  this  spongy  Randschleier  that  the  nerve-cells  send  out  their  axis- 
cylinder  processes  in  the  course  of  the  development  of  the  spinal  and 
cranial  nerves.  The  Randschleier,  or  mantle,  is,  in  fact,  the  first  indica- 
tion of  the  appearance  of  the  embryonic  neuroglia  which  is  to  be  traversed 
by  the  axis-cylinder  fibres  of  the  white  columns  and  bundles,  throughout 
the  peduncles,  medulla,  and  cord. 

The  neuroblasts  or  nerve-cells  of  the  ventral  zone  of  His  throw  out 
from  the  ventro-lateral  face  of  the  latter  the  anterior  or  motor  roots  as 
bundles  of  axis-cylinder  fibres.  These  nerve-cells  also  send  commissural 
fibres  through  the  ventral  or  anterior  plate  across  the  median  line.  From 
the  dorsal  zone  of  His  some  axis-cylinder  fibres  emerge  from  its  dorso- 
lateral  face  to  join  the  sensory  roots  of  the  spinal  nerves,  so  that  the  com- 
missural fibres,  the  motor  fibres  of  the  anterior  root,  and  some  of  the  fibres 
of  the  dorsal  root  grow  out  centrifugally,  or  from  nerve-cells  that  lie  within 
the  medullary  tube.  These  nerve-cells  also  send  axis-cylinder  fibres  length- 
wise along  the  outer  side  of  the  cord  through  the  Randschleier  or  primitive 
outer  stratum  of  embryonic  neuroglia  investing  the  ganglionic  tracts,  the 
lateral  zones  of  His. 

The  process  of  nerve-development  is,  however,  very  different  with  the 
ganglion-cells  of  the  sensory  ganglia.  These  contain  nerve-cells  which  are 
bipolar.  That  is,  each  nerve-cell  of  a  ganglion  sends  out  two  processes  as 
axis-cylinders  in  two  opposite  directions,  and  therefore  from  opposite  ends 
centrifugally  and  centripetally,  or  fibres  are  sent/rom  the  ganglion  outward 
to  the  lateral  parts  of  the  embryo  as  well  as  to  the  central  nervous  system 
from  the  ganglion.  In  other  words,  one  set  of  fibres  grow  out  centrifugally 
from  the  outer  or  external  ends  of  the  cells  in  the  same  way  as  the  nerve- 
cells  of  the  anterior  zone  of  His  send  out  axis-cylinder  fibres  centrifugally 


DEVELOPMENT   OF   THE    EYE.  57 

to  form  the  motor  roots ;  another  set,  on  the  contrary,  grow  inward  from 
the  internally  directed  ends  of  the  nerve-cells  of  the  ganglion  toward  the 
cord  and  penetrate  it  along  the  course  of  the  dorsal  zone  of  His  above 
the  sulcus  of  Monro.  Entering  the  cord,  these  fibres  divide  into  anterior 
and  posterior  longitudinal  branches  which  traverse  the  Randschleier  and 
then  give  off  collatwal  branches  which  subdivide  and  ramify  in  the  gray 
matter. 

A  development  of  nerve-fibres  may  also  proceed  centrifugally  from 
the  extreme  outer  peripheral  sense-cells,  such  as  the  retinal,  olfactory,  and 
auditory  sensory  epithelia,  for  example,  and  pass  by  way  of  the  dorsal 
roots  into  the  brain  and  medulla  in  the  same  way  as  the  centripetal  or 
ingrowing  fibres  from  the  nerve-cells  of  the  sensory  ganglia,  but  directly 
and  without  making  any  connections  with  the  latter.  If  it  be  true,  as 
appears  very  probable,  that  the  optic  nerve  represents  only  a  dorsal  or 
sensory  nerve  the  ganglionic  foundation  of  which  has  been  precociously 
separated  and  carried  outward  from  the  brain,  we  may  possibly  regard 
it  as  representative  of  the  third  type  or  method  of  development  of 
nerve-fibres  as  described  above.  It  is,  however,  very  probable  that  the 
two  types  of  the  centripetal  development  of  nerve-fibres  here  mentioned 
will  be  found  to  have  been  primitively  the  same  as  further  embryological 
research  clears  up  the  many  perplexing  questions  that  still  present 
themselves  in  connection  with  the  development  of  the  nervous  system. 
There  is,  however,  under  all  circumstances,  a  very  broad  distinction  to  be 
observed  between  medullary  nerves  that  grow  out  directly  as  bundles  of 
axis-cylinder  fibres  from  groups  of  nerve-cells  lying  within  the  cerebro- 
spinal  axis  and  those  that  may  be  regarded  as  ganglionic  nerves,  in  which 
the  nerve-fibres  have  grown  into  the  cerebro-spinal  axis  from  groups  of 
nerve-cells  lying  external  to  the  nervous  system.  We  may  now  consider 
the  development  of  the  nerves  that  supply  the  muscles,  etc.,  of  the  orbit, 
as  well  as  the  development  and  connections  of  the  optic  tract. 

The  Oculo-Motor  or  Third  Nerve. — According  to  His,  the  third  nerve 
grows  out  from  the  ventral  zone  in  an  inferior  position,  or,  in  anatomical 
language,  from  the  under  side  of  that  region  of  the  mid-brain  which  corre- 
sponds to  the  posterior  quadrigeminal  bodies  or  testes.  A  pair  of  clusters 
of  nerve-cells  or  neuroblasts  in  the  ventral  zone  send  out  an  axis-cylinder 
process  almost  directly  forward  to  the  future  ventral  edge  of  the  eyeball. 
It  divides  after  reaching  this  point,  and,  as  is  well  known,  innervates  five 
muscles, — rectus  internus,  rectus  superior,  rectus  inferior,  obliquus  inferior, 
and  levator  palpebra?.  Schwalbe  discovered  a  ganglion  belonging  to  this 
nerve  in  Selachians ;  the  true  ganglion  of  the  oculo-motor  of  the  mammalia 
has  not  yet  been  discovered  in  mammalian  embryos,  according  to  Minot. 
In  mammalia  it  appears  to  be  a  purely  medullary,  and  consequently  a  motor, 
nerve,  as  indicated  by  its  development.  It  has  been  suspected  by  Minot 
that  the  transitory  ganglion  of  the  so-called  thalamic  nerve,  discovered  by 
Miss  Platt  in  elasmobranchs,  may  represent  the  true  ganglion  of  the  oculo- 


58  DEVELOPMENT   OF   THE   EYE. 

motor  nerve.  The  oculo-motor  is  distinctly  developed  in  the  human  embryo 
of  ten  millimetres. 

The  Trochlearis,  Patheticus,  or  Fourth  Nerve. — This  nerve  has  a  most 
singular  origin.  It  arises  at  the  isthmus  or  on  the  dorsal  side  of  the  brain 
at  the  posterior  limit  of  the  posterior  quadrigeminal  body,  immediately 
in  front  of  the  rudiment  of  the  cerebellum.  Its  right  and  left  trunks 
decussate  at  the  point  of  exit  from  the  mantle  of  neuroglia.  It  is  quite 
well  developed  in  a  human  embryo  of  ten  millimetres  or  of  the  fifth 
week.  This  at  first  would  appear  to  be  a  medullary  nerve  also,  since 
the  pair  of  groups  of  nerve-cells  from  which  it  arises  are  situated  in  the 
ventral  zone  of  His.  From  these  groups  of  nerve-cells  a  bundle  of  fibres 
passes  upward  on  each  side  through  the  mantle  layer  of  neuroglia  within 
the  medullary  wall  to  their  point  of  exit  and  decussation  mentioned  above. 
It  has,  however,  been  found  that  the  peculiar  course  of  the  fibres  is  due  to 
a  migration  downward  of  the  neuroblasts  within  the  wall  of  the  medullary 
tube,  and  that  the  nerve  of  one  side  does  not  at  first  cross  its  fellow  of  the 
opposite  side.  The  ganglion  of  the  trochlearis  was  discovered  in  elasmo- 
branchs  by  Froriep  and  Miss  Platt ;  it  appears  to  be  for  a  time  continuous 
with  that  of  the  trigeminal.  The  trochlearis  in  the  human  embryo  passes 
straight  forward  toward  the  future  superior  side  of  the  eyeball,  across  the 
course  of  the  oculo-motor,  and  nearly  parallel  with  the  top  of  the  head  of 
an  embryo  two-fifths  of  an  inch  long,  to  join  the  superior  oblique  muscle, 
which  it  innervates. 

The  Trigeminal  or  Fifth  Nerve. — This  nerve,  together  with  its  Gasserian 
ganglion,  is  the  most  strongly  developed  of  all  the  cranial  nerves,  its  pre- 
ponderating size  and  importance  being  apparent  in  all  vertebrates  at  a  very 
early  stage.  In  man,  its  three  branches — from  which  it  derives  its  name 
of  trigeminus — are  already  developed  by  the  fifth  week,  together  with  the 
Gasserian  and  ciliary  ganglia.  Embryologically,  as  well  as  anatomically, 
this  nerve  proves  to  be  of  a  mixed  character;  that  is,  its  origin  is  a 
mixed  one,  since  some  of  its  fibres  are  of  medullary  and  others  of  gan- 
glionic  origin,  in  conformity  with  its  motor  and  sensory  functions.  The 
uppermost  or  ophthalmic  branch,  with  its  associated  ciliary  ganglia,  is  the 
principal  one  that  concerns  us.  It  is  now  known,  however,  that  the  devel- 
opment of  the  ciliary  ganglion  is  intimately  associated  with  that  of  the 
Gasserian,  and  that  the  former  appears  to  be  continuous  with  the  latter  at 
an  early  stage.  In  the  lower  vertebrates  the  ciliary  ganglion  is  associated 
also  with  a  single  great  trunk  growing  out  centrifugally  from  the  ganglion 
and  known  as  the  ramus  ophthalmicus  profundus.  In  man,  however,  the 
branches  emanating  directly  from  this  ganglion  are  the  ciliary  nerves  that 
pass  to  the  eyeball,  these  being  evidently  centrifugal  outgrowths  from  the 
ganglion.  How  the  radix  longa  and  radix  brevis  of  the  ciliary  ganglion 
are  developed  in  man  is  not  known,  but  it  is  very  probable  that  they  are 
developed  centrifugally.  The  frontal,  nasal,  and  lacrymal  branches  of 
the  ophthalmic  are  clearly  in  part  at  least  centrifugal  outgrowths  from 


DEVELOPMENT   OF   THE   EYE.  59 

the  Gasserian  ganglion.  The  orbital  and  palpebral  branches  of  the  max- 
illary or  second  branch  have  a  similar  history. 

The  motor  root  of  the  trigeminal  grows  out  from  a  pair  of  clusters  of 
nerve-cells  situated  in  the  ventral  zones  of  His,  at  the  anterior  end  of  the 
medulla  oblongata  of  the  embryo.  The  point  of  exit  of  the  motor  root  is 
just  below  the  level  of  the  sulcus  of  Monro.  The  ascending  sensory  root 
penetrates  the  lateral  wall  of  the  medulla  of  the  embryo  from  the  Gasserian 
ganglion,  just  external  to  and  above  the  sulcus  of  Monro,  over  the  lower 
margin  of  the  dorsal  zone  of  His.  The  superior  or  first  branch  seems  in 
the  human  embryo  of  the  fifth  week  to  be  principally  associated  with  the 
ciliary  ganglion,  and  arises  and  passes  almost  vertically  from  the  Gasserian 
ganglion  to  a  position  behind  the  eye. 

The  Abducens  or  Sixth  Nerve. — This  nerve  is  developed  wholly  from 
fibres  that  have  grown  out  of  the  medulla  oblongata  centrifugally.  It  is 
therefore  a  medullary  nerve.  His  discovered  the  nuclei  of  this  nerve  in 
the  ventral  zones  that  are  called  after  him,  situated  pretty  close  to  the 
median  line  or  median  ventral  plate  of  the  medulla  oblongata.  The  points 
of  exit  of  this  pair  of  nerves  in  the  embryo  are  already  closer  together  and 
more  decidedly  ventral  than  those  of  any  of  the  cranial  pairs  behind  it. 
The  ventral  or  motor  root  of  the  facial  sends  fibres  into  the  ventral  column 
of  His  which  overarch  the  nuclei  of  the  abducens.  In  the  human  embryo 
of  the  fifth  week  the  sixth  nerve  passes  almost  vertically  from  the  floor  of 
the  medulla  upward  toward  the  eye  and  at  right  angles  to  the  fourth 
nerve. 

DEVELOPMENT  OF   THE  OPTIC  TRACTS. 

It  would  be  superfluous  here  to  discuss  the  gross  anatomy  and  physi- 
ology of  the  optic  tracts,  but  it  has  been  thought  best  to  preface  the  strictly 
embryological  discussion  of  that  region  by  briefly  tracing  the  paths  taken 
by  the  optic  stimuli  and  the  gross  relations  of  the  parts  involved. 

The  path  of  the  visual  stimuli  is  from  the  sensory  epithelium  (the  rods 
and  cones)  of  the  retina  along  the  optic  nerve  to  the  chiasma.  In  the 
chiasma  the  fibres  from  the  nasal  or  internal  half  of  each  retina  cross  to 
the  optic  tract  of  the  opposite  side.  The  optic  tracts  may  be  compared  to- 
a  pair  of  flattened  bundles  of  nerve-fibres  which  embrace  the  peduncles  of 
the  brain  on  each  side  from  a  point  a  little  way  behind  the  optic  chiasma, 
somewhat  as  the  reins  of  a  bridle  in  the  hands  of  a  rider  embrace  the 
sides  of  the  horse's  neck.  The  optic  tracts  pass  obliquely  upward  and 
backward  on  each  side,  and  send  fibres  to  the  internal  and  external  genicu- 
late  bodies,  corpus  subthalamicum,  pulvinar,  optic  thalamus,  and  the  upper 
side  of  the  anterior  quadrigeminal  body ;  also  through  the  anterior  and 
posterior  brachia  of  the  quadrigeminal  bodies,  and  by  way  of  the  latter 
and  the  optic  tract  itself  to  the  internal  capsule,  along  the  optic  radiation 
of  the  cerebral  hemispheres  to  the  visual  cortex  of  the  occipital  lobes.  It 
will  therefore  readily  be  understood  that  destruction  or  failure  of  develop- 


60  DEVELOPMENT   OF   THE    EYE. 

ment  of  the  internal  bundles  of  the  optic  tract  must  lead  to  the  partial 
blindness  known  as  hemianopia. 

Experimentation  in  relation  to  the  development  of  the  optic  tracts  has 
yielded  results  of  great  interest  and  of  some  clinical  value.  Removal  of 
the  eyeball  and  section  of  the  optic  nerve  result  in  degeneration  of  the 
optic  tract.  If  the  removal  is  carried  out  witfy  a  young  animal,  this  cen- 
tripetal degeneration  of  the  nerve-fibres  of  the  tract  of  one  side  is,  however, 
not  the  only  result,  since  it  is  found  that  the  external  geniculate  body,  the 
pulvinar,  and  the  anterior  quadrigeminal  bodies  do  not  undergo  complete 
development.  The  trophic  centre  which  controls  the  development  of  these 
deep-lying  ganglionic  parts  in  a  young  animal  is  therefore  to  be  sought  in 
the  ganglionic  cells  of  the  retina,  or  in  an  extremely  peripheral  sense-organ. 
The  retina,  however,  in  spite  of  its  extremely  peripheral  position,  as  shown 
by  its  development  traced  in  the  earlier  part  of  this  article,  was  primitively 
a  part  of  the  cerebral  cortex  which  has  been  evaginated  upon  the  optic 
stalk  and  greatly  modified  and  specialized. 

The  converse  experiment,  namely,  the  extirpation  or  destruction  of  the 
temporal  portion  of  the  cerebral  cortex  in  newly-born  animals,  leads  to  the 
imperfect  development  of  the  internal  geniculate  bodies  and  portions  of  the 
posterior  corpora  quadrigemina.  A  most  interesting  and  promising  field 
of  investigation  for  students  of  the  development  of  the  eye  has  long  since 
suggested  itself  to  the  writer, — namely,  a  study  of  the  optic  tracts  of  our 
common  native  American  rodents,  shrews  and  moles.  I  should  expect  to 
find,  a  priori,  that  in  the  shrews  and  moles  these  tracts  would  be  very  defec- 
tively developed,  since  their  eyes  are  so  minute — little  over  half  a  millimetre 
in  diameter  in  the  adult — that  they  can  be  of  very  little  functional  use. 
Here  we  might  look  for  a  defective  development  of  the  occipital  visual 
cortex,  the  geniculate  bodies,  pulvinar,  thalamus,  etc.  Here  Nature  has 
made  the  necessary  preliminary  experiment  for  us  in  almost  extirpating  the 
eye,  and,  inasmuch  as  these  types  also  present  us  with  one  of  the  simplest 
forms  of  the  mammalian  brain,  valuable  results  might  be  anticipated  from 
such  an  investigation.  Whether  destruction  of  the  occipital  cortex  in  very 
young  animals  will  produce  degeneration  of  the  geniculate  bodies,  pulvinar, 
and  anterior  quadrigeminal  body  does  not  seem  to  have  been  decided  by 
experiment. 

The  following  account  of  the  development  of  the  optic  tract  and  its 
connections  with  the  corpus  .subthalamicum,  thalamus,  pulvinar,  geniculate 
bodies,  and  corpora  quadrigemina  is  based  upon  the  researches  of  Bern- 
heimer,  the  illustrations  to  whose  memoir  have  been  reproduced  here  by 
photogravure  process,  but  without  the  advantage  of  the  colors  of  the 
originals.  His  results,  as  will  be  seen  from  the  appended  summary  of  his 
conclusions,  have  been  got  by  tracing  the  development  of  the  medullary 
sheaths  of  the  nerve-fibres  which  traverse  the  intervals  between  the  struc- 
tures named.  They  are  necessarily  based  upon  sections  of  the  region  of 
the  optic  tract,  the  materials  being  for  the  most  part  derived  from  advanced 


FIG.  44. 

Oblique  longitudinal  section  through  the  external  geniculate  body  and  base  of  optic 
tract  of  a  human  embryo  of  36-38  weeks.  Enlarged.  (After  Bernheimer.) 

The  upper  free  margin  of  the  figure  corresponds  to  the  infero-external  angle  of  the 
external  geniculate  body. — <S  T.F.,  superficial  tangential  fibres  and  parts  of  fibres  on  the 
free  outer  border  of  the  geniculate  body;  R.F.,  radially  arranged  fibres,  arising  from  a 
root  composed  of  fan-shaped  bundles;  S.F.,  superficially  coursing  fibres;  Ggn.,  ganglionic 
nidulus;  Bv.,  section  through  blood-vessels;  C.g.ex.,  corpus  geniculatum  externum  ;  Tr., 
tractus. 

NOTE.  — The  piece  from  which  the  above  section  was  cut  was  so  embedded  that  the 
plane  of  section  passed  in  a  direction  coincident  with  the  long  axis  of  the  tractus  and 
through  the  oblique  long  axis  of  the  corpus  geniculatum  externum,  in  such  a  way,  how- 
ever, that  the  first  section  was  cut  off  external  to  the  outer  surface  of  the  external  genicu- 
late body ;  the  following  sections,  therefore,  subdivided  the  geniculate  body  from  without 
inward  and  from  above  downward.  The  plane  of  section,  in  other  words,  converged  an- 
teriorly inward  toward  the  median  line,  and  also  inward  above  toward  the  median  line, 
and  therefore  departed  somewhat,  in  a  double  sense,  from  a  plane  parallel  to  a  true  median 
one. 


FIG.  44. 


R.F. 


a 


DEVELOPMENT   OF   THE   EYE.  61 

foetuses.  Figs.  44,  45,  and  46  illustrate  the  development  of  the  connec- 
tions of  the  optic  tract. 

Investigation  of  the  region  of  the  outer  geniculate  body  and  optic  tract 
in  embryos,  mature  fetuses,  children,  and  adults  shows  that  the  fibres  of 
the  tractus  have  a  double  origin  from  the  external  geniculate  body.  First, 
fibres  proceed  from  various  points  on  the  surface  of  the  geniculate  gan- 
glion, mostly  from  above  and  below  outward,  pass  thence  from  without 
inward  obliquely,  and  radiate  into  the  tractus  as  a  whole,  and  after  manifold 
decussation.  Secondly,  all  the  much  more  numerous  remaining  fibres  arise 
from  the  inner  layers  of  the  external  geniculate  ganglion,  in  the  form  of  a 
group  of  fan-shaped  radiating  bundles,  and  pass  to  the  tractus  in  a  some- 
what oblique  longitudinal  plane.  These  fibres  also  pass  from  the  external 
geniculate  body  to  the  beginning  of  the  tractus  in  a  slightly  convergent 
direction.  They  are  to  be  met  with  most  distinctly  within  the  geniculate 
body  itself.  These  two  sets  of  fibres  show  undoubted  evidence  of  union 
with  axis-cylinders  and  processes  of  ganglion-cells  that  decussate  in  every 
direction  in  the  external  geniculate  body.  Fibres  that  simply  passed 
through  the  geniculate  ganglion  without  ending  within  it  could  not  be 
demonstrated.  The  external  geniculate  body  is  therefore  to  be  regarded  as 
the  true  ganglion  of  origin  of  a  great  part  of  the  fibres  of  the  tractus. 

In  embryos  of  from  sixteen  to  twenty  weeks,  none  of  these  fibres  have  yet 
developed  their  medullary  sheaths.  The  first  evidence  of  the  development 
of  the  medullary  sheaths  is  seen  in  embryos  of  from  twenty  to  twenty-two 
weeks,  in  the  form  of  delicate  thickenings  which  extend  for  a  greater  or  less 
distance  along,  the  fibres,  though  this  is  visible  only  in  the  fibres  of  deeper 
origin  that  arise  in  the  form  of  the  fan-shaped  bundles.  The  fibres  of 
superficial  or  external  origin  begin  to  develop  their  medullary  sheaths  after 
the  twenty-eighth  week  of  uterine  existence,  and  after  the  deeper  or  first 
group  of  fibres  has  already  developed  a  distinct,  delicate,  though  by  no 
means  complete  medullary  investment.  In  foetuses  that  are  mature  or 
nearly  so,  the  medullary  sheaths  of  both  sets  are  completely  developed, 
but  they  are  still  very  thin,  and  the  fibres  appear  distinctly  isolated. 

The  same  condition  of  affairs  is  seen  in  the  brains  of  infants  several 
weeks  old ;  but  in  children  several  years  old  and  in  adults  a  change  comes 
about,  in  consequence  of  the  increase  in  the  thickness  of  the  medullary 
sheaths  of  the  fibres  causing  the  spaces  between  the  individual  fibres  and 
bundles  to  become  less  evident.  Single  fibres  can  therefore  no  longer,  in 
the  late  stages,  be  distinguished  and  traced  to  their  sources  of  origin. 

The  optic  tract  receives  a  strong  accession  of  fibres  from  the  so-called 
corpus  Luys  or  corpus  subthalamicum.  The  larger  part  proceeds  directly 
into  the  tractus  after  passing  over  the  intermediate  portion  of  the  pedun- 
cles. The  other  portion  takes  a  longer  route,  through,  over,  and  around  the 
internal  geniculate  body,  to  the  tractus.  Fibres  also  arise  from  the  inner 
geniculate  body,  at  the  beginning  of  the  tractus,  with  a  so-called  short  root, 
without,  however,  radiating  to  the  tractus  in  the  form  of  bundles ;  these 


62  DEVELOPMENT   OF   THE    EYE. 

fibres  originate  and  pass  on  singly  to  their  destinations.  The  tractus  also 
receives  fibres  that  arise  singly  from  the  whole  of  the  external  surface  of 
the  internal  geniculate  body,  with  long  or  short  roots,  according  as  they 
originate  farther  from  or  nearer  to  the  tractus. 

From  the  thalamus  fibres  arise,  as  is  already  known,  which  may  be 
distinguished  as  superficial  and  deep. 

The  deep  root  arises  from  the  gray  substance  of  the  thalamus  in  the 
form  of  long  and  short  fibres  :  the  union  of  the  fibres  with  ganglion-cells 
of  the  thalamus  has  been  traced  especially  in  respect  to  the  latter ;  they 
pass  below  and  between  the  geniculate  bodies,  and  are  best  seen  in  the 
newly-born  subject. 

The  superficial  root  of  the  thalamus  arises  from  the  ganglion-cells  that 
are  scattered  in  the  cortical  layers  of  the  pulvinar  (stratum  zonale).  They 
arise  here  and  form  a  very  dense  and  delicate  plexus  of  fibres ;  fine  fibrils 
of  this  plexus  may  be  seen  with  axis-cylinder  processes  passing  between 
the  ganglion-cells. 

Through  this  plexus  fibres  pass  that  do  not  join  it.  They  may  be  dis- 
tinctly traced  to  the  tractus ;  but  it  cannot  be  definitely  stated  whence  they 
originate.  They  do  not  arise  from  the  basal  ganglia ;  they  are  distinguished 
by  their  somewhat  greater  thickness  and  their  more  prolonged  course.  The 
development  of  their  medullary  sheaths  is  also  characteristic.  The  suppo- 
sition has  been  expressed  that  they  are  perhaps  optic  fibres  that  connect 
some  of  the  cells  of  the  retina  directly  with  cells  of  the  cortex,  and  they 
have  been  compared  to  the  commissural  fibres  of  the  corpus  callosum. 
Their  physiological  significance  cannot  be  discussed  here. 

None  of  these  fibres  give  evidence  of  a  developed  medullary  sheath  in 
embryos  of  the  fourteenth  to  the  sixteenth  week.  The  development  of 
medullary  sheaths  begins,  as  already  stated,  in  embryos  of  from  twenty  to 
twenty-two  weeks,  most  distinctly  in  the  radiation  from  the  corpus  Luys 
or  corpus  subthalamicum,  then  in  the  fibres  from  the  internal  geniculate 
body  and  the  deep  root  from  the  thalamus,  and  not  at  all  in  the  fibres  from 
the  stratum  zonale. 

While  all  the  bundles  of  fibres  in  embryos  of  the  thirtieth  week  show 
the  development  of  medullary  sheaths,  those  proceeding  from  the  stratum 
zonale  are  only  beginning  to  develop  sheaths.,  The  so-called  commissural 
fibres  are  not  yet  visible. 

In  embryos  of  the  thirty-fourth  to  the  thirty-sixth  week  the  medul- 
lary sheaths  of  all  the  bundles  of  fibres  just  mentioned  are  apparent ;  those 
covering  the  fibres  from  the  corpus  subthalamicum  are,  however,  the  most 
distinctly  developed.  The  superficial  commissural  fibres  are  not  yet  covered 
with  a  medullary  sheath.  These  fibres  are  first  distinctly  developed  in  the 
brains  of  children  several  weeks  old. 

In  the  quadrigeminal  region  of  the  brain  in  embryos  of  from  fourteen 
to  sixteen  weeks  the  nerve-fibres  are  still  entirely  without  medullary  sheaths. 
The  first  indications  of  medullary  sheaths  become  apparent  at  the  twentieth 


FIG.  45. 


Horizontal  section  through  the  optic  thalamus  and  deeper  parts  of  the  optic  tract  of  a  34-36  weeks 
human  embryo.  (Enlarged,  after  Bernheimer.) 

The  section  cuts  through  the  optic  thalamus,  corpus  subthalamicum  (or  corpus  Luys),  and  tractus. 
The  transverse  bundles  from  the  thalamus  plainly  converge  toward  the  tractus.  Bv.,  sections  through 
blood-vessels.  The  section  passes  in  an  almost  horizontal  plane,  parallel  to  the  longitudinal  fibres  of 
the  tract,  cutting  the  internal  geniculate  body  tangentially,  and  so  as  to  carry  away  a  segment  of  the 
thalamus. 


DEVELOPMENT  OF   THE   EYE.  63 

to  the  twenty-second  week,  but  exclusively  in  the  region  of  the  deeper 
bundles  from  the  gray  layer  of  both  pairs  of  the  corpora  quadrigemina. 
Embryos  of  the  thirtieth  to  the  thirty-second  week  show  a  development  of 
the  medullary  sheaths  of  the  fibres  of  the  stratum  zonale  for  the  first  time. 
It  would  therefore  seem  that  the  appearance  of  the  medullary  sheaths  of  the 
deeper  nerve-bundles  that  pass  to  the  optic  tract  precedes  that  of  the  medul- 
lary sheaths  of  the  more  superficial  bundles,  on  an  average,  by  from  four 
to  eight  weeks.  By  the  thirty-fourth  to  the  thirty-sixth  week  all  the  fibres 
from  the  quadrigeminal  region  have  acquired  medullary  sheaths. 

There  can  be  no  doubt  that  the  corpora  quadrigemina  are  far  less  im- 
portant as  centres  of  origin  of  optic  fibres  than  are  the  ganglionic  masses 
previously  discussed.  The  importance  of  the  corpora  quadrigemina  has 
been,  in  fact,  as  much  overrated  as  the  thalami  and  geniculate  bodies  have 
been  underrated  in  this  connection.  The  division  into  two  bundles  of  the 
fibres  that  in  the  adult  pass  from  the  anterior  pair  of  quadrigeminal  bodies 
by  way  of  the  anterior  brachium  to  the  optic  tract  is  not  to  be  discovered 
in  the  brains  of  immature  foetuses  or  of  recently-born  children.  It  appears 
that  not  a  very  great  many  fibres  can  be  traced  as  a  radiation  in  part  from 
the  tractus  to  the  surface  of  the  anterior  quadrigeminal  bodies  and  in  part 
to  the  deeper-lying  gray  substance  of  the  latter.  A  superficial  net-work 
of  quadrigeminal  fibres,  forming  a  plexus  somewhat  similar  to  that  seen  at 
the  posterior  end  of  the  thalamus  (stratum  zonale),  could  not  be  traced  to 
ganglion-cells,  nor  could  the  centrifugal  terminations  of  the  fibres  be  satis- 
factorily made  out.  That  these  fibres  pass  into  the  optic  tract  cannot  be 
stated  as  certain  :  all  that  can  be  said  is  that  some  of  them  converge  toward 
the  tract.  Of  the  fibres  that  originate  from  the  gray  substance  of  the 
anterior  quadrigeminal  bodies  some  undoubtedly  pass  into  the  tract,  though 
it  may  be  said  in  a  general  way  that,  in  comparison  with  those  arising  from 
the  corpus  subthalamicum  (corpus  Luys),  the  external  geniculate  body,  and 
the  deep  root  of  the  thalamus,  they  are  very  few  in  number. 

The  relations  of  the  posterior  quadrigeminal  bodies  to  the  optic  tract 
appear  to  be  somewhat  similar  to  those  of  the  anterior,  and  the  fibres  that 
pass  from  them  to  the  optic  tract  seem  to  be  few  in  number.  There  are 
probably  fibres  that  pass  by  way  of  the  posterior  brachium  over  the  internal 
geniculate  body  to  the  tractus. 

It  may  be  added  here  that  the  cerebral  hemispheres  are  produced  from 
the  extreme  anterior  portion  of  the  dorsal  zone  of  His,  so  that  the  basal 
ganglion  of  the  latter,  the  corpus  striatum,  may  be  said  to  originate  above 
the  sulcus  of  Monro.  In  the  region  of  the  mid- brain  of  the  embryo  the  optic 
thalami  are  developed  as  thickenings  of  the  dorsal  zones  of  His.  The  thick- 
enings that  lead  to  the  development  of  the  thalami,  and  presumably  to  the 
development  also  of  the  pulvinar  and  corpus  subthalamicum,  are  continuous 
anteriorly  with  the  thickenings  that  develop  into  the  corpora  striata  of  the 
hemispheres.  The  sulci  of  Monro  persist  in  the  aqueduct  or  mid-brain, 
and  it  may  be  said  that  even  in  the  adult  the  parts  of  the  corpora  quadri- 


64  DEVELOPMENT   OF   THE    EYE. 

gemina  related  to  the  innervation  of  the  optic  tract  lie  above  those  sulci. 
The  probabilities  are  therefore  greatly  in  favor  of  the  view  that  the  major 
portions  of  the  ganglionic  centres,  here  as  elsewhere, — viz.,  the  corpora 
quadrigemina  and  the  geniculate  bodies, — have  also  arisen  in  the  embryo 
from  the  dorsal  zone  of  His. 

DEVELOPMENT  OF  THE   MUSCLES  OF   THE   EYEBALL,  OE  THE 
ORBITAL  MUSCLES. 

The  development  of  the  muscles  that  move  the  eyeball  has  not  been 
genetically  traced  in  the  mammalian  embryo.  Since  the  acceptance  of  the 
doctrine  of  descent,  it  is,  however,  generally  admitted  that  the  less  modi- 
fied and  abridged  method  of  development  of  organs  seen  in  the  simpler 
fish-like  vertebrates  must  be  the  primitive  one.  This  primitive  method 
of  development  in  the  lower  forms  often  gives  the  clue  to  an  understanding 
of  the  more  complex  and  obscure  processes  of  the  embryonic  growth  of 
homologous  parts  in  the  higher  types.  Since  the  orbital  muscles  are  homol- 
ogous throughout  almost  all  the  classes  of  vertebrates,  and  since  the  primi- 
tive segments  or  somites  of  the  embryos  of  vertebrates  generally,  from 
which  they  are  derived,  are  serially  homologous,  I  have  not  hesitated  to 
utilize  the  knowledge  gained  in  regard  to  the  history  of  those  muscles 
through  a  study  of  their  development  in  the  lower  forms. 

All  the  voluntary  muscles  of  the  trunk  and  limbs  of  vertebrates  are 
genetically  derived  from  the  inner  stratum  of  the  so-called  myotomes  or 
muscle-plates  that  are  parts  of  the  paired  blocks  of  mesoderm,  sometimes 
called  protovertebrse,  well  seen  in  the  embryo  bird  of  the  second  or  the 
embryo  rabbit  of  the  eighth  day.  Embryological  investigation  has  shown 
that  the  orbital  muscles  are  no  exception  to  this  rule,  and  that,  although  it 
is  difficult  or  perhaps  impossible  to  trace  such  a  genetic  connection  in  the 
higher  vertebrates,  such  a  connection  in  all  probability  originally  existed. 
Curiously  enough,  the  representatives  of  the  somites,  muscle-plates,  or  myo- 
tomes of  the  head  reach  their  fullest  development  only  in  the  shark-like 
vertebrates,  where  they  exist  temporarily  in  the  very  young  embryos,  in 
which  they  were  first  discovered  by  Balfour  and  called  by  him  the  head- 
cavities.  The  cephalic  myotomes  from  which  the  orbital  muscles  are  known 
to  be  derived  are  parts  of  these  head-cavities.  The  head-cavities  are  em- 
bedded in  the  indifferent  mesoderm  of  the  head,  and  undergo  a  most  com- 
plex series  of  transformations  in  the  course  of  the  metamorphosis  of  por- 
tions of  their  walls  into  the  orbital  muscles. 

The  accompanying  Fig.  47  will  give  a  fairly  good  idea  of  the  relations 
of  the  mesoderm  of  the  head  to  the  optic  cup  in  an  advanced  stage  of 
development  in  the  bird.  The  optic  nerve,  II,  is  seen  to  pass  from  the 
base  of  the  optic  cup  to  the  optjc  chiasma,  crossing  below  the  optic  lobes 
and  the  infundibulum,  V.  Two  dark  bodies  above  and  below  the  optic 
nerve  represent  portions  of  the  orbital  muscles,  the  recti,  that  have  been 
cut  through  in  place.  Two  nerves,  the  oculo-motor,  oc  and  n,  are  also 


F.  p.  a.  gen.  corp.  gen.  int. 


FIG.  46. 


"-.         ~\      '-"c    •"•'•' 

'  •     ^     .     .  '-. 

-v  v     •  *    ^ 


;*        X  .        >"      ^'"      ' 


Horizontal  section  through  the  thalamus,  inner  geniculate  body,  and  optic  tract  of  a  34-36  weeks 
human  embryo.  (Enlarged,  after  Bernheimer.) 

The  section  passes  through  the  thalamus,  corpus  subthalamicum,  internal  geniculate  body,  and 
tract.  It  is  nearly  on  the  same  plane  as  the  preceding,  Fig.  15,  except  that  it  cuts  the  thalamus  at  a 
point  lower  down.—/1,  p.  a.  gen.,  fibres  of  the  posterior  angle  of  the  internal  geniculate  body;  Bv., 
blood-vessels;  TV.,  tractus. 


DEVELOPMENT   OF   THE   EYE. 


65 


shown  cut  across.     At  the  upper  inner  side  of  the  optic  cup  a  blood- 
vessel has  been  cut  across.     The  important  points  to  notice  are  the  direc- 
tions which  these  muscles  and  nerves  are  taking, 
and  the  fact  that  they  are  surrounded  on  all  sides  FIG.  47. 

by  an  indifferent  matrix  of  mesoderm  with  very 
few  blood-vessels  or  lymph-spaces.  In  fact, 
at  this  stage  of  development  in  the  chick,  as 
well  as  in  the  mammalian  embryo,  there  are  as 
yet  no  capillaries.  All  the  vessels  now  present 
represent  in  reality  what  are  to  become,  gener- 
ally speaking,  much  larger  trunks ;  the  perma- 
nent capillary  circulation  of  the  adult  still  re- 
mains to  be  developed.  Into  this  mesodermal 
matrix  the  rudiments  of  the  orbital  muscles, 
nerves,  and  vessels  grow  and  make  their  way 
at  first  as  solid  cellular  processes  or  outgrowths. 
The  discussion  of  the  development  of  the  orbital 
muscles  may  now  be  entered  upon. 

It  may  be  well,  however,  to  note  that  the 
retractor  bulbi  muscle,  so  well  developed  in  the 
lower  mammalia  (ungulates),  is  wanting  in  man. 
This  hollow,  conical  muscle  has  not,  moreover, 
been  genetically  traced  to  the  "  head-cavities," 
though  it  is  not  improbable  that  it,  together 
with  several  other  small  intracrauial  muscles, 
may  be  found  ultimately  to  have  such  a  genetic 
history. 

A  very  thorough  study  of  the  development 
of  the  muscles  that  move  the  eyeball  has  been 
carried  out  by  Miss  Julia  B.  Platt  upon  a  shark 
(Acanthias  vulgar  is).  These  studies  enable  us 
to  indicate  at  least  what  were  the  primitive  con- 
ditions that  attended  the  development  of  these 
muscles,  though  it  is  probable  that  the  process 

has  been  greatly  abbreviated  and  obscured  in  the  case  of  the  higher  verte- 
brates, especially  birds  and  mammals.  In  fact,  in  these  forms  no  satisfac- 
tory studies  of  consecutive  stages  have  yet  been  made. 

It  appears  from  the  work  of  the  author  cited  that  there  are  on  either 
side  of  the  head  of  the  embryo  of  Acanthias  four  "  head-cavities,"  as  the 
spaces  first  discovered  by  Balfour  in  the  heads  of  embryo  sharks  are  called. 
These  paired  cavities  unquestionably  represent  spaces  within  the  head  which 
are  serially  homologous  with  the  mesodermal  somites  of  the  body  farther 
back  in  the  trunk  of  the  embryo.  They  are  structures  that  are  evidently 
undergoing  retrogressive  development,  since  they  do  not  persist  to  adult 
life  as  cavities,  but  undergo  a  complicated  series  of  transformations  which 
VOL.  I.— 5 


Transverse  section  through  the 
head  of  a  chick  embryo  at  the  end 
of  the  sixth  day.  Enlarged  11 
times.  (Reduced,  from  Duval.)-^ 
Fj,  posterior  part  of  corpora  bi- 
gemina,  or  second  cerebral  vesicle, 
FI,  ventral  and  infundibular  por- 
tion of  first  cerebral  vesicle,  from 
the  ventral  side  of  which  the 
optic  nerves,  II,  pass  outward  to 
the  eye ;  oc,  oculo-motor  nerve,  cut 
through  obliquely;  n,  ophthalmic 
branch  of  fifth  nerve  (the  darker 
oblong  bodies  represent  oblique 
sections  through  the  superior  and 
inferior  recti  muscles);  o,  olfactory 
pit.  The  retina  and  the  lens  are 
formed,  and  a  large  vitreous  space 
is  now  developed,  but  there  are  no 
vessels  yet  apparent  in  it. 


66 


DEVELOPMENT   OF   THE   EYE. 


FIG.  48. 


will   be   best    understood   by   reference   to   the   accompanying   series   of 
figures. 

It  appears  that  two  (a  and  1)  of  the  four  pairs  of  head-cavities  in  the 
embryos  of  sharks  (Fig.  48)  are  premandibular  in  position.  One  pair — the 
third  (2) — is  closely  associated  with  the  mandible,  and  is  spoken  of  as  the 
mandibular  cavity.  The  fourth  pair  (3)  is  behind  the  mandibular.  The 
relations  of  these  four  structures  can  very  easily  be  understood  from  the 

accompanying  figures,  which  also 
elucidate  the  relations  of  these  struc- 
tures to  the  development  of  the 
muscles  of  the  eyeball.  These  mus- 
cles are,  in  fact,  differentiated  from 
parts  of  the  walls  of  these  cavities, 
and  in  the  course  of  this  process  the 
walls  of  the  cavities  are  pushed  out 
at  definite  points  and  in  definite 
directions  as  prolongations,  which 
are  variously  and  appropriately  bent 
about  the  eyeball  toward  its  grow- 
ing mesodermal  coat  (sclerotic),  to 
which  they  eventually  become  affixed 
by  their  distal  ends  as  a  motor  ap- 
paratus. In  the  very  earliest  stages 
of  the  development  of  these  cavities 
they  are  present  as  four  spaces  on 
either  side  of  the  head,  ranged  some- 
what in  conformity  with  the  curva- 
ture of  the  cranial  flexure  of  the 
head  of  the  embryo,  and  at  first 
appear  as  globular  or  oval  spaces, 
with  no  processes  growing  from  their  walls.  The  first  of  the  cavities  to 
throw  out  a  process  is  cavity  2  of  Fig.  49,  or  the  mandibular.  This 
process  is  extended  backward  and  downward,  and  gives  rise  to  a  man- 
dibular muscle  which  afterward  degenerates. 

The  further  steps  by  which  the  eye-muscles  are  evolved  are  illustrated 
by  the  remaining  figures.  It  appears  that  the  premandibular  cavity,  or 
that  marked  1  in  the  figures,  grows  out  into  a  concavo-convex  plate  some- 
what conformably  to  the  inner  convexity  of  the  eyeball.  From  it  then 
arise  as  outgrowths  four  of  the  eye-muscles.  These  are  the  superior,  in- 
ferior, and  internal  recti  muscles,  and  the  inferior  oblique  (see  Figs.  50  and 
51).  The  superior  and  internal  recti  arise  from  its  anterior  extremity, 
while  the  inferior  rectus  and  inferior  oblique  arise  as  outgrowths  from  the 
postero-inferior  angle  of  the  cavity.  From  the  third  or  mandibular  cavity, 
marked  2  in  the  figures,  and  as  an  anterior  outgrowth  that  bends  toward 
the  eyeball,  the  superior  oblique  muscle  arises.  From  the  fourth  and  last 


Th 


Side  of  part  of  head  of  an  Acanthias  embryo 
6  millimetres  long,  showing  the  region  of  the 
cranial  flexure  as  a  transparent  object,  with 
the  four  head-cavities,  a,  1,  2,  3,  reconstructed 
from  serial  sections.  These  are  shown  in  their 
primitive  relation  to  the  eye,  opt,  lying  in  a  row 
behind  and  above  the  latter.  IV,  V,  VII,  cranial 
nerves ;  Th,  thalamic  nerve ;  eg,  rudimentary  cil- 
iary ganglion ;  ch,  anterior  end  of  notochord ;  and, 
auditory  vesicle;  sp,  spiracular  cleft.  (Enlarged, 
after  Miss  Platt.) 


DEVELOPMENT   OP   THE   EYE. 


67 


head-cavity,  marked  3  in  the  figures,  the  external  rectus  is  formed,  about 
all  of  the  blastema  of  this  cavity  being  consumed  in  the  development  of 


FIG.  49. 


hr. 


opt 


The  four  anterior  head-cavities  of  an  embryo  of  Acanthias  12  millimetres  long,  more  advanced 
in  their  development  than  in  the  preceding  figure. — III  to  VII,  cranial  nerves;  a,  1,  2,  3,  the  head- 
cavities  reconstructed;  mandibular  cavity  2  gives  rise  to  a  strong  process  extending  backward,  des- 
tined to  form  the  transient  maudibular  muscle ;  I",  part  of  the  premandibular  head-cavity,  1,  that 
gives  rise  to  the  inferior  oblique  muscle ;  opt,  outline  of  upper  part  of  eyeball ;  prof,  ramus  profundua 
ophthalmicus  trigemini ;  eg,  ciliary  ganglion ;  vbr,  floor  of  brain  in  region  of  cranial  flexure ;  ch,  ante- 
rior end  of  notochord ;  Van,  branch  from  fifth,  finally  anastomosing  withtrochlearis ;  prof*,  branch  from 
profundus  opththalmicus  toward  anterior  head-cavity.  (Enlarged,  after  Miss  Platt.) 

this  muscle.     The  rudimentary  muscle  developed  as  a  backward  extension 
of  the  mandibular  cavity,  close  to  the  external  rectus,  afterward  degener- 

FIG.  50. 


'-cpt. 


The  four  head-cavities  of  an  embryo  of  Acanthias  16  millimetres  long  and  advanced  beyond  the 
condition  shown  in  the  preceding  figure.  Only  the  anterior  part  of  cavity  2  is  now  shown ;  mandibu- 
lar cavity  2  is  growing  forward  anteriorly  at  2»°  into  the  rudiment  of  the  superior  oblique  muscle ;  pre- 
mandibular cavity  l  is  differentiating  into  the  inferior  oblique  posteriorly  at  1'°.  Other  letters  as 
before.  Oph.  sup.,  superior  ophthalmic  branch  of  seventh  cranial  nerve.  (Enlarged,  after  Miss  Platt.) 

ates,  and  is  lost  in  indiiferent  mesoderm.     The  first  cavity,  or  that  marked 
a  in  the  figures,  also  appears  to  degenerate  into  indiiferent  mesoderm. 


68 


DEVELOPMENT   OF   THE   EYE. 


Miss  Plata's  studies  also  disclosed  the  following  facts.  The  muscle-cells 
of  the  external  rectus  first  appear  in  the  median  wall  of  cavity  3,  and  they 
pass  from  this  wall  into  the  cavity,  ultimately  filling  it.  The  muscular 
tissue  also  first  appears  in  the  median  or  internal  wall  of  the  mandibular 
cavity  (2)  which  is  to  form  the  superior  oblique  and  the  rudimentary  jaw 
muscle.  The  history  of  the  premandibular  cavity  is  more  complex.  It 
appears  to  be  formed  by  the  fusion  of  a  pair  of  lateral  cavities  with  a 


FIG.  61. 


VI- 


-sup.  obi. 


infrec 


Still  more  advanced  condition  of  the  head-cavities  of  an  embryo  of  Acanthias,  from  the  side,  as 
before.— Premandibular  cavity  1  is  giving  rise  to  the  rudiments  of  the  inferior  oblique,  in/,  obi. ;  inferior, 
superior,  and  internal  recti,  inf.  rec.,  sup.  rec.,  and  int.  rec. ;  mandibular  cavity  2  is  showing  the  extension 
of  the  superior  oblique,  sup.  obi.,  still  more  prominently  than  in  the  preceding  figure;  cavity  3  is  ex- 
tending as  the  rudiment  of  the  external  rectus.  Other  letters  as  before.  (Enlarged,  after  Miss  Platt.) 

median  space,  which  is  generally  supposed  to  have  a  morphological  value 
widely  different  from  that  of  the  two  cavities  which  it  unites.  The  four 
premandibular  eye-muscles  are  said  by  Miss  Platt  to  arise  from  the  dorsal 
wall  of  the  lateral  portions  of  the  premandibular  cavity,  approximating 
closely,  in  their  place  of  origin,  the  line  of  fusion  between  the  paired 
cavities  and  the  central  space. 

The  transformations  attending  the  innervation  of  the  external  rectus 
by  the  sixth  or  abducens  nerve  can  be  very  satisfactorily  traced  from  the 
accompanying  figures,  as  well  as  the  manner  in  which  the  relations  of  the 
third  or  oculo-motor  nerve  to  the  inferior  rectus,  inferior  oblique,  and 
superior  rectus  are  brought  about  in  the  progress  of  development.  The 
development  of  the  relations  of  the  fourth  or  trochlearis  to  the  superior 
oblique  muscle  is  also  shown,  as  well  as  its  mode  of  anastomosis  with 
the  trigeminus  or  fifth  nerve  at  Van.  It  also  appears  that  the  troch- 
learis and  the  trigeminus  have  at  first  a  common  origin,  but  soon  become 
divided. 

These  figures,  including  the  last,  Fig.  52,  enable  one  to  trace  very  satis- 


DEVELOPMENT   OF   THE   EYE.  69 

factorily  the  history  of  essentially  important  orbital  muscles,  which,  as  will 
be  seen  from  the  last  stage  represented,  present  a  strong  analogy  to  those  of 
the  human  subject  in  their  mode  of  arrangement.  The  wonderful  manner 
in  which  these  outgrowths  from  the  head-cavities  grope  their  way  through 
the  mesoderm  and  embrace  the  eyeball,  seeking,  so  to  speak,  for  their  proper 
points  of  insertion  upon  it,  as  shown  in  Fig.  51,  is,  to  say  the  least,  a  most 

FIG.  62. 


extrec. 


The  transformation  of  the  head-cavities  nearly  completed  in  an  embryo  of  Acanthias  55  milli- 
metres long. — Cavity  1  has  given  rise  to  inferior  oblique,  inferior  rectus,  interior  and  superior  recti ; 
cavity  2  has  given  rise  to  superior  oblique,  and  cavity  3  to  external  rectus.  The  mode  in  which  the 
innervation  of  these  muscles  is  achieved  by  III,  TV,  and  VI,  pairs  of  cranial  nerves,  is  also  shown. 
The  eyeball,  opt,  is  supposed  to  be  transparent,  and  all  the  foregoing  structures  lie  behind  or  beneath 
it.  II  indicates  the  position  of  the  point  of  union  of  the  optic  nerve  with  the  eyeball.  (Enlarged,  after 
Miss  Platt.) 

remarkable  fact.  How  these  parts,  as  well  as  all  others  in  the  embryo,  are 
guided  to  grow  to  just  the  right  proportions  in  respect  to  one  another,  how 
they  are  guided  in  the  course  of  their  extension  through  its  mesodermal 
tissues  to  exactly  the  right  places,  must  ever  remain  an  unsolved  problem. 
We  may  say  that  the  hereditary  tendencies  of  the  germinal  matter  of  the 
embryo  determine  these  processes,  and  that  this  germinal  matter  is  trans- 
mitted as  an  actual,  visible  substance  from  generation  to  generation  con- 
tinuously. This  manner  of  disposing  of  the  questions  raised  is,  however, 
unsatisfactory,  since  it  is  not  really  an  explanation.  It  is  too  general  a 
statement.  When  it  is  attempted  to  particularize,  as  has  recently  been 
done  by  an  eminent  biologist,  we  become  hopelessly  involved  in  a  maze 
of  speculation. 

The  development  of  the  muscles  of  the  face,  including  the  orbicularis, 
has  been  traced  by  Huge  to  the  platysma  myoides  (musculus  subcutaneus 
colli),  which  is  to  be  regarded  as  a  part  of  a  muscle  which  in  the  ancestors 


70  DEVELOPMENT   OF   THE   EYE. 

of  mammals  extended  forward  from  the  neck  over  the  face,  where  the 
whole  have  been  innervated  by  the  facial  or  seventh  nerve.  Nothing  is 
known  of  the  development  of  the  tensor  tarsi  (Horner's  muscle)  at  the 
inner  angle  of  the  orbit.  This  muscle,  in  common  with  the  orbicularis 
palpebrarum,  corrugator  supercilii,  etc.,  is  innervated  by  the  facialis,  and 
its  development  must  therefore  be  associated  with  that  nerve  and  the  pla- 
tysma  myoides  muscle. 


THE  ANATOMY  OF  THE  ORBIT  AND 
THE  APPENDAGES  OF  THE  EYE. 

BY  THOMAS  DWIGHT,   M.D.,   LL.D., 

Parkman  Professor  of  Anatomy  at  Harvard  University,  Cambridge,  Massachusetts,  U.S.A. 


THE  following  order  has  been  pursued  : — I.  The  bony  walls  of  the  orbit. 
II.  The  anterior  fibrous  wall, — namely,  the  septum  orbitale  and  the  tarsal 
plates.  III.  The  skin,  the  surface  anatomy,  including  the  movements  of 
the  lids,  asymmetry,  the  vascular  and  nervous  supply  of  the  skin.  IV. 
The  orbicularis,  the  conjunctiva,  and  the  intermediate  structures  of  the  lids, 
including  the  expansion  of  the  levator  palpebree.  V.  The  lacrymal  appa- 
ratus. VI.  The  course  of  the  optic  nerve.  VII.  The  muscles.  VIII. 
Tenon's  capsule,  the  expansions  from  the  sheaths  of  the  muscles,  the  fasciae 
and  fat  of  the  orbit.  IX.  The  arteries,  veins,  and  lymphatics.  X.  The 
nerves.  XI.  Synopsis  of  the  topography  of  the  contents  of  the  orbit. 

It  has  seemed  wisest  to  pass  lightly  over  the  more  elementary  facts  of 
anatomy,  paying  attention  rather  to  those  not  generally  stated,  and  to  treat 
the  matter  to  a  great  extent  topographically.1 

I. 

The  bony  framework  of  the  orbit  is  important  as  a  whole.  The  prac- 
titioner needs  to  know  which  parts  are  strong  and  which  are  weak,  also 
what  is  to  be  found  on  the  other  side  of  the  walls.  The  orbits  are  roughly 
described  as  pyramids,  the  bases  of  which  are  at  the  openings,  so  placed 
that  the  inner  walls  are  parallel  and  that  their  axes  diverge  as  they  go  for- 
ward. This  is  in  the  main  true,  only  it  must  be  observed  that  though  the 
base  is  quadrilateral  the  angles  inside  are  rounded  off,  and  that  most  trans- 
verse sections  show  the  orbit  to  be  more  of  a  cone  than  of  a  pyramid.  The 
outline  of  the  base  is  formed  above  by  the  frontal,  the  prominent  external 
angular  process  of  which  joins  the  ascending  process  of  the  malar.  At 
the  upper  inner  angle  this  border  shows  usually  something  of  a  prominence, 
caused  by  the  frontal  sinus.  At  about  the  junction  of  the  middle  and 
inner  thirds  is  the  supra-orbital  notch,  or  foramen.  The  vertical  outer 
border  and  the  outer  half  of  the  base  are  made  by  the  malar,  which  has  a 

1  I  wish  to  acknowledge  the  valuable  services  of  Dr.  Benjamin  Tenney,  who  has  made 
many  dissections  and  measurements  for  this  work. 

•      71 


72  THE   ANATOMY   OF   THE   ORBIT 

sharp  orbital  edge  throughout.  This  is  continued  by  the  ascending  sharp 
edge  of  the  superior  maxillary,  which  at  the  inner  side  of  the  orbit  forms 
the  front  border  of  the  lacrymal  canal.  This  sharp  ridge  ends  just  before 
meeting  the  internal  angular  process  of  the  frontal. 

The  inner  border  of  the  orbit  is  by  no  means  always  so  easy  to  define. 
The  preceding  description  applies  very  well  to  the  female  skull  depicted  in 
Fig.  1,  but  the  other  skull  (Fig.  2)  is  very  different.  The  superior  border 
followed  downward  at  the  inner  side  of  the  orbit  is  continuous  with  the 
ridge  of  the  lacrymal  bone,  forming  the  posterior  border  of  the  groove. 
As  will  be  seen  presently,  this  has  an  important  bearing  on  measurements 
of  the  base  of  the  orbit. 

The  lower  inner  angle  is  more  rounded  than  the  others.  The  one 
above  it  is  the  sharpest.  The  upper  and  lower  borders  generally  slant 
downward  as  they  pass  outward.  Both  the  shape  and  size  of  the  base  of 
the  orbit  vary  considerably.  Broca  introduced  an  orbital  index  which  is  the 

ratio  of  the  height  to  the  breadth  of  the  base,  thus :  index  =  — p — . 

breadth 

If  the  index  is  below  84  it  is  microseme,  from  84  to  89  mesoseme,  and 
above  89  megaseme.  A  large  index  means  a  high  orbit.  The  index  of 
English  skulls  is  given  as  88.  Fig.  1  shows  a  Roman  female  skull  in 
which  the  orbital  index  is  remarkably  high,  106.  Fig.  2  is  that  of  a 
Caucasian  with  a  pretty  low  index,  73.  These  may  serve  to  show  two 
extreme  forms,  but  the  latter  is  much  less  noteworthy  than  the  former. 
The  height  of  the  orbital  opening  is  easily  measured.  Unfortunately,  there 
is  a  vagueness  and  discrepancy  in  the  directions  for  measuring  the  breadth. 
Flower  gives  the  inner  point  as  that  where  the  crest  bounding  posteriorly 
the  lacrymal  groove  strikes  the  suture  below  the  frontal  bone.  This  has 
the  great  fault  of  excluding  the  lacrymal  canal  from  the  orbit,  in  which  it 
certainly  belongs.  Broca  takes  the  dacryon, — i.e.,  the  point  at  which  frontal, 
dafofio'r  maxillary,  and  lacrymal  bones  touch.  This  would  include  the 
groove,  and  yet  he  states  that  it  is  not  in  the  orbit.  The  skulls  represented 
were  measured  from  the  dacryon.1 

The  axes  of  the  orbits  are  not  parallel  like  those  of  the  eyeballs.  They 
diverge  considerably.  If  prolonged  backward  they  meet  near  the  sella 
turcica  at  an  angle  of  from  forty-two  to  forty-four  degrees.  The  axes, 
moreover,  run  downward  as  well  as  outward  in  their  course  from  the  apex 
to  the  base,  forming  an  angle  of  from  fifteen  to  twenty  degrees  with  the 
horizon.  It  has  been  thought  that  greater  divergence  is  found  in  shorter 
skulls,  but  this  is  not  certain.  The  roof  of  the  orbit  is  in  the  main  more 
nearly  horizontal  from  before  backward  than  the  floor.  It  is  not  a  straight 
line,  but  a  sinuous  one  curving  upward  in  the  middle.  The  cavity  is  more 
or  less  overhung  by  the  superior  border.  The  degree  of  overhang  is  very 
uncertain.  There  is  next  to  none  in  the  Roman  skull,  and  a  great  deal  in 

1  Topinard,  Elements  d'Anthropologie  generate,  1885. 


FIG.  1. 


AND  THE   APPENDAGES   OF   THE   EYE.  73 

the  other.  It  is  deepest  at  the  outer  angle,  but  it  is  not  unlikely  that  it  is 
most  marked  in  skulls  with  large  frontal  sinuses,  like  that  of  Fig.  2.  When 
large,  these  may  expand  throughout  the  front  half  of  the  roof.  This  is 
otherwise  thin  as  paper,  though  thickened  by  irregular  ridges  on  its  cere- 
bral surface.  The  inner  wall  is  vertical.  It  joins  the  roof  above  at  almost 
a  right  angle,  while  below  it  passes  by  a  gradual  curve  into  the  floor.  It 
is  made  chiefly  by  the  orbital  plate  of  the  ethmoid.  A  small  part  of  the 
sphenoid  is  behind  this.  In  front  of  it  is  the  lacrymal  bone,  which  con- 
sists of  two  parts.  The  hind  one  is  in  the  same  plane  as  the  ethmoid. 
The  front  one,  separated  from  the  other  by  a  ridge,  forms  a  part  of  the 
lacrymal  groove,  which  is  completed  by  the  ascending  process  of  the 
superior  maxillary.  The  anterior  and  posterior  ethmoidal  foramina  are 
between  the  ethmoid  and  the  frontal.  Almost  the  whole  of  this  wall  is  of 
extreme  thinness,  quite  unable  to  resist  pressure,  as  of  a  tumor,  from  either 
within  or  without.  Anteriorly  the  frontal  sinus  descends  into  it.  Below 
and  behind  this  the  wall  separates  the  orbit  from  the  nasal  cavity.  Occa- 
sionally the  ethmoidal  plate  bulges  into  the  orbit.  Probably  this,  if  marked, 
is  pathological.  The  floor  slopes  more  or  less  downward  and  outward.  It 
is  made  by  the  superior  maxilla,  excepting  a  small  triangular  piece  near  the 
apex  made  by  the  palatal,  and  its  outer  anterior  angle  made  by  the  malar. 
The  spheno-maxillary  fissure  opening  into  the  zygomatic  fossa  bounds  the 
floor  externally  in  the  posterior  two-thirds  of  its  extent.  From  this  fissure 
the  infra-orbital  groove  runs  to  within  one  or  one  and  a  half  centimetres  of 
the  anterior  wall,  when  it  becomes  a  canal  passing  under  the  orbit.  Very 
often  its  course  is  marked  by  a  suture  in  the  floor.  The  outer  wall  is  made 
by  the  great  wing  of  the  sphenoid  with  the  malar  in  front.  At  the  very 
back  part,  the  edge  of  the  great  wing,  just  below  the  sphenoidal  fissure, 
separates  the  orbit  from  the  middle  fossa  of  the  skull.  Beyond  that  the 
great  wing  is  very  thick,  but  the  posterior  portion  of  the  malar  is  very 
thin.  Rather  more  than  the  front  half  of  this  wall  is  between  the  orbit  and 
the  temporal  fossa.  Two  very  small  foramina  may  be  seen  in  the  malar 
bone.  The  upper  leads  into  the  temporal  fossa,  the  lower  to  the  cheek.  The 
apex  of  the  orbit  is  near  the  lower  end  of  the  sphenoidal  fissure,  which  runs 
upward  and  outward  between  the  greater  and  lesser  wings  of  the  sphenoid. 
It  opens  into  the  middle  fossa  and  transmits  the  ophthalmic  vein  and  all 
the  nerves  of  the  orbit  except  the  optic  nerve  and  one  or  two  small  branches 
from  the  second  division  of  the  fifth  pair.  It  is  always  largest  at  the  inner 
end.  It  then  narrows  to  a  small  slit,  and  usually,  but  not  always,  ex- 
pands at  the  outer  end.  Sometimes  the  narrowing  is  slight  and  the  en- 
largement wanting.  The  variations  in  shape  depend  chiefly  on  the  lower 
edge.  Just  to  the  outside  of  the  enlargement  a  more  or  less  prominent 
spine  serves  for  the  attachment  of  a  part  of  the  external  rectus  (Fig.  1, 
left  orbit).  The  optic  foramen  (Fig.  2,  left  orbit),  the  orifice  of  a  short 
canal  through  the  lesser  wing  of  the  sphenoid,  opens  just  above  and  inter- 
nal to  the  inner  end  of  this  fissure.  Its  diameter  is  about  five  millimetres 


74 


THE   ANATOMY   OF   THE   ORBIT 


transversely.  It  is  often  a  little  larger  from  above  downward.  The  spheno- 
maxillary  fissure  is  longer  than  the  other,  which  it  almost  meets  at  the  apex. 
It  runs  downward,  outward,  and  forward,  bounded  by  the  sharp  lower  bor- 
der of  the  great  wing  of  the  sphenoid  above  and  the  border  of  the  maxil- 
lary below.  It  is  closed  in  front  by  the  malar  bone.  It  often  is  enlarged 
at  this  point. 

In  view  of  the  importance  of  the  topographical  relations  of  the  bony 
walls  of  the  orbit,  three  frontal  sections  through  this  region  have  been 
made  for  this  work.  The  vertical  lines  A,  B,  and  C  on  Fig.  3  show  their 

FIG.  3. 


positions  on  the  surface.  Each  of  the  three  following  figures  shows 
the  front  of  each  slice  at  the  three  lines :  thus,  in  Fig.  4,  all  before  A  is 
taken  off. l 

The  first  cut,  Fig.  4,  at  A,  strikes  the  bone  at  pretty  nearly  the  middle 
of  the  orbital  opening,  a  little  outside  of  the  supra-orbital  notch.  The 
outline  of  the  orbit  is  even  more  quadrilateral  than  at  the  front,  which 

1  Figs.  3,  4,  5,  and  6  are  a  little  larger  than  nature. 


AND    THE    APPENDAGES    OF    THE    EYE. 


75 


is  partly  due  to  the  section  striking  the  lacrymal  groove  and  canal,  L, 
which  appear  at  the  inner  side  of  the  orbit.  The  canal  opens  into  the  in- 
ferior meatus  just  back  of  this  plane.  Above,  at  the  inner  side,  is  seen 
a  well-developed  frontal  sinus,  F;  it  reaches  a  little  farther  outward  just 
back  of  this  section.  Below,  we  see  a  little  of  the  antrum,  A  ;  external  to 
this  is  the  opening  of  the  infra-orbital  canal,  C,  four  millimetres  below 


FIG.  4. 


the  floor,  with  a  rod  passed  through  it.  Internal  to  this,  under  the  floor, 
is  a  side  branch,  the  anterior  dental  canal  for  the  artery  and  nerve  of  the 
incisors. 

Fig.  5  shows  the  section  at  J5.  The  inner  angles  are  much  rounded  off. 
The  obliquity  of  both  roof  and  floor  is  striking.  At  the  inside  are  the 
ethmoidal  cells,  E.  Above  and  external  to  these,  hardly  visible,  is  the  ex^ 
treme  posterior  point  of  the  frontal  sinus,  F.  In  the  floor  is  the  infra- 
orbital  groove,  (7,  which  becomes  a  canal  in  the  thickness  of  the  slice  just 
removed.  The  extreme  delicacy  of  this  floor  separating  the  orbit  from  the 
antrum,  A,  needs  no  comment.  The  apparently  great  thickness  of  the  outer 


76 


THE   ANATOMY   OF   THE   ORBIT 


wall  is  owing  to  the  way  in  which  the  section  has  divided  the  orbital 
process  of  the  malar.  Behind  this  is  seen  the  flaring  outer  wall,  and  in 
the  lower  outer  angle  the  front  of  the  spheno-maxillary  fissure,  S.  M.  F. 

Fig.  6,  at  C,  cuts  the  orbit  near  its  apex.  The  thickness  of  the  outer 
wall  is  here  due  to  the  saw  having  passed  through  the  great  wing  of  the 
sphenoid  just  where  it  forms  the  front  of  the  middle  fossa  of  the  skull. 
In  fact,  it  has  opened  this  at  one  point,  M.  F.  Had  it  struck  even  one 
millimetre  farther  back,  this  cavity  would  have  been  shown  as  a  rent 

FIG.  5. 


S.M.F. 


between  the  outer  wall  of  the  orbit  and  that  of  the  skull.  The  spheno- 
maxillary  fissure,  S.  M.  F.}  runs  into  the  orbit  below.  Internal  to  this  is 
the  back  of  the  antrum,  A,  with  the  last  molar  tooth  just  beneath  it.  The 
posterior  ethmoidal  cells,  E,  appear  above  the  antrum  at  the  inner  wall 
of  the  orbit.  The  roof  of  the  orbit  is  thicker  here  than  in  the  preceding 
sections.  At  the  apex  of  the  orbit  a  part  of  the  optic  foramen,  O,  can 
be  seen.  External  to  this  is  the  sphenoid  fissure,  S.  F.,  opening  into  the 
middle  fossa  of  the  skull  between  the  greater  and  lesser  wings  of  the  sphe- 
noid bone. 


AND   THE   APPENDAGES   OF   THE   EYE. 


77 


The  actual  size  of  the  orbit  varies  both  with  races  and  with  individuals. 
It  could  hardly  be  otherwise,  depending  as  it  does  on  the  shape  and  size  of 
the  face,  one  of  the  most  variable  parts  of  the  body.  Merkel  puts  the 
depth  of  the  orbit  at  43  millimetres  in  male  skulls  and  40.5  in  female 
ones,  but  this  applies  only  to  those  from  certain  parts  of  Germany.  Other 
statements  range  from  below  4  centimetres  to  5  centimetres.  The  breadth 
at  the  base  is  given  by  Merkel  at  40.5  millimetres  for  men  and  40  for 
women,  and  the  height  at  35  millimetres  for  men  and  34.5  for  women. 
The  height  varies  from  3  to  4  centimetres,  and  the  breadth  from  36  to  50 


FIG 


M.  F. 


S.  M.  F. 


millimetres.  It  is  to  be  remembered  that  the  two  orbits  often  differ  in  size 
and  position.  The  female  orbit,  though  absolutely  rather  smaller,  is  larger 
relatively  to  the  face  than  the  male  one.  Its  outlines  usually  show  the 
greater  delicacy  which  is  characteristic  of  the  bones  of  the  female  face. 

"  If  we  contrast  the  front  view  of  the  face  and  cranium  of  the  infant 
and  the  adult  by  counting  as  face  all  below  a  line  at  the  tops  of  the  orbital 
arches  and  as  skull  all  that  is  seen  above  that  line,  considering  it  projected 
on  a  vertical  plane  as  in  a  photograph,  we  find  that  in  the  infant  the  skull 
forms  about  one-half  and  in  the  adult  much  less.  Coming  to  details,  we 


78  THE  ANATOMY  OF  THE  ORBIT 

find  that  the  height  of  the  orbit  bears  pretty  nearly  the  same  proportion  to 
the  skull  at  all  ages,  but  that  it  equals  barely  a  third  of  the  adult  face, 
while  it  makes  nearly  a  half  of  it  at  birth.  While  the  top  of  the  nasal 
opening  retains  pretty  nearly  the  same  relation  to  the  orbit  at  all  ages,  its 
lower  border  is  but  veiy  little  below  the  lowest  point  of  the  orbit  at  birth 
and  much  below  it  in  the  adult." l 

Merkel,  who  has  paid  particular  attention  to  the  growth  of  the  head 
in  childhood,  states  that  at  five  years  the  base  of  the  orbit  lacks  only  2  or 
3  millimetres  of  its  adult  height,  which  it  gains  usually  in  the  next  two 
years.  The  breadth,  however,  is  not  yet  reached,  so  that  the  orbital  index 
of  the  child  is  higher,  or,  in  other  words,  the  diameters  are  more  nearly 
equal.  The  female  orbit  is  more  like  that  of  the  child.  Moreover,  in 
infancy  the  axis  of  the  orbit  is  horizontal  instead  of  slanting  downward. 

The  walls  of  the  orbit  are  lined  with  periosteum,  which  is  firmly  at- 
tached at  the  borders  and  loosely  to  the  smooth  surfaces  of  the  bones. 
At  the  optic  foramen  the  dura  is  continuous  both  with  the  outer  sheath  of 
the  optic  nerve  and  with  the  periosteum  of  the  orbit.  Both  the  fissures 
are  closed  by  membrane.  That  of  the  spheno-maxillary  fissure  consists 
in  great  part  of  involuntary  muscular  fibres. 

II. 

The  base  of  the  cavity  of  the  orbit,  open  in  the  skeleton,  is  closed  in 
life  by  the  lids,  using  the  word  in  its  widest  possible  sense.  These  consist 
of  the  tarsal  plates  (miscalled  cartilages)  which  form  the  greater  part  of  the 
lids  proper,  and  of  the  membrane  attaching  them  to  the  walls  of  the  orbit. 
This  whole  structure  may  be  likened  to  an  optical  diaphragm,  the  opening 
of  which  is  the  slit  of  the  lids.  It  is  covered  on  the  outside  by  the  orbicu- 
laris  and  the  skin.  The  plates,  lined  on  the  inside  by  the  conjunctiva,  are 
made  of  dense  connective  tissue  which  gets  thinner  at  the  periphery,  so  that 
it  passes  insensibly  into  the  surrounding  membrane.  Any  strict  definition 
of  their  boundaries  is  therefore  artificial.  They  are  about  as  long  as  the 
opening  of  the  lids  exclusive  of  the  lacrymal  bay.  The  breadth  verti- 
cally of  the  upper  is  given  as  10  or  12  millimetres,  which  is  as  good  a 
conventional  statement  as  is  needed.  The  inferior,  which  is  altogether  less 
well  defined,  may  be  said  to  be  half  as  broad.  Both  are  convex,  so  as  to 
fit  over  the  eyeball.  The  membrane  attaching  these  to  the  orbit  is  best 
called  septum  orbitale  (ligament  large  des  pauptires).  This,  when  dissected, 
appears  as  a  strong  unmistakable  membrane  in  the  upper  lid.  It  is  much 
more  delicate  in  the  lower.  In  microscopic  sections  it  is  hardly  to  be  dis- 
tinguished (Fig.  17,  Sep.  O.).  It  is  treated  diagrammatically  in  Figs.  18 
to  21. 

The  septum  orbitale  is  attached  along  the  border  of  the  orbit  at  the  lower 

1  Archives  of  Pediatrics,  September,  1891.  The  Neck  and  Head  in  Infancy,  by 
Dwight  and  Kotch. 


AND   THE   APPENDAGES   OF   THE    EYE.  79 

and  outer  sides,  and  more  or  less  within  it  at  the  inner  side  and  above. 
It  is  generally  incorrectly  represented  both  as  to  its  origin,  which  is  put  at 
the  edge  or  even  at  the  outer  surface  of  the  top  of  the  orbit,  and  as  to  its 
direction.  The  upper  part  is  not  vertical  in  any  position  of  the  lid,  but 
slants  backward  and  then  turns  forward  at  an  angle  with  its  former  course 
before  it  is  lost  in  the  tarsus.  The  external  palpebral  ligament  (Fig.  16, 
E.  P.  L.)  is  a  badly-marked  thickening  of  some  fibres  of  this  membrane 
at  the  outer  angle  of  the  lids,  running  to  the  malar  bone.  The  inner  palpe- 
bral ligament  (Fig.  16,  I.  P.  L.}  is  a  true  and  important  band.  It  lies  on 
the  septum  orbitale,  with  which  it  is  inseparably  connected.  It  runs  from 
the  inner  ends  of  the  tarsi  to  the  superior  maxilla  in  front  of  the  nasal 
groove.  When  made  tense  by  pulling  the  lids  outward,  it  shows  clearly 
through  the  skin  which  is  attached  to  it.  It  is  attached  posteriorly  through 
the  septum  orbitale  to  the  anterior  surface  of  the  lacrymal  sac.  This 
ligament  is  the  same  thing  as  the  direct  tendon  of  the  orbicularis.  There 
is  also  a  membranous  expansion  passing  behind  the  lacrymal  sac  to  the 
crest  on  the  lacrymal  bone,  which  is  often  called  the  reflected  tendon  of  the 
orbicularis. 

III. 

The  region  of  the  eye  as  studied  on  the  living  is  bounded  above  by  the 
eyebrows,  externally  by  the  border  of  the  orbit.  It  is  separated  from  the 
cheek  below  by  a  curved  line,  hardly  to  be  seen  in  the  young  and  fat,  but 
very  clear  in  others.  Internally  it  is  bounded  by  the  projecting  nose.  The 
eyebrows,  generally  nearly  straight,  except  in  the  outer  part  which  slants 
downward,  but  sometimes  decidedly  arched  throughout,  are  of  very  vary- 
ing development.  They  are  composed  of  coarse  stiif  hairs  pointing  out- 
ward. The  inner  half  corresponds  pretty  closely  to  the  upper  border  of 
the  orbit,  but  the  outer  half,  on  account  of  the  downward  slope  of  the  orbit, 
is  above  it,  resting  against  the  forehead.  Sometimes  the  outer  half  is  want- 
ing. Sometimes,  especially  in  dark-haired  races,  the  eyebrows  meet  at 
the  root  of  the  nose.  The  inner  half  is  the  strongest  and  thickest.  At  the 
outer  end  the  hairs  are  fewer  and  smaller.  The  lower  hairs  slant  upward, 
and  the  upper  downward  as  well  as  outward.  Thus  they  meet  to  make  a 
raised  crest  in  the  middle.  The  shape  depends  largely  on  the  direction  of  the 
outer  end.  The  eyebrows  are  but  little  developed  in  infancy.  They  rarely 
are  strong  in  childhood.  At  about  puberty  they  become  more  marked. 
The  hairs  grow  longer  and  coarser  throughout  life,  especially  in  men.  In 
women  this  feature  is  more  delicate.  Individual  differences  are  endless. 

Just  above  the  eyebrows  over  the  inner  half  of  the  orbit  may  be  felt 
the  superciliary  eminences.  The  skin  here  is  thick  and  but  very  loosely 
attached  to  the  bone,  so  that  it  follows  the  pull  of  the  muscles  which  are 
practically  in  it.  Thus  the  eyebrows  may  be  raised  well  on  to  the  forehead 
by  the  frontalis,  or  brought  far  down  over  the  orbits  by  the  orbicularis. 
The  skin  at  the  outside  of  the  orbit,  as  well  as  below  it,  is  thinner,  and  is 
also  loosely  fastened  so  as  to  be  easily  thrown  into  folds.  The  slit  of  the 


80  THE   ANATOMY   OF   THE   ORBIT 

eyelids  may  be  closed  by  drawing  the  skin  directly  outward.  At  the  side 
of  the  nose  the  thin  skin  is  more  adherent  than  elsewhere,  but  even  here 
some  displacement  is  possible.  The  skin,  becoming  thinner,  turns  in  under 
the  upper  border  of  the  orbit  to  form  a  deep  furrow  where  it  again  turns 
forward  over  the  tarsus.  The  amount  of  overhanging  tissue  varies  much. 
It  is  one  of  the  characteristic  points  of  a  face,  generally  increasing  with 
age.  Photographs  from  life  show  how  this  fold  is  deepened  as  the  eye 
is  turned  up.  When  the  eye  is  closed  the  fold  is  practically  effaced,  but 
after  early  youth  one  or  more  creases  remain  (Fig.  8).  The  lid  of  the 
infant  (Fig.  22)  contains  much  fat,  which  later  generally  disappears.  With 
its  absorption  and  the  loss  of  elasticity  of  the  tissues,  folds  and  wrinkles 
increase.  Towards  the  edge  of  the  lids  the  skin  is  closely  adherent  to  the 
tissues  beneath  it.  The  furrow  already  alluded  to  below  the  eye,  though 
following  the  general  curve  of  the  lower  border  of  the  orbit,  is  not  oppo- 
site to  it,  but  distinctly  (almost  one  centimetre)  lower  down.  After  middle 
age  another  smaller  fold  above  it  shows  approximately  the  lower  border 
of  the  tarsal  plate.  The  peculiar  dark  discoloration  which  often  is  seen 
below  the  inner  angle  in  varying  degrees  under  different  circumstances  is 
not  easy  to  account  for.  It  has  been  ascribed  to  venous  stasis.  In  some 
cases  this  explanation  may  be  satisfactory,  but  not  in  all.  It  seems  as  if 
there  were  an  actual  change  in  the  color  of  the  skin.  The  opening  of  the 
lids  is,  roughly  speaking,  oval,  the  length  being  from  twenty-five  to  thirty 
millimetres  and  the  greatest  breadth  from  twelve  or  less  to  fourteen  milli- 
metres. Both  the  actual  size  and  the  proportions  of  the  opening  vary  con- 
siderably. Usually  all  but  the  upper  part  of  the  cornea  is  uncovered  in 
most  persons.  The  inner  angle  or  canthus  presents  a  little  bay,  due  to  a 
change  in  direction  in  each  lid,  in  which  lies  a  raised  pinkish  little  body, 
the  lacrymal  caruncle.  In  infancy  the  lacrymal  bay  is  rudimentary  and 
the  vertical  height  of  the  opening  large  relatively  to  the  length,  which 
gives  the  well-known  appearance  of  large  eyes  of  babies,  The  lids  of  the 
open  eye  meet  at  the  same  level.  That  is  to  say  that,  the  eye  being  open, 
the  upper  lid  does  not  overlap  the  lower  at  the  outer  angle,  though  a  little 
fold  and  the  line  of  the  eyelashes  seem  to  suggest  it.  The  lashes  of  both 
lids  stop  a  little  short  of  the  lacrymal  bay.  Those  of  the  upper  lid,  much 
the  larger  and  more  numerous,  spring  in  several  rows  from  the  under  edge 
of  the  lid  and  turn  upward.  Those  of  the  lower  lid  arise  from  its  anterior 
surface  near  the  edge  and  are  directed  forward  and  downward.  There  is 
a  peculiarity  of  the  lower  lashes  that  is  significant  when  the  closing  of 
the  lids  is  considered.  It  is  that  the  line  of  insertion  of  these  lashes  is 
farther  from  the  edge  of  the  lid  at  the  outer  part  than  at  the  inner.  Both 
lids  have  the  longest  lashes  in  the  middle.  The  curve  of  the  edge  of 
the  upper  lid  is  much  greater  than  that  of  the  lower.  Not  only  is  the 
outer  canthus  generally  the  higher,  but  the  axis  of  the  lacrymal  part 
often  slants  a  little  downward  and  inward.  When  the  eye  is  closed  certain 
remarkable  changes  in  these  relations  occur.  The  upper  lid  falls  a  great 


Fia.  7. 


FIG.  8. 


FIG.  9. 


FIG.  10. 


AND   THE   APPENDAGES   OF   THE   EYE.  81 

deal ;  the  lower  lid  rises  a  little.  At  the  same  time  the  lower  lid  is  drawn 
somewhat  inward,  and  thus  at  its  outer  end  is  for  a  short  distance  over- 
lapped by  the  upper.  The  line  of  the  joined  lids  describes  a  slight  curve, 
rising  and  becoming  straight  at  the  inner  end.  The  outer  end  of  the  slit 
has  become  distinctly  lower  than  the  inner.  (See  Figs.  7  and  8.)  In 
watching  the  movements,  one  is  struck  by  the  fact  that  the  inner  canthus 
is  practically  still.  A  series  of  frozen  sections  through  the  closed  lids 
(Figs.  18  to  21)  shows — besides  other  things — that  at  the  outer  canthus 
the  upper  lid  overhangs  the  lower  when  closed.  It  continues  to  do  so, 
though  in  a  constantly  decreasing  degree,  as  we  pass  inward.  Thus  the  slit 
between  the  lids,  from  an  angle  of  forty-five  degrees,  becomes  nearly  hori- 
zontal before  the  inner  canthus  is  reached.  It  is  to  be  noted  that  the  edges 
of  the  lids  are  applied  closely  one  to  another  throughout.  There  is  no 

FIG.  11. 


reason  to  believe  in  the  three-sided  canal  which  they  have  been  supposed  to 
form  with  the  cornea  for  the  passage  of  the  tears.  The  changes  in  the 
position  and  shape  of  the  lids  in  the  other  movements  of  the  eyes  deserve 
more  attention  than  is  generally  paid  to  them.  Every  one  knows  how  the 
rise  and  fall  of  the  upper  lid  are  associated  with  the  corresponding  move- 
ments of  the  globe,  but  it  is  not  generally  stated  that  the  lids  take  part  in 
the  lateral  movements.  It  is,  however,  certain  that  they  do  so.  When  the 
eye  is  turned  towards  the  nose  (Fig.  9)  the  inner  canthus  is  drawn  back- 
ward and  inward ;  Avhen  the  eye  is  turned  out  (Fig.  10)  the  outer  canthus 
is  pulled  outward.  The  latter  movement  is  the  greater.  The  change  in 
relation  to  the  outer  border  of  the  orbit  is  shown  conclusively  in  the  profile 
views  Figs.  11 l  and  12.  This  is  the  necessary  result  of  the  insertion  into 

1  Figs.  11,  12,  and  13  are  from  actual  photographs. 
VOL.  I.— 6 


82  THE   ANATOMY    OF   THE   ORBIT 

the  lids  and  conjunctiva  of  the  expansions  from  the  sheaths  of  the  outer 
and  inner  recti.  The  front  views  show  also  certain  changes  in  the  shape 
of  the  opening.  When  the  eye  is  turned  strongly  outward  (Fig.  10)  the 
distance  between  the  lids  is  greater  than  when  it  is  turned  strongly  inward, 
except  at  the  lacrymal  bay,  where  the  reverse  occurs.  When  the  pupil 
is  turned  out  this  is  stretched  and  narrowed ;  when  turned  in  (Fig.  9)  it  is 
expanded.  The  profile  view  of  the  eye  looking  up  (Fig.  13)  is  a  very  in- 
structive one  in  several  respects.  Apart  from  displaying  the  folding  in  of 
the  upper  lid,  it  shows  that  the  lower  lid  rises  also  and  becomes  more  promi- 
nent. The  latter  fact  is  due  partly  to  its  being  in  closer  relation  to  the 
globe  in  this  position  and  partly,  perhaps,  to  its  being  crowded  forward  by 
a  mass  of  solid  fat  in  the  lower  part  of  the  base  of  the  orbit.  These  photo- 
graphs suggest  very  strongly  that  in  this  forced  looking  upward  the  eyeball 

FIG.  12. 


not  merely  rotates  on  a  transverse  axis,  but  is  carried  to  a  slight  extent 
bodily  upward.  This  effect  is  presumably  an  illusion.1  The  lids,  both 
when  open  and  when  shut,  are  applied  very  closely  to  the  globe  of  the  eye, 
except  at  the  lacrymal  bay,  where  they  leave  it.  The  hardness  and  to 
some  degree  the  shape  of  the  eye  are  to  be  felt  through  the  closed  lids. 
The  prominence  of  the  cornea  can  in  some  cases  be  detected  even  by  sight. 
The  inner  edge  of  the  entire  border  of  the  orbit  can  easily  be  explored  by 
the  finger.  Its  relation  to  the  surface  is  indicated  in  Fig.  8.  The  supra- 
orbital  notch,  when  present,  is  more  easily  felt  by  carrying  the  finger  out- 
ward, as  its  outer  border  rises  the  more  suddenly.  If  there  is  a  foramen 

1  This  statement  is  made  out  of  deference  to  the  views  of  ophthalmologists.  I  can  see 
no  anatomical  reason  why  in  forced  raising  of  the  lid  there  should  not  be  also  a  slight 
upward  movement  of  the  entire  globe. 


AND   THE    APPENDAGES   OF   THE   EYE.  83 

it  can  be  made  out  only  by  the  sensations  of  the  patient  from  pressure  on 
the  nerve,  or  possibly  by  the  pulsation  of  the  artery.  The  foramen  may 
be  as  much  as  five  millimetres  above  the  edge.  It  is  worth  noting  that  in 
such  cases  the  canal  leading  to  the  foramen  does  not  necessarily  begin  far 
back  in  the  orbit,  but,  on  the  contrary,  may  begin  just  within  its  margin. 
The  upper  edge  of  the  zygoma  and  the  posterior  border  of  the  malar  bone 
are  felt  rather  less  distinctly.  The  external  angular  process  of  the  frontal, 
however,  is  very  plain,  and  usually  the  suture  between  it  and  the  malar  can 
be  made  out.  It  is  recognized  by  a  change  of  level,  the  angular  process 
being  a  little  more  prominent  than  the  top  of  the  malar.  Inside  the  upper 
inner  angle  the  trochlea  can  be  felt  with  some  difficulty.  Two  very  im- 
portant practical  points  call  for  notice :  first,  the  large  size  of  the  globe 
in  proportion  to  the  size  of  the  orbit,  and  next  its  prominence.  A  vertical 

FIG.  13. 


^^MlfHiiiK^^f^^^fHii^^^Hi( 
, 

plane  from  the  upper  to  the  lower  border  of  the  front  of  the  orbit  would 
in  some  cases  touch  the  front  of  the  cornea,  and  in  very  prominent  eyes 
pass  through  it.  At  the  outside,  owing  to  the  divergence  of  the  outer  wall 
of  the  orbit,  the  globe  is  much  uncovered.  A  vertical  transverse  plane  at 
the  outer  edge  of  the  orbit  would  pass  near  its  equator.  Fig.  29  is  just  in 
front  of  this.  The  projection  of  the  upper  angle  of  the  orbit  gives  it  a 
certain  amount  of  protection,  but  still  this  side  is  comparatively  unguarded. 
Various  attempts  have  been  made  to  recognize  on  the  living  the  position 
of  the  optic  foramina.  One  of  the  latest  suggestions  is  that  they  are  very 
nearly  in  a  vertical  plane  passing  through  the  two  points  at  the  greatest 
outward  convexity  of  the  zygomata.  This  will  do  for  a  rough  method.  If 
we  evert  the  lids  we  see  the  little  lacrymal  papillae  with  the  minute  orifices 
of  the  tear-ducts  at  the  beginning  of  the  lacrymal  bay.  They  are  directed 


84 


THE   ANATOMY   OF   THE  ORBIT 


FIG.  14. 


somewhat  backward,  that  they  may  be  the  better  applied  to  the  surface  of  the 
conjunctiva  so  as  to  suck  up  the  tears.  They  are  not  precisely  in  the  same 
vertical  plane,  but  the  upper  is  a  little  the  internal,  which  admits  of  the 
more  accurate  closing  of  the  lids.  The  little  ducts  run  close  to  the  edge  of 
the  lid  on  their  way  to  the  sac.  This  sac  is  so  placed  that  its  highest  part 
extends  somewhat  above  the  internal  palpebral  ligament.  The  line  of  the 
axis  and  of  its  continuation,  the  nasal  duct,  must  be  studied  both  from  the 
front  and  from  the  side.  In  a  front  view  (Fig.  8)  it  crosses  at  the  middle 
of  the  internal  palpebral  ligament  and  runs  to  near  the  end  of  a  line  ex- 
tending into  the  furrow  between 
the  cheek  and  the  ala  of  the 
nose  at  its  widest  point.  Ac- 
cording to  the  breadth  of  the 
nose,  this  line  may  be  vertical, 
or  incline  somewhat  outward, 
or  even  a  little  inward.  It  al- 
ways slants  more  or  less  back- 
ward, and  in  a  varying  degree. 
Merkel  would  have  the  line 
representing  it  on  a  side  view 
extend  from  the  preceding 
starting-point  to  the  space  be- 
tween the  last  bicuspid  and  the 
first  molar.  Luschka  drew  it  to 
the  space  between  the  first  and 
second  molars.  I  incline  to 
the  latter  view.  It  is  needless 
to  say  that  only  the  first  part 
of  this  line  corresponds  to  the 
duct. 

Before  leaving  the  surface 
anatomy,  the  vexed  question 
of  asymmetry  of  the  eyes  must 

be  touched  lightly.  This  is  to  be  seen  almost  always  in  the  living  if  the 
face  be  looked  at  through  a  screen  of  wires  at  right  angles.  "  The  right 
eye  is  probably  usually  the  higher.  According  to  Hasse,  the  left  eye  is 
nearer  the  middle  line,  whether  it  be  the  higher  or  the  lower.  One  eye  is 
often  more  open  than  the  other.  A  want  of  symmetry  is  often  found  in  the 
skull,  but,  for  obvious  reasons,  it  is  less  than  in  the  flesh.  The  left  orbit 
shows  no  approximation  to  the  middle.  One  important  factor  in  this  ques- 
tion is  generally  overlooked, — namely,  that  there  is  not  only  a  difference 
in  height  between  the  eyes,  but  that  one  orbit  and  cheek  are  anterior  to  the 
others.  This  complicates  the  problem  strangely,  making  it  often  almost  im- 
possible to  decide  which  position  of  the  head  is  to  be  called  normal.  A 
striking  instance  is  the  above  extremely  uneven  head  and  face  (Fig.  14),  of 


AND   THE   APPENDAGES   OF   THE   EYE. 


85 


which,  nevertheless,  the  asymmetry  would  easily  pass  unnoticed.  To  de- 
termine how  much  habitual  position  of  the  trunk  or  habitual  use  of  one 
eye  may  account  for  this  is  by  no  means  so  simple  as  at  first  appears. 

The  arteries  (Fig.  15)  of  the  lids  and  adjacent  parts  of  the  face  come 
from  many  sources,  making  a  series  of  anastomoses  beyond  the  margins  of 
the  orbit.  The  continuations  of  the  ophthalmic  and  facial  arteries  at  the 
inner  side  of  the  nose  and  a  branch  of  the  temporal  near  the  outer  upper 
angle  are  usually  the  largest.  Branches  running  upward  into  the  lower 
lid  come  from  the  facial,  reinforced  by  anastomoses  with  the  infra-orbital 
artery.  At  the  outer  angle  are  branches  from  the  transverse  facial  and, 


Fro.  15. 


A-  Sufrraorb. 


ATempor. 


A&ngul 


A.  lacrym. 


VTempoK 
•Ainfraorb. 


Arteries  dark,  veins  light.    (After  Merkel.) 

perhaps,  from  the  orbital  branch  of  the  middle  temporal.  The  ophthalmic 
artery  of  the  internal  carotid  sends  blood  through  the  lacrymal  branch  to 
the  outer  part  of  the  lids,  through  the  frontal  and  nasal  to  the  inner  upper 
angle  of  the  base  of  the  orbit,  and  through  the  little  supra-orbital  to  the 
upper  part  of  this  region.  A  delicate  arch  is  found  in  each  lid  between  the 
tarsus  and  the  orbicularis.  That  of  the  upper  lid  is  nearer  the  edge  than 
that  of  the  lower.  These  arches  are  formed  by  the  meeting  of  the  palpe- 
bral  arteries  from  the  inner  side  with  branches  of  the  lacrymal  from  the 
outer.  A  less  regular  arch  in  the  upper  lid  may  be  found  near  the  top  of 
the  tarsus.  This  system  of  vessels  supplies  all  the  structures  of  the  lid 
and  communicates  in  the  conjunctiva  lining  it  with  the  conjunctival  vessels 


86  THE  ANATOMY  OF  THE  ORBIT 

proper.  Small  arterial  twigs  are  to  be  found  both  over  and  beneath  the 
orbicularis. 

The  large  frontal  vein  at  the  inner  side  of  the  orbit  communicates  with 
branches  of  the  ophthalmic  vein.  (See,  also,  Fig.  32.)  A  branch  connect- 
ing it  with  the  anterior  temporal  forms  an  arch  along  the  top  of  the  orbit. 
The  facial  vein  receives  some  distance  below  the  orbit  a  vein  from  its  outer 
border.  The  branches  in  the  lids  do  not  form  definite  arches  like  the 
arteries,  but  run  in  the  main  at  right  angles  to  the  palpebral  opening.  It 
is,  however,  worthy  of  notice  that  the  vessel  marked  Fin  Fig.  17,  though 
of  about  the  size  and  position  of  the  one  which  Merkel  describes  as  an 
artery  forming  the  tarsal  arch,  is  undoubtedly  a  vein.  The  artery  lies  a 
little  higher.  Merkel  points  out  that  most  of  the  superior  branches  and  all 
the  internal  ones  pass  through  the  orbicularis,  so  that  its  continued  con- 
traction must  cause  a  congestion.  Probably  under  these  circumstances  more 
of  the  blood  passes  off  into  the  cranium  or  into  the  system  of  the  internal 
maxillary  vein,  but  under  ordinary  circumstances  the  current  is  superficial. 

The  lymphatics  of  the  lids  form  two  nets  before  and  behind  the  tarsal 
plates.  Of  the  few  vessels  crossing  the  face,  some  empty  into  the  system 
of  the  parotid  glands,  and  probably  some  into  that  of  the  submaxillary. 

The  sensory  nerves  of  the  upper  lid  and  neighborhood  come  from  the 
first  division  of  the  fifth  pair.  Above  the  orbit  are  the  supra-orbital  nerve, 
passing  through  the  notch,  and  the  supra-trochlear  at  the  inner  angle.  Both 
of  these  send  branches  downward  to  the  upper  lid,  which  may  also  receive 
twigs  externally  from  the  lacrymal  nerve.  The  infra-trochlear  branch  of 
the  nasal  nerve  reaches  the  surface  at  the  inner  side  of  the  upper  lid  above 
the  palpebral  ligament,  to  which  it  may  or  may  not  give  branches,  going 
chiefly  to  the  side  of  the  nose.  The  lower  lid  is  supplied  by  several  branches 
of  the  infra-orbital.  The  palpebral  branches  in  both  lids  run  in  the  main 
towards  the  slit,  near  which,  according  to  Mises,  they  make  a  series  of 
communicating  arches.  The  motor  branches  from  the  facial  reach  the 
orbicularis  from  the  outer  side. 

IV. 

Beneath  the  skin  lies  the  orbicularis  palpebrarum,  the  superficial  part 
of  which  may  be  divided  into  the  palpebral  and  the  orbital  portion.  The 
former  is  confined  to  the  lids  proper ;  the  latter  spreads  out  beyond  the 
margin  of  the  orbit,  mingling  with  the  muscles  of  the  forehead  and  of  the 
cheek.  The  palpebral  portion  arises  from  the  internal  palpebral  ligament 
and  from  the  front  of  the  lacrymal  sac  as  a  series  of  delicate  bundles  of 
pale  fibres  somewhat  scattered,  so  as  not  to  form  a  continuous  layer,  which 
spread  themselves  in  arches  over  the  front  of  the  tarsal  plates,  the  inner 
ones  ending  in  the  fibrous  tissue  at  the  outer  canthus  called  the  external 
palpebral  ligament,  the  outer  ones  describing  complete  loops.  Fibres  from 
the  beginning  of  the  band  run  upward  to  the  forehead  and  downward  to 
the  skin  of  the  cheek.  Some  few  bands  of  fibres  running  in  a  group  by 


AND   THE   APPENDAGES   OF   THE   EYE. 


87 


themselves  near  the  margin  of  the  lid  have  been  called  the  ciliary  muscle 
of  Riolan.  It  is  well  shown  in  the  sections  of  the  lid,  Figs.  1 7  and  22,  R. 
What  is  usually  described  as  Horner's  muscle  (5  in  Fig.  23)  is  best  con- 
sidered as  a  deep  head  of  the  orbicularis  which  arises  from  the  crest  of  the 
lacrymal  bone  behind  the  sac,  over  which  it  spreads,  to  send  some  fibres  to 
the  inner  ends  of  the  tarsi,  while  others  mix  with  those  of  the  superficial 
set.  Some  fibres  twine  themselves  around  the  tear-ducts.  Horner's  muscle 
should  be  dissected  from  behind.  The  portion  going  to  the  upper  lid  is 
much  the  larger,  and  placed  back  of  the  lower.1  The  palpebral  portion  of 
the  orbicularis  closes  the  lids  gently,  as  in  unperceived  winking.  The  deep 


E.  P.  L. 


I.P.L 


Orbicularis  palpebrarum.— The  palpebral  and  orbital  portions  are  easily  recognized,  though  the 
line  of  separation  is  not  always  to  be  seen.  C.  S.  points  to  the  corrugator  supercilii ;  /.  P.  L.,  internal 
palpebral  ligament ;  E.  P.  L.,  position  of  external  palpebral  ligament.  (After  Henle.) 

portion  draws  the  lids  inward,  which  is  sometimes  evident  in  the  early  part 
of  the  process  of  closing  them.  A  few  people  have  the  power  of  contract- 
ing it  independently,  so  as  to  draw  the  lids  inward  and  narrow  the  slit 
without  closing  the  eye.  The  tonicity  of  the  whole  palpebral  portion 
affords  a  support  to  the  globe.  The  orbital  portion  is  continuous  with  the 
former.  Some  of  its  inner  fibres  arise  from  the  inner  ligament,  others  from 
the  inner  border  of  the  orbit  above  and  below  it.  It  overlaps  the  borders 


1  Fig.  23  is  useful,  but  very  diagrammatic, 
not  in  one  plane,  as  there  represented. 


The  two  parts  of  Horner's  muscle  are 


88  THE   ANATOMY   OF   THE   ORBIT 

considerably,  except  at  the  inner  side.  Some  of  its  fibres  make  loops  sur- 
rounding the  orbit,  open  only  at  the  inner  end ;  others  mingle  with  the 
neighboring  muscles, as  shown  in  Fig.  16.  The  corrugator  supercilii  (C.  S., 
Fig.  16)  arises  from  the  superciliary  ridge  just  outside  of  the  glabella.  Its 
fibres  run  outward  through  those  of  the  frontalis  and  orbicularis,  which 
cover  it,  into  the  skin  over  the  middle  of  the  orbit,  to  draw  it  by  its  con- 
traction into  vertical  folds.  These  muscles  are  supplied  by  the  facial  nerve. 
The  lids  must  now  be  considered  comprehensively.  They  contain  the 
following  layers :  first,  the  skin ;  second,  the  orbicularis ;  third,  the  tarsus 
and  septum  orbitale ;  fourth,  the  conjunctiva  near  the  opening,  and  farther 
from  it  Miiller's  muscle,  and  in  the  upper  lid  the  expansions  of  the  levator 
palpebrae.  The  skin  and  the  orbicularis  have  been  described;  the  pal- 
pebral  portion  of  the  latter  rests  on  the  tarsus.  There  is  but  little  areolar 
tissue  between  them  in  the  adult.  It  is  different,  however,  beyond  the  tarsi. 
There  is  much  loose  areolar  tissue  on  both  sides  of  the  septum  orbitale,  which 
is  the  seat  of  effusion  in  oedema.  The  layer  of  the  tarsi  and  the  septum  has 
also  been  described,  with  the  exception  of  certain  features  at  the  border  of 
the  lids.  It  is  important  to  observe  that  in  microscopic  sections  the  septum 
disappears.  It  seems  to  have  no  recognizable  limits.  At  the  border  most 
anteriorly  is  the  layer  of  the  roots  of  the  eyelashes  (Fig.  17,  (7).  These, 
two  or  three  deep,  lie  in  front  of  the  lowest  part  of  the  tarsus.  Sebaceous 
follicles  ($.  6r.)  and  the  so-called  glands  of  Moll  (G.  Ml.) — modified  sweat- 
glands — are  among  them.  In  a  deeper  layer,  actually  embedded  in  the  tarsus 
(£),  are  the  Meibomian  (M )  glands  (see  also  Figs.  22  and  23),  opening  in  a 
row  in  the  deeper  part  of  the  border  of  the  eyelid.  These  two  rows  are  so 
distinct  that  the  lid  can  be  split  by  an  incision  between  them  without  hurting 
either.  The  lower  lid  shows  the  same  features  less  developed.  The  delicate 
conjunctiva  becomes  continuous  with  the  skin  at  the  border  of  the  lids.  The 
attachment  here  is  very  close,  but  it  gradually  becomes  less,  till  near  the  line 
of  its  reflection  towards  the  globe  it  is  very  loose.  There  is  here  more  or 
less  subconjunctival  areolar  tissue.  The  course  of  the  line  of  reflection  along 
the  lids  is  worth  noting.  It  extends  one  or  two  millimetres  beyond  the 
outer  canthus,  but  does  not  pass  the  inner  canthus  at  all.  Beginning  at  its 
outer  end,  just  beyond  the  canthus,  it  rises  and  falls  rapidly  behind  the 
upper  and  lower  lids  respectively.  Its  reflection  may  be  traced  on  the 
series  of  sections  shown  in  Figs.  18,  19,  20,  and  21.  In  Fig.  18,  some  2 
millimetres  inside  the  outer  canthus,  it  is  14  millimetres  above  the  outer 
line  of  closure  of  the  eyelids  and  sinks  2  millimetres  below  it.  At  about 
the  middle  of  the  opening  (Fig.  20)  it  is  17  millimetres  above  and  4  milli- 
metres below.  At  the  beginning  of  the  lacrymal  bay  (Fig.  21)  it  is  9 
millimetres  above  and  about  3  millimetres  below.  (These  measurements 
show  the  line  of  reflection  projected  on  a  vertical  plane  measured  from 
the  edge  of  the  upper  lid,  which,  as  these  sections  prove,  overlaps  the 
lower.)  If  we  measure  the  breadth  of  the  inner  surface  of  the  lids  to 
the  reflection  of  the  conjunctiva,  we  find  the  greatest  distance  in  the  upper 


AND   THE   APPENDAGES   OF   THE    EYE. 


89 


lid  18  millimetres  and  in  the  lower  8.5  millimetres.  The  line  of  its  re- 
flection is  dotted  on  Fig.  8.  The  explanation  of  the  short  distance  which 
it  reaches  below  the  eye  is  in  the  foreshortening  resulting  from  the  for- 
ward projection  of  the  lower  lid.  It  is  not  impossible,  however,  to  hold 


Fro.  17. 


Sep.O 


GMl. 


Microscopic  section  of  the  upper  lid  and  front  of  the  eye,  by  Dr.  H.  P.  Quincy.  C,  eyelash;  F,  fat; 
<?.  Ml.,  gland  of  Moll ;  L,  levator  palpebrse ;  if,  Meibomian  glands ;  0,  orbicularis ;  R,  ciliary  muscle  of 
Riolan;  Sep.  O.,  septum  orbitale;  S.G.,  sebaceous  gland;  S.R.,  superior  rectus;  S.S.R.,  sheath  of  superior 
rectus ;  V,  vein. 

that  this  line  might  have  been  drawn  a  trifle  lower.  The  conjunctiva, 
after  its  reflection,  comes  soon  into  contact  with  Tenon's  capsule.  At  the 
distance  of  some  three  millimetres  from  the  cornea  they  are  inseparably 


90 


THE   ANATOMY   OF   THE   ORBIT 


connected  into  a  single  membrane  closely  attached  to  the  front  of  the 
eye.  At  the  inner  canthus,  external  to  the  lacrymal  bay,  there  is  a  ver- 
tical curved  fold  of  conjunctiva,  called  from  its  shape  the  plica  semi- 
lunaris,  which  is  drawn  forward  when  the  eye  is  turned  outward  (Fig. 


FIG.  18. 


8ep.O. 


Fio.  19. 


Sep.0.- 


Conj. 


L.  O.,  lacrymal  gland;  A.  L.  O.,  acces- 
sory gland.  The  expansion  of  the  levator 
is  seen  between  these.  E.  R.,  external  rec- 
tus;  Inf.  0.,  inferior  oblique;  Conj.,  con- 
junctiva; Sep.  O.,  septum  orbitale. 


Conj 


L.  P.,  levator;  S.  R.,  superior  rectus; 
Sep.  0.,  septum  orbitale ;  Conj.,  conjunctiva  ; 
L  R.,  inferior  rectus;  Inf.  O.,  inferior  ob- 
lique. 


10).  There  are  no  arteries  of  any  size  in  the  conjunctiva,  but  there  is  a 
very  rich  net-work  of  minute  vessels  which  come  into  view  in  inflamma- 
tion. It  is  noted  for  its  irregular  arrangement,  and  may  be  distinguished 
by  this  and  its  mobility  from  a  deeper  system  of  vessels  connected  with  the 


FIG.  20. 


FIG.  21. 


Sep.  O. 


Conj 


Sep.  O.,  septum  orbitale;  L.  P.,  levator; 
S.  R.,  superior  rectus ;  S.  O.,  superior  oblique ; 
Oonj.,  conjunctiva;  Inf.  R.,  inferior  rectus; 
Inf.  0.,  inferior  oblique. 


L.C 


Sep.  O.,  septum  orbitale;  L.  P.,  ex- 
pansion from  levator ;  S.  R.,  superior  rec- 
tus; S.  0.,  tendon  of  superior  oblique. 
The  'oblique  course  of  this  tendon  ac- 
counts for  its  apparently  excessive  thick- 
ness. Conj.,  conjunctiva ;  L.  C.,  lacrymal 
canals. 


interior  of  the  eyeball.  There  is  a  considerable  plexus  of  lymphatics 
which  is  in  communication  with  lymph-spaces  of  the  cornea.  The  nerves 
of  the  conjunctiva  are  from  the  lacrymal  and  from  the  supra-  and  infra- 
trochlear  branches  of  the  nasal. 


AND   THE   APPENDAGES   OF   THE   EYE. 


91 


(Figs.  18  to  21  are  inserted  here  because,  among  other  things,  they 
show  the  levator  and  the  superior  rectus.  They  show  many  points  of 
the  topography  of  the  orbit.  They  are  in  series  from  without  inward, 
18  being  just  inside  the  outer  can  thus  and  21  at  the  beginning  of  the 
lacrymal  bay.  They  are  made  from  actual  frozen  sections.  The  treat- 
ment is  necessarily  diagrammatic.) 

We  come  now  to  Mulleins  muscle,  and  to  the  expansions  into  the  lids 

FIG.  22. 


Microscopic  section  through  the  upper  lid  of  a  negro  infant,  by  Dr.  H.  P.  Quincy. 
M,  Meibomian  gland  ;  0,  orbicularis ;  R,  ciliary  muscle  of  Riolan. 


P.fat; 


from  the  levator  palpebrse  and  from  the  superior  and  inferior  recti.  First 
as  to  the  upper  lid.  The  sheath  of  the  superior  rectus  is  at  first  closely 
connected  with  the  under  surface  of  the  levator  (Figs.  20  and  21).  The 
superior  rectus,  S.  R.,  besides  its  insertion  into  the  globe,  sends  fibres  to  the 
top  of  the  fold  of  the  conjunctiva,  which  is  thus  pulled  upward  and  back- 
ward in  harmony  with  the  turning  up  of  the  eye,  and  from  its  sheath, 
8.  8.  R.  (Fig.  17),  fibres  pass  to  the  top  of  the  tarsus.  The  levator,  L, 


92  THE  ANATOMY  OF  THE  ORBIT 

broadens  out  into  an  expansion  stretching  across  the  whole  orbit  from  one 
bony  wall  to  the  other,  which  by  its  outer  portion  separates  the  greater 
lacrymal  gland  from  the  accessory  portion  below  it  (Fig.  18).  This  ex- 
pansion splits  into  two  layers.  The  greater  portion,  consisting  of  invol- 
untary muscular  fibres  (Midler's  muscle),  is  inserted  into  the  upper  portion 
of  the  tarsus,  while  certain  anterior  fibres  pass  into  or  through  the  fibres 
of  the  orbicularis  to  the  skin  of  the  lid.  Their  function  is  to  draw  the  skin 
to  the  fold  above  the  tarsus  when  the  lids  are  opened.  The  expansion 
of  the  levator  passing  to  the  tarsus  consists  largely  of  unstriped  muscular 
fibres  mingled  with  elastic  tissue.  This  is  connected  with  other  involun- 
tary fibres  arranged  transversely,  the  whole  constituting  what  is  known  as 
Midler's  muscle. 

The  inferior  tarsus  is  smaller  than  the  upper,  the  Meibomian  glands 
less  developed.  There  is  a  collection  of  involuntary  fibres  known  also  as 
Midler's  muscle  which  is  of  little  importance.  Fibres  from  the  sheath  of 
the  inferior  rectus  may  be  traced,  in  sections  of  the  lid,  to  the  tarsus.  Ac- 
cording to  Schwalbe,  the  termination  of  the  sheath  of  the  inferior  rectus 
may  be  divided  into  three  layers  somewhat  analogous  to  those  of  the  upper 
lid,  the  middle  one  containing  unstriped  muscular  fibres  and  going  to  the 
tarsus.  The  lid  of  the  infant  differs  in  some  respects  from  that  of  the 
adult.  Fig.  22  represents  a  section  through  the  lid  of  a  negro  child  at 
birth.  As  Figs.  17  and  22  are  equally  magnified,  they  show  that  the 
thickness  of  the  lid  at  the  edge  is  nearly  as  great  at  birth  as  in  the  adult. 
The  most  striking  differences  are  the  relatively  small  size  of  the  infant's 
muscle  and  the  great  collections  of  fat  in  the  areolar  tissue.  This  accounts 
in  a  great  measure  for  the  thickness.  In  the  infant  also  the  Meibomian 
glands  are  very  large. 

V. 

The  tears  are  secreted  by  the  lacrymal  gland,  which,  enveloped  in  its 
sheath,  is  situated  under  cover  of  the  external  angular  process  in  a  shallow 
pit  in  the  top  of  the  orbit.  It  consists  of  two  parts,  of  which  the  larger 
is  placed  above  and  external  to  the  smaller,  which  is  sometimes  called 
the  accessory  gland.  A  part  of  the  expansion  of  the  levator  palpebrse 
separates  the  smaller  gland  from  the  larger,  without,  however,  quite  di- 
viding them  (Fig.  18).  The  greatest  diameter  of  the  gland  is,  on  the 
average,  2  centimetres.  It  is  situated  above  the  conjunctiva  and  outside 
of  the  expansion  of  Tenon's  capsule.  The  ducts  of  the  larger  gland  pass 
through  the  smaller  and  receive  communications  from  it.  The  chief  ducts 
are  some  three  or  five  in  number,  but  with  them  are  a  few  smaller  ones 
(Sappey).  These  pierce  the  conjunctiva  under  the  outer  part  of  the  upper" 
lids.  The  lacrymal  canals  (Figs.  21,  24,  and  25)  which  collect  the  tears 
begin  at  the  points  of  the  minute  lacrymal  papillae,  which  are  almost  oppo- 
site to  each  other  at  the  beginning  of  the  lacrymal  bay.  The  papillae  are 
directed  obliquely  inward  towards  the  globe.  The  opening  in  the  upper  is 


AND   THE   APPENDAGES   OF   THE   EYE. 


93 


somewhat  the  smaller.  The  first  part  of  the  duct  is  vertical,  running  upward 
in  the  upper  lid  and  downward  in  the  lower  for  some  2  millimetres. 
They  then,  turning  at  a  slightly  acute  angle,  run  horizontally  for  5  or 


FIQ.  23. 


PIG.  24. 


1,  Inner  wall  of  orbit  ;  2,  inner  part  of  orbicularis  ;  3,  its  origin  from  inner  wall  ;  4,  small  opening 
for  nasal  artery  and  infra-trochlear  nerve  ;  5,  Homer's  muscle  (note  remarks  In  text)  ;  6,  Meibomian 
glands  ;  7,  lacrymal  gland  ;  8,  its  accessory  portion  ;  9,  chief  ducts  ;  10,  ducts  of  accessory  portion  ;  11, 
openings  of  the  ducts  ;  12  and  13,  lacrymal  papillae.  (From  Sappey.) 

6  millimetres  to  the  sac,  which  they  may  enter  either  separately  or,  as  is 

probably  the  more  usual  occurrence,  by  a  common  termination  (Fig.  24). 

There  is  a  dilatation  at  the  junction  of  the  vertical  and 

horizontal  portions  which  is  called  a  diverticulum.      The 

horizontal  part  hardly  deserves  this  name,  for  the  ducts 

converge  to  their  point  of  junction.     It  is  evident  that 

if  the  eye  is  closed  they  are  more  nearly  horizontal  and 

parallel  than  if  it  is  open.     Horizontal  sections  (Fig.  25) 

show  that  the  superior  canal  is  curved,  with  the  convexity 

in  front,  while  the  lower  is  about  straight.      They  enter 

the  tear-sac  just  behind  the  outer  end  of  the  internal  pal- 

pebral  ligament.     On  their  way  they  run  close  under  the 

thin  skin  at  the  edge  of  the  lids,  surrounded  by  spiral 

muscular  fibres  from  Homer's  muscle,  by  which  they  are 

J  ... 

compressed  many  times  a  minute  in  unconscious  winking 
(Krehbiel).  As  to  size,  the  superior  point  is  .2  millimetre 
in  diameter,  the  inferior,  .25.  Less  than  a  millimetre 
farther  on,  both  are  narrowed  to  .1  millimetre.  The  diameter  then  in- 
creases to  .6  millimetre.  The  widest  portion  is  the  collecting  tube,  the 
width  of  which  is  in  inverse  ratio  to  its  length.  The  lacrymal  sac  is 
situated  in  the  groove  at  the  inner  side  of  the  orbit  (Fig.  25).  Its  con- 
tinuation in  the  canal  is  called  the  nasal  duct.  There  is  a  pretty  well 
marked  contraction  at  the  end  of  the  sac,  the  duct  being  smaller.  A  vari- 


Tear-sac  from  a 

metal  cast   in  the 

warren  Museum  of 
Medi" 


94 


THE   ANATOMY   OF   THE   ORBIT 


FlQ.  25. 


L.  C.  Conj. 


L.  8. 


able  fold  is  sometimes  found  between  the  two.     The  sac  extends  upward 
some  2  or  3  millimetres  above  the  internal  palpebral  ligament.     The  direc- 
tion of  the  canal  and  the  position  of  the  sac  have  already  been  described 
in  the  surface  anatomy.     On  the  back  of  the  sac  lies  the  belly  of  Homer's 
muscle,  arising  from  the  so-called  reflected  tendon  of  the  orbicularis.     It  is 
very  doubtful  if  its  contraction  compresses  the  sac.     The  diameter  of  the 
sac  when  distended  is  some  6  or  7  millimetres,  its  length  about  12  milli- 
metres.    The  length  of  the  succeeding  nasal 
duct  is  very  variable.     According   to  Von 
Gerlach,  it  is  from  12  to  14  millimetres, — 
about  the  same  as  that  of  the  sac.     He  states 
that  it  is  often  prolonged  at  the  lower  end 
in  the  thickness  of  the  mucous  membrane,  so 
as  to  reach  a  length  of  20  millimetres.     The 
nasal  duct  in  the  cast  which  we  figure  is  even 
longer,  23  millimetres.    Its  greatest  diameter 
is  4  millimetres.      Frozen  sections  (Fig.  25) 
show  clearly  that  in  the  'undistended  state 
the  antero-posterior  diameter  of  both  sac  and 
duct  exceeds  the  transverse.     A  system  of 
veins  surrounds  the  duct  in  its  canal.     Henle 
cited  it  as  an  instance  of  what  he  called  com- 
pressible cavernous  tissue.     It  gives  a  sup- 
port to  the  walls,  and  probably  when  con- 
gested can  quite  compress  the  cavity.     The 

opening  of  the  duct  under  cover  of  the  inferior  turbinate  bone  varies  in 
shape  and  position.  Perhaps  it  is  most  frequently  a  vertical  or  an  oblique 
slit.  Sometimes  it  is  a  transverse  line  under  a  fold  of  mucous  membrane 
which  is  practically  a  valve.  It  does  not  always  end  with  the  bony  canal, 
but  may  run  for  a  variable  distance  in  the  mucous  membrane.  The  dis- 
tance from  the  posterior  border  of  the  nasal  opening  is  given  by  Arlt  as 
30  or  35  millimetres,  by  Von  Gerlach  as  28  or  30  millimetres.  In  no  one 
of  eight  measurements  by  Dr.  Tenney  was  it  as  much  as  28  millimetres.1 
The  distance  from  the  front  of  the  inferior  turbinate  is  given  by  Von  Ger- 
lach as  8  or  10  millimetres.  Our  measurements  of  ten  specimens  gave  an 
average  of  a  little  less  than  12  millimetres.  The  extremes  were  10  and  16 
millimetres.  Valves  have  been  described  in  various  parts  of  the  tear- 
passages.  They  are  for  the  most  part  irregular  and  inconstant  folds.  The 
most  constant  and  the  best  marked  seems  to  be  at  the  junction  of  the  sac 
and  the  nasal  duct.  It  is  rarely,  however,  a  perfect  valve.  The  opening 
at  the  duct  into  the  inferior  meatus,  while  presenting  no  true  valve,  is  of  a 
nature  to  impede  regurgitation. 


inf.  a 


Horizontal  section  through  inner 
half  of  right  orbit.— L.  S.,  lacrymal 
sac ;  L.  C.,  lacrymal  canal ;  Conj.,  con- 
junctiva; Int.  R.,  internal  rectus  and 
expansion  from  its  sheath;  Inf.  R.,  in- 
ferior rectus. 


1  Perhaps  the  reason  for  this  discrepancy  is  that  these  measurements  were  made  from 
the  junction  of  skin  and  mucous  membrane. 


AND   THE   APPENDAGES   OF  THE   EYE. 


95 


VI. 

The  optic  nerve  passes  from  the  optic  foramen  in  a  slightly  sinuous 
course  downward  and  outward  to  the  globe.  As  it  descends  it  passes  some- 
what beyond  what  would  be  a  straight  course,  so  that  it  has  to  bend  in 
again.  There  is  much  variation  in  the  amount  of  these  curves,  which,  in 
fact,  may  differ  in  the  two  eyes.  The  outer  sheath  of  the  nerve,  which  is 
continued  into  the  sclerotic,  comes  from  the  dura  through  the  optic  foramen. 
It  is  therefore,  of  course,  attached  to  the  edges  of  that  opening,  and  also  at 
the  same  place  to  the  periosteum  lining  the  orbit. 

VII. 

The  muscles  inside  the  orbit  are  the  levator  palpebrse  superioris,  the 
four  straight,  and  the  superior  and  inferior  oblique  muscles.  All  except  the 
last-named  arise  near  the  optic  foramen.  The  origin  of  the  four  recti  has 
been  variously  described  with  very  unnecessary  complications.  They  spring 
in  common  from  a  short  tendinous  tube  which  includes  the  optic  foramen 
and  the  inner  part  of  the  sphenoidal  fissure.  It  therefore  is  oval  on  section, 
the  long  diameter  being  placed  transversely.  It  springs  from  the  edges  of 
the  optic  foramen  on  its  inner  side  and  above,  inseparably  connected  with 


FIG.  26. 


F.N 


Frontal  frozen  section  of  left  orbit,  seen  from          Section  about  five   millimetres  behind  globe, 
before,  about  twelve  millimetres  behind  globe.—  Letters  as  in  preceding  figure. 

S.  R.,  superior  rectus;  L.  P.,  levator;  S.  O.,  supe- 
rior oblique;  In.  R.,  internal  rectus;  Inf.  R.,  in- 
ferior rectus;  E.  R.,  external  rectus;  O.  N.,  optic 
nerve;  F.  N.,  frontal  nerve;  Inf.  0.  N.,  infra- 
orbital  nerve;  6th,  sixth  nerve. 

the  sheath  of  the  optic  nerve,  giving  origin  to  the  internal  and  superior 
recti.  Below,  it  passes  from  under  the  foramen  transversely  outward  across 
the  sphenoidal  fissure,  giving  origin  to  the  inferior  rectus.  The  external 
rectus  springs  from  the  outer  border  of  the  fissure  and  then  from  a  fibrous 
band  which  crosses  it  again,  having  its  lower  attachment  to  the  spine  (Figs. 
1  and  6),  usually  found  on  the  outer  border  of  the  fissure.  The  upper  part 
of  the  external  rectus  is  continuous  with  the  outer  border  of  the  superior 
rectus.  In  fact,  the  edges  of  these  four  muscles  are  continuous  along 

7  o  (~~3 

this  fibrous  band.     When   the  origin  of  the  external  rectus  is  dissected 


96 


THE   ANATOMY   OF   THE   ORBIT 


FIG.  28 


8.  O. 


thoroughly  from  the  outside,  the  fibrous  band  crossing  the  fissure  and  arch- 
ing over  the  third  nerve  gives  the  muscle  the  appearance  of  springing  from 
either  side  of  the  fissure ;  hence  it  is  usually  called  two-headed,  but  this  is 
improper,  for  the  muscular  fibres  arise  in  an  unbroken  series,  or  at  least 
any  gap  between  them  is  filled 
by  membrane.  The  levator 
arises  just  above  the  inner  part 
of  the  superior  rectus,  some- 
what overlapping  it  to  that 
side.  Very  near  it,  but  lower, 
inside  the  upper  part  of  the 
internal  rectus,  arises  the  su- 
perior oblique.  Having  given 
the  origin  of  these  muscles,  we 


In.  R. 


In£R 


IniO.N. 


Section  about  three  millimetres  in  front  of  back  of  globe. 
Letters  as  before.— S.  O.  N.,  supra-orbital  nerve. 


L.P. 


FIG.  29. 


S.  O.  N. 


leave  them  to  return  to  the  recti, 
which  form  part  of  a  muscular 
cone,  outside  of  which  lie  the 
others.  The  fibrous  ring  from  which  they  arise  sends  tendinous  fibres  into 
the  ocular  side  of  the  muscles.  On  the  outer  aspect  they  speedily  become 
muscular.  They  are  composed  of  straight  parallel  fibres  with  very  little 
connective  tissue  between  them.  The  muscular  bellies  soon  reach  their 
greatest  thickness,  and,  as  is  well  shown  in  Fig.  26,  they  occupy  much 
of  the  "space  of  the  orbit  near  the  apex.  Fig.  27,  near  the  back  of  the 
eye,  shows  some  of  them  smaller.  Fig.  28,  through  the  posterior  portion  of 

the  globe,  shows  them  much 
reduced  in  thickness  and  more 
separated.  In  Fig.  29  they  are 
still  outside  of  the  capsule  of 
Tenon  and  becoming  tendinous. 
The  fibrous  tissue  forming  the 
sheaths  becomes  more  delicate 
as  they  pass  forward.  The  re- 
lations of  these  muscles  may  be 
followed  on  the  frontal  sections 
(Figs.  26  to  29)  and  also  on  the 
sagittal  sections  (Figs.  18  to 
21).  Certain  very  important 
expansions  from  the  sheaths  will 
be  considered  later.  The  muscles,  having  become  tendinous,  perforate  or, 
more  properly,  invaginate  the  capsule  of  Tenon,  and,  passing  under  the 
conjunctiva,  are  inserted  into  the  sclerotic  at  from  5  to  8  millimetres  from 
the  cornea.  The  recti  differ  among  themselves  in  many  respects  :  in  size,  in 
the  length  of  the  anterior  tendon,  in  its  breadth  at  the  line  of  insertion, 
and  in  the  distance  of  this  line  from  the  cornea.  These  points  are  shown 
in  the  following  table,  compiled  from  Volkmann,  Merkel,  and  Fuchs  : 


8.0 


S.  R. 


E.E. 


Inf.  E. 


Section  near  equator  of  globe.   Letters  as  before.— IvJ.O., 
inferior  oblique;   L.  S.,  lacrymal  sac. 


AND   THE   APPENDAGES   OF   THE    EYE. 


97 


Weight  of 
Muscle. 

Length  of 
Tendon. 

Breadth  at 
Insertion. 

Distance  from 
Cornea. 

Authority                       Volkmann. 

Merkel 

Fuchs 

Fuchs 

Gramme. 
Internal  rectus.           .        .    .        .            .747 

Millimetres. 
8.8 

Millimetres. 
103 

Millimetres. 
6  5 

Inferior  rectus  .        .    .        ....            .671 

6.6 

98 

6  5 

Kxi'Tnal  rectus            .        .        .                .715 

3  7 

92 

6  9 

Superior  rectus         .514 

68 

106 

7  7 

It  is  needless  to  say  that  in  several  of  these  respects  the  variation  is 
considerable.  It  is  worth  noting  that  the  rectus  internus  is  the  heaviest 
(presumably,  therefore,  the  strongest)  muscle,  and  that  being  attached  near- 
est to  the  cornea  it  has  a  decided  mechanical  advantage.  The  superior  is 
both  the  weakest  and  the  worst  placed.  It  appears  from  the  last  column 
of  the  above  table  that  the  insertions  of  the  recti  may  be  diagrammatically 
represented  by  thickenings  on  a  spiral  line  round  the  cornea,  which,  starting 
at  5.5  millimetres  from  its  inner  border,  passes  downward  and  outward, 
upward  and  inward,  gradually  receding  from  it.  There  is,  of  course,  much 
individual  variation.  Fuchs  found  no  difference  in  the  distance  of  insertion 
between  normal  and  myopic  eyes,  but  the  breadth  of  the  line  of  insertion 
is,  on  the  average,  greater  in  the  latter.  As  a  rule,  whether  inserted  nearer 
or  farther,  the  muscles  retain  their  relative  distances.  According  to  Fuchs, 
the  greatest  irregularity  is  in  the  inferior  and  external  recti,  which  are  often 
at  the  same  distance.  In  a  number  of  measurements  made  for  this  work 
it  was  found  that  the  external  and  superior  were,  about  the  same  distance 
from  the  cornea  at  a  point  about  half-way  between  Fuchs's  figures, — that  is, 
from  7.1  to  7.3  millimetres.  MerkeFs  measurements  agreed  with  those  of 
Fuchs  for  the  inferior  and  external,  but  he  found  the  internal  and  inferior 
more  distant  from  the  cornea.  Fuchs's  results  deserve  to  remain  the  stand- 
ard ones,  but  it  is  clear  that  there  is  a  good  deal  of  irregularity.  The 
lines  of  insertion  are  not  parallel  to  the  edge  of  the  cornea.  The  superior 
has  the  end  of  its  inner  edge  much  in  advance  of  the  other,  and  most  of 
its  insertion  is  usually  on  the  outer  half  of  the  eye.  The  inferior,  though 
more  symmetrical,  has  its  outer  end  a  little  in  front.  There  is  a  consider- 
able amount  of  loose  connective  tissue  between  the  eyeball  and  the  inser- 
tions of  the  tendons  and  also  at  their  sides,  so  that  it  requires  dissection  to 
ascertain  the  precise  position  of  their  borders.  As  the  axes  of  the  orbits 
diverge  considerably,  while  the  axes  of  the  eyes  are  parallel,  it  is  easy  to 
see  that  botli  the  superior  and  inferior  recti,  being  inserted  far  in  front  of 
the  equator,  must,  in  addition  to  the  obvious  action  of  either,  turn  the  eye 
inward.  At  the  same  time  they  probably  cause  a  certain  amount  of  rota- 
tion, the  superior  rolling  a  point  on  the  upper  border  of  the  cornea  inward 
and  downward,  the  inferior  rolling  it  outward  and  downward.  Evidently 
peculiarities  in  the  insertion  of  one  of  these  muscles  must  affect  the  range 
and  direction  of  these  movements.  The  internal  and  external  recti  move 
VOL.  I.— 7 


98  THE  ANATOMY   OF  THE  ORBIT 

the  pupils  directly  inward  or  outward.  The  origin  of  the  superior  oblique 
muscle  has  been  given.  It  quickly  gets  into  the  upper  inner  part  of  the 
orbit  (Fig.  33),  lying  well  outside  the  cone  formed  by  the  four  recti,  be- 
comes a  small  round  fibrous  band  as  it  plays  through  the  pulley,  and  then 
expands  into  a  tendon  which,  passing  backward  and  outward  between  the 
globe  and  the  superior  rectus,  is  inserted  as  follows.  The  anterior  point  of 
the  line  of  insertion  lies  about  as  far  outward  as  the  outer  end  of  the  supe- 
rior rectus,  and  is  about  as  far  behind  the  equator  as  the  latter  is  before  it. 
From  this  point  the  line,  according  to  the  more  usual  arrangement,  runs  in 
a  curve  backward  and  inward,  to  end  on  the  inner  side  of  the  meridian.  In 
certain  cases  it  runs  more  directly  backward,  being  throughout  its  course 
external  to  the  meridian.  In  the  former  class  the  line  of  insertion  ap- 
proaches the  direction  of  the  equator,  in  the  latter  that  of  the  meridian. 
Either  form  may  be  found  in  a  normal  or  in  a  myopic  eye,  but  the  latter 
is  essentially  that  of  the  myopic.  The  breadth  of  the  insertion  ranges 
from  6.8  to  14  millimetres,  being  narrower  on  the  average  in  myopic  eyes 
(Fuchs).  The  inferior  oblique  springs  from  the  floor  of  the  orbit  just  in- 
side the  opening  for  the  nasal  duct.  It  is  almost  entirely  muscular.  Pass- 
ing between  the  floor  of  the  orbit  and  the  inferior  rectus,  it  curls  round  the 
globe,  to  be  attached  far  back  in  the  outer  and  inferior  region  of  the  back 
of  the  eye.  The  distance  of  its  insertion  from  the  outer  sheath  of  the  optic 
nerve  averages  5.2  millimetres  for  normal  eyes  and  7.1  for  myopic  ones.  In 
the  former  the  breadth  of  the  insertion  is  9.4  millimetres,  in  the  latter,  10.5 
millimetres.  The  superior  oblique  as  a  factor  in  the  movements  of  the 
eye  must,  for  obvious  mechanical  reasons,  be  supposed  to  start  from  the 
pulley.  Thus  the  two  obliques  practically  leave  the  upper  and  lower  inner 
angles  of  the  base  of  the  orbit  and  pass  backward  and  outward  to  the  pos- 
terior outer  portion  of  the  globe.  Each  oblique  turns  the  pupil  outward. 
The  superior,  in  addition,  turns  it  downward  and  rotates  (a  point  at  the 
top  of  the  cornea)  inward.  The  inferior  turns  it  upward  and  rotates  out- 
ward. It  is  customary  to  teach  that  the  inferior  oblique  corrects  the  effect 
of  the  obliquity  of  the  superior  rectus,  so  that,  both  acting  together,  the  eye 
is  turned  straight  upward,  while  the  inferior  rectus  and  the  superior  oblique 
turn  it  straight  downward.  Theoretically  this  is  tolerably  certain,  but  prac- 
tically it  is  to  be  remembered  that  it  is  very  unlikely  that  one  or  even  two 
of  these  muscles  ever  act  alone.  The  accuracy  of  any  movement  is  due  not 
alone  to  the  pull  of  the  muscle  to  which  it  is  usually  ascribed,  but  also  to 
the  graduated  resistance  of  the  antagonists.  Probably  the  simplest  move- 
ment of  the  eye  is  made  by  the  more  or  less  active  concurrence  of  all  the 
muscles.  The  levator  palpebrae  superioris  runs  close  above  the  inner  por- 
tion of  the  superior  rectus.  The  areolar  tissue  between  them  is  so  slight  that 
transverse  vertical  sections  show  them  in  the  back  part  of  the  orbit  almost 
as  one  muscle.  Anteriorly  they  are  quite  distinct,  as  the  rectus  (except  its 
expansion)  sinks  to  the  eye  and  the  levator  expands  into  the  broad  layer 
already  described,  stretching  pretty  nearly  across  the  roof  of  the  orbit. 


AND  THE   APPENDAGES   OF   THE   EYE.  99 

VIII. 

The  eyeball,  from  the  optic  nerve  to  near  the  border  of  the  cornea,  is 
enclosed  by  a  delicate  membrane,  called  the  capsule  of  Tenon.  As  the  com- 
plications of  this  membrane  are  limited  only  by  the  perverted  ingenuity  of 
those  who  describe  it,  let  it  be  understood  that  in  this  paper  is  meant  only 
the  capsule  around  the  globe.  Near  the  cornea  the  conjunctiva  and  the 
capsule  of  Tenon  fuse  into  a  single  membrane.  The  arrangement  of  the 
capsule  may  be  understood  by  following  the  steps  of  the  dissection  shown 
in  Fig.  30.  A  cut  with  a  very  sharp  knife  was  made  close  around  the 
cornea,  dividing  the  membrane  formed  by  the  capsule  of  Tenon  and  the 
conjunctiva.  This  was  then  turned  back  from  the  eyeball  by  a  blunt  in- 
strument. The  outer  canthus  was  cut  to  gain  room,  and  the  membrane  re- 
flected and  stitched  to  the  integument. 

This   exposes  a  cavity  between   the  FlG-  80- 

globe  and  the  membrane.  At  the  free 
edge,  of  course,  this  membrane  is  made 
of  both  conjunctiva  and  Tenon's  cap- 
sule, but  these  two  separate  within 
three  millimetres  from  the  edge  of  the 
cut.  What,  therefore,  is  seen  on  the 
inside  of  the  reflected  membrane  is 
wholly  Tenon's  capsule.  It  is  shown 
diagrammatically  in  Figs.  18  to  21.  It 

is    a    delicate    membrane    enclosing    a      Dissection  of  capsule  of  Tenon,  by  Dr.  Tenney. 

lymph-space  between   itself  and  the 

globe  and  separating  the  latter  from  the  fat  of  the  orbit.  Bands  of  con- 
nective tissue  run  through  this  space.  They  are  particularly  numerous  in 
the  posterior  part.  Authorities  differ  as  to  the  place  at  which  the  capsule 
ends  behind.  It  goes  to  the  optic  nerve.  Nevertheless,  Schwalbe  has 
shown  that  a  lymph-space  around  the  optic  nerve  can  be  injected  from  it. 
It  would  seem  that  this  could  take  place  through  small  openings,  especially 
at  the  points  of  entrance  of  the  ciliary  vessels  and  nerves,  and  that  the  cap- 
sule may  be  said  to  reach  the  optic  nerve,  as. gross  appearances  indicate. 
It  is  taught  as  by  common  consent  that  the  capsule  of  Tenon  is  a  socket 
in  which  the  eyeball  rotates  without  change  of  position,  except,  perhaps, 
that  under  certain  circumstances  it  may  move  a  minute  distance  forward  or 
backward.  Anatomy  shows  that  this  is  impossible.  It  is  easy  to  see  that 
as  Tenon's  capsule  is  closely  attached  to  the  globe  near  the  cornea,  it  is  out 
of  the  question  that  the  former  should  stand  still  while  the  latter  moves  in 
it.  Undoubtedly  the  two  move  together  on  the  cushion  of  fat  behind  them, 
and  perhaps  some  slight  motion  may  occur  between  them.  The  muscles 
pierce  or,  more  properly,  invaginate  this  membrane.  The  folds  shown  in 
the  figure  at  their  points  of  passage  through  it  are  caused  by  its  position  as 
dissected. 


100  THE   ANATOMY   OF   THE   ORBIT 

From  the  fibrous  tissue  forming  the  sheaths  of  the  recti  strong  expan- 
sions pass  off  before  the  muscles  pass  through  the  capsule  of  Tenon.  Those 
of  the  superior  and  inferior  recti  have  been  described  with  the  lids.  Analo- 
gous but  stronger  ones  pass  off  laterally  not  only  to  the  bones  of  the  base 
of  the  orbit,  but  to  the  angles  of  the  lids  and  the  conjunctiva.  The  ex- 
ternal one  goes  also  to  the  so-called  external  palpebral  ligament,  and  the 
inner  goes  also  to  the  reflected  tendon  behind  the  tear-sac  (Fig.  25).  These 
not  only  admit  of  dissection,  but  are  beautifully  shown  on  frozen  sections. 
Although  the  fibrous  tissue  of  the  sheaths  of  the  muscles  is  undoubtedly 
continuous  with  that  of  the  capsule  of  Tenon,  it  seems  unnecessary  to  make 
them  expansions  of  the  latter,  as  is  done  by  Sappey  and  others.  Fig.  25, 
though  somewhat  diagrammatic  in  execution,  is  from  an  actual  section,  and 
gives  a  true  representation  of  the  fibres.  A  similar  view  is  to  be  found  in 
Von  Gerlach's  work.  Almost  all  other  writers  represent  these  expansions 
as  nearly  transverse  bands  passing  more  to  the  walls  of  the  orbit  than  to 
the  soft  parts.  The  effect  of  these  expansions  is  partly  to  steady  the  eye- 
ball and  to  resist  the  backward  pull  of  the  recti,  partly  to  draw  the  lids 
and  the  fold  of  the  conjunctiva  in  harmony  with  the  movements  of  the 
eye  (Fig.  12).  Quite  a  strong  fascia  covers  the  tendon  of  the  superior 
oblique  from  the  globe  to  the  pulley.  Besides  these,  Lockwood  has  de- 
scribed a  hammock-shaped  sling  of  fibrous  tissue  more  or  less  connected 
with  the  capsule  and  with  other  processes  of  orbital  fascia,  which,  fastened 
at  either  side  of  the  orbit,  supports  the  globe.  It  is  best  seen  when  dis- 
sected from  below. 

The  orbit  .is  filled  with  fat,  which  is  bounded  in  front  by  the  capsule 
of  Tenon  and  the  fibrous  expansions.  The  fat  of  the  orbit  deserves 
special  notice.  It  is  not  all  of  the  same  kind.  Inside  the  space  bounded 
by  the  muscular  cone,  and  in  front  by  the  capsule  of  Tenon  enclosing 
the  eye,  the  fat  is  very  delicate  and  loosely  packed,  especially  around  the 
optic  nerve  and  the  delicate  ciliary  vessels  and  nerves.  Frozen  sections, 
when  fresh,  show  in  many  places  most  delicate  fasciae  dividing  the  fat. 
Both  transverse  and  longitudinal  sections  show  a  thin  irregular  membrane 
at  some  distance,  say  from  three  to  ten  millimetres,  from  the  optic  nerve 
bounding  Schwalbe's  supra-vaginal  space.  It  may  be  doubted  whether 
this  constitutes  an  absolutely  continuous  tube.  It  may  also  be  doubted 
whether  it  entirely  limits  this  most  delicate  fat  which  forms  an  almost 
fluid  support  for  the  eye,  well  adapted  for  its  movements.  At  certain 
places  in  the  orbit  the  fat  is  very  different,  being  collected  into  large  and 
comparatively  hard  masses.  One  of  these  extends  from  the  inside  of 
the  lacrymal  gland  to  the  posterior  surface  of  that  organ,  another  is  in 
the  lower  part  of  the  front  of  the  orbit.  It  probably  helps  to  make  the 
prominence  of  the  lower  lid  which  is  so  striking  when  the  eye  is  turned 
upward.  In  frozen  sections  lines  of  thin  fascia  are  seen  cutting  off  these 
and  similar  collections.  There  is  also  fat  of  various  degrees  of  density 
between  these  two  extremes. 


AND   THE   APPENDAGES   OF   THE   EYE.  101 

IX. 

The  ophthalmic  artery  arises  inside  the  skull  from  the  internal  caro- 
tid just  after  that  vessel  has  pierced  the  dura.  It  passes  into  the  orbit 
through  the  optic  foramen  below  and  to  the  outer  side  of  the  optic  nerve. 
If  we  remember  that  the  outer  sheath  of  the  optic  nerve  is  continuous 
with  the  dura  along  the  margin  of  the  optic  foramen,  it  is  clear  that  at 
first  the  artery  must  lie  within  the  sheath.  It  leaves  it  very  soon,  run- 
ning between  the  nerve  and  the  beginning  of  the  external  rectus;  then 
curving  forward  and  inward  to  run  nearly  transversely,  it  crosses  over  the 
nerve  below  the  superior  rectus  and  reaches  the  upper  inner  angle1  of  the 
orbit  far  back.  It  then  runs  forward  below  the  superior  oblique,  perforates 
the  upper  lid  below  the  trochlea,  and  divides  into  the  nasal  and  frontal 
arteries.  As  Sappey  remarks,  its  branches  may  be  divided  into  three  sets, 
— those  going  to  the  globe,  those  of  the  appendages,  and  those  which  merely 
pass  through  the  orbit  to  go  elsewhere. 

The  first  class  comprises  the  central  artery  of  the  retina  and  the  ciliary 
arteries ;  the  second,  the  branches  to  the  muscles,  the  lacrymal  apparatus, 
and  the  lids ;  the  third,  the  terminal  branches  and  the  ethmoidal  arteries. 
Be  it  noted,  however,  that  the  anterior  ciliaries  come  from  branches  of  the 
second  set.  There  are  usually  two  ciliary  arteries,  an  outer  and  an  inner, 
each  of  which  divides  into  many  branches  which  pierce  the  sclerotic  near 
the  optic  nerve.  The  number  of  these  secondary  branches  may  reach 
twenty,  but  this  is  uncommon.  Two,  called  the  long  ciliaries,  after  enter- 
ing the  globe,  run  to  the  anterior  part  of  the  eyeball,  but  they  are  not  easy 
to  distinguish  from  the  other  branches  while  in  the  orbit,  though  said  to  be 
larger.  The  anterior  ciliary  arteries,  some  half-dozen  in  number,  are  twigs 
of  the  muscular  branches  and  perhaps  of  the  lacrymal  and  supra-orbital, 
which,  having  divided  once  or  twice,  pierce  the  sclerotic  in  a  ring  near  the 
margin  of  the  cornea.  The  two  posterior  ciliaries  and  the  central  artery 
of  the  retina  are  the  first  branches  of  the  ophthalmic.  According  to  Meyer,2 
who  has  written  a  very  valuable  paper  on  the  orbital  arteries,  the  normal 
arrangement  is  for  the  central  artery  and  the  inner  of  the  posterior  ciliaries 
to  arise  in  common,  very  often  within  the  sheath  of  the  optic  nerve,  while 
the  outer  one  arises  a  little  later.  The  central  artery  pierces  the  nerve 
within  1.5  centimetres  of  the  globe.  The  next  branch  is  the  lacrymal, 
which  arises  from  the  outer  side  of  the  vessel  to  run  forward  to  the  gland 
along  the  upper  border  of  the  outer  rectus.  It  supplies  by  its  terminal 
branches  the  outer  part  of  the  lids  and  of  the  conjunctiva,  anastomosing 
with  arteries  of  the  face.  It  has  an  important  connection  soon  after  its  origin 
with  a  branch  from  the  middle  meningeal  which  enters  the  orbit  through  the 

sphenoidal  fissure.    The  importance  of  this  lies  in  the  changes  in  the  plan  of 

. 1 ~ 

1  The  word  "angle"  is  used  for  convenience.  Sections  show  that  the  term  is  inexact 
as  expressing  the  shape  of  the  orbit. 

8  Morphologisches  Jahrbuch,  Bd.  xii.,  1887. 


102 


THE   ANATOMY   OF   THE   ORBIT 


the  orbital  arteries  which  may  spring  from  an  early  variation  in  the  course 
of  blood  through  this  system.  This  may  lead  to  the  lacrymal  arising  from 
the  middle  meningeal,  or,  more  important  still,  to  the  ophthalmic  becoming 
a  branch  of  that  artery  and  reaching  the  orbit  through  the  fissure,  or  through 
a  special  canal.  The  muscular  branches,  two  or  three  in  number,  are  pretty 
uncertain  in  their  mode  of  origin.  The  supra-orbital,  which  is  neither  con- 
stant nor  important,  runs  in  the  upper  part  of  the  orbit  at  first  to  the  inner 

FIG.  31. 


Frontal. 


Nasal. 


Ant.  ethm. 

Post.  ethm. 
Ophthal. 

Inf.  int.  muse. 
Opt.  for. 


Ext.  palp. 


Cent.  ret. 
Lacrym. 

Ciliary. 


Sup.  ext.  muse. 
To  meuingeal. 
Sphen.  fissure. 


Int.  carotid. 
The  orbital  arteries.    (From  Quain's  Anatomy,  after  Meyer.) 

side  of  the  levator  palpebrae  and  then  above  it  to  the  supra-orbital  notch. 
The  two  ethmoidal  arteries  leave  the  orbit  by  the  little  ethmoidal  foramina 
between  the  frontal  bone  and  the  os  planum  of  the  ethmoid  ;  the  posterior 
may  be  a  branch  of  the  supra-orbital,  t{ie  anterior  comes  from  the  main 
trunk.  The  palpebral  branches  have  been  described  with  the  lids.  It  re- 
mains only  to  state  that  they  may  arise  either  separately  or  in  common,  and 
pass  out  of  the  orbit  above  the  internal  palpebral  ligament.  Meyer  gives 


AND   THE   APPENDAGES   OF   THE   EYE. 


103 


the  following  as  the  order  of  the  origin  of  the  branches  of  the  ophthalmic 
artery :  1,  the  retinal  artery  with  the  inner  ciliary ;  2,  outer  ciliary ;  3, 
lacrymal ;  4,  upper  and  outer  muscular ;  5,  supra-orbital  with  posterior 
ethmoidal ;  6,  inner  and  lower  muscular ;  7,  anterior  ethmoidal ;  8,  final 
division  into  palpebral,  frontal,  and  nasal.  Variations  of  the  lacrymal 
artery  have  been  discussed.  The  other  important  variation  is  of  the  course 
of  the  ophthalmic  itself,  which  sometimes  passes  under  the  optic  nerve  in- 
stead of  over  it.  The  arteries  of  the  orbit  are  tortuous  and  but  lightly 
connected  with  the  fat  of  the  orbit,  so  that  it  is  easy  for  them  to  yield  to 
the  varying  pressure  depending  on  the  swelling  of  the  muscles,  on  the  move- 
ments of  the  globe,  or  on  increased  pressure  from  the  heart  or  increased 
resistance  from  distended  veins. 


FIG.  32. 


CarS. 


F.V. 


Veins  of  the  orbit.  (From  Sesemann.)  Cav.  S.,  cavernous  sinus ;  Com.  V.,  communicating  branch 
between  Sup.  Oph.  V.,  superior  ophthalmic  vein,  and  Ir\f.  Oph.  V.,  inferior  ophthalmic  vein  ;  P.  V.,  facial 
vein  ;  V.  Oph.fac.,  ophthalmo- facial  vein  passing  through  spheno-maxillary  fissure. 

The  veins  of  the  orbit  form  a  very  free  system  of  anastomosing  vessels 
between  the  sinus  cavernosus  inside  the  skull,  the  facial  vein  on  the  sur- 
face of  the  face,  and  the  tributaries  of  the  internal  maxillary  through  the 
spheno-maxillary  fissure.  It  seems  reasonably  certain  that  the  blood  from 
the  lids  returns  by  the  superficial  veins,  that  the  blood  from  the  eyeball 
returns  through  the  sinus  cavernosus,  and  that  most  of  the  blood  in  the 
orbit  can  in  case  of  need  flow  either  into  the  sinus  or  into  the  superficial 
or  deep  veins  of  the  face.  We  incline  to  the  opinion  that  under  ordinary 
circumstances  the  general  current  is  inward.  The  chief  vein  of  the  orbit 
is  the  superior  ophthalmic,  which  begins  at  the  upper  inner  angle  of  the 
orbit  by  communicating  branches  with  the  frontal  and  nasal  veins.  It  runs 


104  THE  ANATOMY   OF  THE  ORBIT 

backward  across  the  optic  nerve,  under  the  superior  rectus,  through  the 
sphenoidal  fissure,  to  the  cavernous  sinus.  The  vein  may  enter  the  sinus 
on  either  side  of  the  external  rectus.  Most  writers  ignore  this  point,  and 
the  statements  of  others  do  not  agree.  We  incline  to  the  opinion  that  the 
vein  usually  passes  inside  of  the  rectus.  This  vein  presents  a  remarkable 
narrowing  just  before  its  end,  and  sometimes  a  considerable  dilatation  near 
the  middle.  According  to  Sesemann,1  it  has  no  valves,  though  each  of  its 
tributaries  has  a  valve  at  the  point  of  entrance.  Its  contents,  therefore,  can 
escape  in  either  direction.  It  has,  moreover,  constant  connections  with  the 
inferior  ophthalmic  vein  by  a  small  communicating  branch  (Com.  V.,  Fig. 
32).  The  inferior  ophthalmic  vein,  which  is  much  smaller  than  the  pre- 
ceding, runs  near  the  floor  of  the  orbit  above  the  inferior  rectus  to  the 
cavernous  sinus,  or  sometimes  to  a  vein  called  by  Hyrtl  the  ophthalmo- 
meningeal.  It  begins  at  the  front  of  the  orbit  by  a  plexus  about  the 
inferior  oblique  which  drains  the  inferior  part  of  the  conjunctiva.  It  com- 
municates with  the  veins  of  the  face  through  the  spheno-maxillary  fissure. 
The  lacrymal  vein  opens  usually  into  the  superior  ophthalmic  by  a 
branch  called  the  ophthalmo-facial,  taking  the  blood  from  the  upper  con- 
junctiva as  well  as  from  the  gland.  The  smaller  veins  of  the  orbit  cor- 
respond in  the  main  to  the  arteries,  and,  with  two  exceptions,  call  for  no 
special  description.  The  venae  vorticosae,  four  or  five  in  number,  leave  the 
orbit  near  its  equator  and  pass  backward,  becoming  the  ciliary  veins,  to  open 
into  one  or  both  of  the  ophthalmic  veins.  The  question  has  been  raised 
as  to  whether  they  are  compressed  by  the  action  of  the  muscles.  Fuchs 
describes  them  as  in  two  pairs,  an  upper  and  a  lower,  each  with  an  inner 
and  an  outer  vein.  He  finds  that  the  recti  exert  no  action  on  them,  but 
that  one  upper  and  one  lower  may  be  compressed  by  the  oblique  muscles, 
especially  when  the  eye  is  in  the  position  for  looking  at  near  objects.  The 
alternate  compression  and  release  of  the  veins  help  the  circulation  of  the 
eyeball.  The  central  vein  of  the  retina  deserves  consideration  for  its  ob- 
vious importance.  At  first  it  runs  in  the  optic  nerve,  but  it  reaches  its 
outer  side  about  half-way  back.  It  soon  pierces  the  sheath  and  runs  to  the 
sinus  cavernosus.  Whatever  suction  may  occur  in  the  sinus  is  therefore 
easily  felt  by  the  blood  in  this  vein.  Sesemann  insists  on  the  point  that  the 
central  vein  has  always  at  least  one  side  branch.  The  most  common  connec- 
tion is  with  the  superior  ophthalmic.  Indeed,  this  branch  may  be  so  large 
as  to  make  it  doubtful  into  which  the  vein  may  be  said  to  empty.  This  pro- 
vides an  escape  for  the  blood  of  the  retina  in  case  of  thrombosis  of  the  sinus. 
Sometimes  this  vein  runs  into  a  delicate  plexus  around  the  optic  nerve. 

The  lymphatic  circulation  of  the  orbit  proper  is  partly  through  vessels 
and  partly  through  spaces.  The  vessels  are  said  to  be  few.  They  pass 
through  the  spheno-maxillary  fissure  to  the  deep  facial  glands,  a  small 
group  by  the  back  part  of  the  buccinator  and  the  side  of  the  pharynx. 

1  Archiv  fur  Anatomic  und  Physiologic,  1869, 


AND   THE    APPENDAGES   OF   THE    EYE. 


105 


The  spaces  through  which  lymph  passes  are  Tenon's  capsule  and  the  lymph- 
space  around  the  optic  nerve  into  which  it  drains  (Schwalbe's  supra- 
vaginal  space). 

X. 

The  nerves  of  the  orbit  are  the  third,  fourth,  and  sixth  pairs,  the  first 
division  of  the  fifth  pair,  and  some  sympathetic  fibres  from  the  cavernous 
plexus.  Besides  these,  but  hardly  belonging  to  this  subject,  may  be  men- 
tioned the  infra-orbital  while  in  its  groove,  and  the  orbital  branch  of  the 
superior  maxillary,  which  enters  the  orbit  through  the  spheno-maxillary 
fissure  to  divide  into  the  temporal  and  malar  branches,  which  leave  the  orbit 
through  their  respective  minute  canals  in  the  malar  bone.  According  to 
Sappey,  it  gives  another  branch  running  to  the  lacrymal  gland  and  to  the 
outer  angle  of  the  lids.  The  nerves  of  the  orbit  enter  it  through  the  sphe- 
noidal  fissure,  but  on  or  before  reaching  it  the  third  divides  into  an  upper 
and  a  lower  division,  and  the  fifth  into  the  lacrymal,  frontal,  and  nasal. 
They  pass  through  the  fissure  as  follows :  most  externally  and  highest  the 
lacrymal,  then  the  frontal,  and  then  the  fourth,  these  three  being  external 
to  the  outer  rectus.  Internal  to  that  muscle,  and  therefore  entering  the 
cone  formed  by  the  recti,  come  the  others.  The  upper  division  of  the  third 
passes  highest.  Below  it  comes  the  nasal,  then  the  lower  division  of  the  third 
and  the  sixth  on  about  the  same  level, 
the  sixth  being  external.  The  sym- 
pathetic filaments  do  not  seem  to  be 
very  regular  in  their  entrance.  They 
are,  at  all  events,  near  the  nasal  nerve. 
The  division  of  the  third  pair  oc- 
curs at  just  about  its  entrance  into 
the  orbit.  The  upper  division,  which 
is  the  smaller,  supplies  the  superior 
rectus  and  the  levator.  The  branch 
to  the  former  enters  the  muscle  on  its 
lower  side,  that  to  the  latter  passes  to 
the  inner  side  of  that  muscle  (some- 
times through  it)  to  the  under  sur- 
face of  the  levator.  The  larger  lower 
division,  passing  downward,  gives 
branches  to  the  inferior  and  internal 


FIG.  33. 


branch  of  same ;  7,  supra -trochlear ;  8,  nasal ;  9, 
infra-trochlear;  10,  nasal,  after  leaving  the  orbit. 


Nerves  seen  from  above.  (From  Sappey,  after 
Hirschfeld.)    The  Roman  numerals  indicate  the 
cranial  nerves  of  the  corresponding  numbers.    1, 
recti,  which  enter  them  through  their     Gasserian  ganglion ;  2,  first  division  of  fifth  pair ; 

superior  and  external  surfaces  respec-    3'  lacrymal;  4'  frontal:  5'  8"Pra-°rbital ; 

tively.     The  longest  branch  is  to  the 

inferior  oblique,  which  it  enters  from 

below.     This  gives  off  at  its  origin  the  short,  motor  root  of  the  lenticular 

ganglion. 

The  fourth  nerve  passes  above  the  optic  nerve  to  the  superior  oblique, 
which  it  enters  from  above  in  several  branches. 


106 


THE   ANATOMY   OF  THE   ORBIT 


The  sixth  nerve  has  a  short  course  in  the  orbit.  It  at  once  reaches  the 
inner  side  of  the  external  rectus,  and  soon  breaks  up  into  many  branches 
which  enter  that  muscle  (Fig.  34). 

There  are  certain  points  which  the  nerves  of  the  recti  have  in  common 
worthy  of  mention.  They  are  all  very  large  in  proportion  to  their  mus- 
cles. The  external  rectus  probably  receives  relatively  to  its  size  the  largest 
nerve  in  the  body.  The  nerves  all  break  up  into  a  sheaf  of  diverging 
branches  before  entering  the  muscles,  and  finally  each  enters  its  muscle  on 
the  side  nearest  to  the  optic  nerve. 

The  three  branches  of  the  first  division  of  the  fifth  pair  are,  of  course, 
purely  sensory.  The  frontal  nerve  runs  close  against  the  periosteum  of  the 
roof  of  the  orbit.  At  about  the  middle  of  the  orbit  it  gives  off  the  supra- 
trochlear,  which  reaches  the  upper  inner  angle  of  the  base  of  the  orbit, 
where  it  sends  a  twig  to  the  infra-trochlear  branch  of  the  nasal  and  leaves 
the  orbit.  Its  distribution  to  the  lids  and  conjunctiva  has  been  described. 
Beyond  the  origin  of  the  supra-trochlear  the  frontal  changes  its  name  to 
the  supra-orbital,  which  passes  through  the  notch  or  foramen,  dividing 
either  before  or  after  its  passage  into  two  chief  branches.  The  lacrymal 
nerve  (Fig.  33),  which  enters  higher  and  more  externally  than  any  other, 
gains  at  once  the  upper  and  outer  part  of  the  orbit  and  runs  above  the  ex- 
ternal rectus  to  the  gland  which  it  supplies.  Before  reaching  the  gland  it 
sends  down  a  communication  to  thev  orbital  branch  which  enters  through 
the  spheno-maxillary  fissure.  The  lacrymal  supplies  also  a  part  of  the 

conjunctiva  and  ends  in  the  upper 
eyelid.  The  nasal  branch  (Fig. 
34),  which  the  Germans  more  hap- 
pily call  the  naso-ciliary,  passes 
into  the  orbit  between  the  muscles 
much  lower  than  the  two  preceding. 
It  is  at  first  external  to  the  optic 
nerve  and  at  a  lower  level,  but  it 
passes  upward  and  inward,  cross- 
ing over  the  nerve  below  the  supe- 
rior  rectus  to  the  inner  wall  of  the 

,_,  orbit,  which  it  leaves  by  the  ante- 

Nerves  dissected  from  the  outside.  (From  Sappey,  • 

after  Hirschfeld.)    1,  optic  nerve  before  entering  the  rior  ethmoidal  foramen,  going  be- 

optic  foramen;  2,  third  pair;  3,  branch  of  same  to  npflfi^  ^P  minprinr   obi  in  HP  musclp 

levator  and  to  superior  rectus;  4,  branch  of  same  to  ] 

inferior  oblique ;  5,  sixth  nerve  to  external  rectus,  In    the    early    part   of    its    COlirse, 

which  is  turned  down ;  6,  fifth  nerve;  7,  first  division        ••  .,         ..,-•  ,    -j         /«    j.r  *.• 

of  same;  8,  nasal  nerve,  branch  of  preceding;  9,  len-  while    Still     Olltside    of    the    OptlC 

ticular  ganglion  ;  10,  short,  motor  root  of  same ;  11.  nerve,  it  Sends  the  long  root  to  the 

long,  sensory  root  of  same ;  12,  sympathetic  root  of  .  . .  .  „ 

same;  13,  ciliary  nerves;  14,  supra-orbital  branch.  lenticular  ganglion.       Alter   CTOSS- 

ing  the  optic  nerve  one  or  two  long 

ciliary  filaments  pass  from  it  to  the  eyeball.  Before  leaving  the  orbit  it 
sends  off  the  infra-trochlear  nerve,  an  important  branch  which  supplies  the 
lacrymal  sac,  the  conjunctiva,  the  lids,  and  a  bit  of  the  nose,  leaving  the 


FIG.  34. 


AND   THE   APPENDAGES   OF   THE   EYE.  107 

orbit  above  the  internal  palpebral  ligament.  The  main  nerve,  after  leaving 
the  orbit,  passes  for  a  short  distance  through  the  cranial  cavity  and  then 
plunges  into  the  nose,  supplying  a  part  both  of  the  mucous  membrane  and 
of  the  skin.  It  is  often  mentioned  in  explanation  of  reflexes  passing  be- 
tween eye  and  nose.  The  lenticular  ganglion  (Fig.  34)  is  diagrammatic  in 
its  simplicity.  It  has  a  short  root  (motor)  from  the  third,  springing  from 
the  branch  to  the  inferior  oblique,  a  long  root  (sensory)  from  the  nasal,  and 
one  or  two  long  sympathetic  fibres  from  the  cavernous  plexus.  It  is  situ- 
ated very  close  to  the  optic  nerve  on  its  outer  side,  rather  below  than  above 
it,  at  one-third  (or  a  little  more)  of  the  distance  from  the  foramen  to  the 
globe.  It  is  irregular  in  size  and  shape,  its  diameter  being  some  two  or 
three  millimetres.  The  short  ciliary  nerves  which  spring  from  it  tend  to 
be  divided  into  an  upper  and  a  lower  group.  The  number  of  primary 
nerves  as  well  as  the  number  of  branches  they  break  up  into  is  very  vari- 
able. If  we  may  believe  all  the  statements,  the  latter  range  from  ten  to 
twenty.  They  pierce  the  sclerotic  with  the  ciliary  arteries  in  a  circle  round 
the  optic  nerve. 

XI. 

If  the  contents  of  the  orbit  be  examined  from  above  by  breaking  and 
removing  the  pieces  of  the  roof,  one  is  struck  by  the  looseness  of  the  at- 
tachment of  the  periosteum  which  remains  intact  covering  the  soft  parts. 
The  frontal  nerve  and  its  continuation,  the  supra-orbital,  are  immediately 
below  it  and  easily  seen  through  it.  At  the  same  level,  but  less  evident, 
are,  towards  the  inside,  the  supra-trochlear  branch  running  near  the  outer 
border  of  the  superior  oblique  in  the  front  part  of  the  orbit,  and  in  the 
back  part  the  fourth  nerve  running  above  the  same  muscle.  At  the  outside 
of  the  orbit  the  lacrymal  nerve  runs  above  the  external  rectus  along  the 
wall.  The  lacrymal  artery  is  very  near  this  nerve,  but  the  supra-orbital 
artery  arises  from  the  ophthalmic  within  the  cone  of  muscles  (or  after  its 
main  continuation  has  passed  out  of  it  to  the  inside  of  the  superior  rectus), 
so  that  it  does  not  reach  the  roof  in  the  posterior  part  of  the  orbit.  The 
main  artery,  lower  than  the  nerve,  is  against  the  upper  part  of  the  inner 
wall,  in  the  anterior  half,  or  rather  more,  of  the  orbit.  The  veins  of  the 
same  names  as  these  arteries  have  the  same  general  course.  In  the  upper 
outer  angle  of  the  base  the  lacrymal  gland  lies  beneath  the  periosteum, 
enclosed  in  its  sheath.  Beneath  these  structures  lies  the  levator  palpebrae 
superioris,  narrow  behind  and  closely  attached  to  the  subjacent  superior 
rectus.  Farther  forward  its  muscular  belly  lies  somewhat  internal  to  that 
of  the  latter,  but  its  expansion  spreads  anteriorly  across  the  orbit,  mingling 
with  transversely  placed  fibres  to  form  Midler's  muscle,  which  sinks  into 
the  lid  and  partially  divides  the  lacrymal  gland.  At  the  top  of  the  inner 
wall  of  the  orbit  runs  the  superior  oblique.  At  the  inner  wall  are  the 
ethmoidal  arteries  and  veins,  the  nasal  nerve  passing  through  the  anterior 
ethmoidal  foramen  and  its  mfra-trochlear  branch.  Almost  all  the  space 


108        ANATOMY   OF   THE   ORBIT   AND   APPENDAGES   OF   THE   EYE. 

near  the  apex  of  the  orbit  is  taken  up  by  the  recti  forming  a  cone  round 
the  optic  nerve.  As  they  diverge  they  leave  room  for  much  fat,  through 
which  pass  the  ciliary  arteries  and  nerves  and  the  continuations  of  the  venae 
vorticosse  of  the  eyeball.  The  artery,  at  first  below  and  then  outside  of  the 
nerve,  passes  over  it,  as  does  also  the  nasal  nerve.  The  ophthalmic  vein 
is  in  this  cone  beneath  the  superior  rectus,  and  the  inferior  ophthalmic  vein 
below  the  optic  nerve.  The  third  pair,  the  sixth,  and  the  nasal  nerve  pass 
at  once  from  behind  into  this  cone.  The  ophthalmic  veins,  or  the  single 
one  formed  by  their  union,  leave  the  cone  to  pass  into  the  sinus.  Below 
the  globe  lie  the  inferior  oblique  and  Lockwood's  suspensory  ligament  of 
the  eye. 


THE  ANATOMY  OF  THE  EYEBALL  AND 
OF  THE  INTRA-ORBITAL  PORTION  OF 
THE  OPTIC  NERVE. 

BY  FRANK   BAKER,  M.D.,  PH.D., 
Professor  of  Anatomy,  University  of  Georgetown,  Washington,  D.C.,  U.S.A. 


THE   EYEBALL. 


GENERAL   CHARACTERS. 


THE  eyeball l  is  a  spheroidal  body  situated  in  the  anterior  portion  of 
the  orbit  and  attached  to  the  brain  by  a  stem-like  structure  called  the  optic 


B 

Camera  obscura  of  a  photographic  apparatus.— AB,  object ;  D,  diaphragm  for  shutting  off  too  divergent 
rays ;  L,  lens  for  refracting  the  rays  so  that  they  will  form  the  image  aft  upon  the  sensitive  plate  R. 

Flo.  2. 


The  eye  as  a  camera.— AB,  object ;  C,  cornea,  where  the  rays  undergo  a  first  refraction ;  D,  the  iris, 
that  acts  as  a  diaphragm  for  shutting  off  too  divergent  rays ;  L,  lens,  where  the  rays  are  again  refracted ; 
K,  retina,  upon  which  the  image  ab  is  projected;  a'b'  represents  the  surface  of  a  hypermetropic  eye,  and 
shows  that  the  rays  are  not  completely  focussed,  and  consequently  the  image  must  be  blurred  and  in- 
distinct; a"b"  represents  the  surface  of  a  myopic  eye,  and  shows  a  similar  condition. 

nerve.     It  consists  of  three  concentric  envelopes,  known  as  the  coats  or 
tunics  of  the  eye,  which  enclose  certain  transparent  contents,  the  whole 


1  Syn.  :  globe  of  the  e}-e  ;  apple  of  the  eye  ;  bulbus  oculi. 


109 


110 


THE  ANATOMY  OF  THE  EYEBALL. 


forming  a  dark  chamber  or  camera  obscura  like  that  of  a  photographic 
apparatus,  in  which  the  light  from  any  object  within  its  field  is  by  refract- 
ing media  thrown  upon  a  background  so  placed  that  the  rays  diverging 
from  any  given  point  converge  to  the  corresponding  point  of  a  small  in- 
verted image.  (Figs.  1  and  2.) 

The  external  coat  (see  Fig.  3)  is  composed  of  condensed,  fibrous  con- 

Fia.  3. 


POSTERIOR 

C  H AMBER 


SCLERAL 
SULCUS 


CONJUNCTIVA 


ILIARV 
BODY 


Diagram  of  a  horizontal  section  of  the  left  eye  drawn  to  scale. 

nective  tissue  which  by  its  resistance  to  internal  pressure  gives  shape  to 
the  eye ;  its  posterior  portion,  comprising  about  five-sixths  of  the  whole, 
is  white  and  opaque,  and  is  known  as  the  sdera;  its  anterior  one-sixth 
is  clear  and  transparent,  and  is  called  the  cornea. 

The  middle  coat  is  essentially  vascular,  and  serves  as  a  nutritive  organ 
for  the  other  coats.  It  also  contains  pigmented  cells  that  prevent  reflections 
within  the  cavity  of  the  ball,  as  well  as  plain  muscle-fibres  that  serve  for 


THE   ANATOMY   OF   THE    EYEBALL.  Ill 

adjusting  the  apparatus  of  vision.  Its  posterior  part  is  known  as  the 
chorioid,  its  anterior  part — where  it  forms  a  delicate  curtain  pierced  by  an 
aperture,  the  pupil — as  the  iris. 

The  inner  coat  is  essentially  nervous  in  character,  being  an  outgrowth 
from  the  brain  and  containing  the  ganglionic  elements  that  supply  the 
optic  nerve.  It  is  known  as  the  retina  ;  upon  it  the  rays  of  light  are  con- 
centrated, and  by  the  action  of  its  cells  the  sensations  of  vision  are  set  in 
train. 

The  spaces  within  the  globe  are  occupied  by  fluid  or  semi-fluid  contents, 
and  are  known  as  the  chambers  of  the  eye.  Two  principal  compartments 
are  established  by  the  interposition  of  a  transparent  lenticular  structure 
called  the  crystalline  lens,  which  is  placed  transversely  across  the  globe,  just 
behind  the  pupil.  The  space  in  front,  which  is  filled  with  a  watery  fluid, 
— the  aqueous  humor, — is  called  the  aqueous  chamber  ;  that  behind,  occupied 
by  a  soft  and  jelly-like  material, — the  vitreous  humor, — is  the  vitreous 
chamber.  During  a  considerable  part  of  foetal  life  the  aqueous  chamber  is 
divided  by  a  vertical  transverse  partition  composed  of  the  iris  and  a  delicate 
sheet — the  pupillary  membrane — stretched  across  the  site  of  the  future 
pupil.  These  subdivisions  are  known  as  the  anterior  and  posterior  chambers. 
When  the  pupillary  membrane  disappears,  as  it  usually  does  before  birth, 
they  communicate  through  the  pupil. 

The  terms  usually  applied  to  a  spheroid  are  often  used  in  describing 
the  eye.  The  axis  is  the  antero-posterior  diameter  passing  through  the 
summit  of  the  cornea.  For  purposes  of  precision  that  part  of  this  diame- 
ter between  opposite  points  of  the  internal  surface  of  the  capsule  is  termed 
the  internal  axis,  the  entire  diameter  being  then  known  as  the  external  axis. 
The  points  where  this  axis  cuts  the  surface  are  termed  poles,  distinguished 
as  anterior  and  posterior.  The  equator  is  the 
great  circle  equidistant  from  the  poles,  and  the 
meridians  are  great  circles  passing  through  both 
poles. 

In  popular  language  the  eyeball  is  considered 
as  a  globe,  and  when  we  reflect  that  its  external 
coat  is  a  capsule  in  a  state  of  tension  over  fluid  or 
semi-fluid  contents,  it  would  seem  natural  that 

this   should   be   the   shape   assumed.      Yet   On   ex-  The  eyeball  compared  to  a 

•    •        •,     j'  •  j        i     .,   •     r>        JJ.LU        sphere.— A,  sclera;   B,   cornea; 

aminmg  its  dimensions  closely  it  is  found  to  be  by    oiSiiilinUaBp. 
no  means  exactly  spherical.     The  principal  mus- 
cles that  move  the  ball  are  inserted  upon  the  capsule  by  tendinous  thick- 
enings arranged  about  the  region  where  the  sclera  and  the  cornea  join. 
This  causes  a  distinct  change  in  the  curvature  at  this  place,  with  the  for- 
mation of  a  slight  groove,  the  scleral  sulcus.1     This  change  of  curvature 
occasions  an  apparent  projection  of  the  cornea  beyond  the  limits  of  the 

1  Syn. :  sulcus  sclerce ;  sulcus  sclerce  externus  (Schwalbe). 


112 


THE  ANATOMY  OF  THE  EYEBALL. 


larger  sphere.  The  latter  would,  however,  if  continued,  enclose  the  whole 
of  the  cornea,  as  shown  by  the  diagram,  Fig.  4.  Any  increase  of  intra- 
ocular pressure  diminishes  the  convexity  of  the  cornea  and  tends  to  obliter- 
ate this  groove. 

The  eyeball  also  shows  other  slight  deviations  from  the  spherical  form, 
and  these  are  by  no  means  easy  to  determine  with  precision,  as  may  be 
judged  from  the  fact  that  some  of  the  most  recent  anatomical  works  differ 
in  their  statements  as  to  which  diameter  of  the  eye  is  the  greatest.  The 
following  citations  from  principal  treatises  on  anatomy  show  variations  of 
statement  on  this  subject : 


Antero- 
Posterior 
Diameter. 

Transverse 
Diameter. 

Vertical 
Diameter. 

Quain's  Anatomy,  10th  edition,  1894    

Millimetres. 
24 

Millimetres. 
24.5 

Millimetres. 
23  5 

Gray's  Anatomy,  13th  edition,  1893  

22.9 

25.4 

22.9 

Morris's  Anatomy,  1894    

24.5 

23.9 

23.5 

Macalister's  Anatomy,  1889     

24.27 

24.25 

23.65 

Leidy's  Anatomy,  1889      

25.4 

25.4 

254 

Testut,  Anatomic  Humaine,  1894  .... 

25.26 

23.6 

23 

Debierre,  Anatomic  de  1'Homme,  1890              

24 

23.5 

23 

Kauber,  Anatomic  des  Menschen,  1893-94  .... 

24.27 

24.32 

23.6 

(Cites  the  older  work  of  Krause.) 
Gerlach,  Specielle  Anatomic  des  Menschen,  1891    .... 
Vierordt  Daten  und  Tabellen,  1893  

24.2 
24 

23.8 
24.3 

23.5 

Merkel,  Topographische  Anatomic,  1887  

24.3 

23.6 

23.3 

These  variations  probably  arise  from  the  fact  that  the  eyes  measured 
were  not  all  in  a  perfectly  fresh  condition,  and  that  the  changes  due  to 
age  were  not  always  properly  considered.  Evaporation  of  the  fluid  con- 
tents of  the  eyeball  commences  at  once  after  death,  and  soon  removes 
a  sufficient  quantity  to  make  a  perceptible  difference  in  the  tension  of 
the  coats  and  consequently  in  the  measurement  of  the  diameters.  Dif- 
ferences in  manipulation  have  doubtless  some  effect,  as  if  the  ball  is  left 
to  lie  for  any  time  upon  a  hard  surface  it  becomes  flattened  sufficiently 
to  affect  the  measurements.  Among  the  most  careful  determinations  re- 
corded are  those  of  Sappey,  who  measured  a  large  number  of  eyes  belong- 
ing to  both  sexes  and  various  ages.  He  has  published  the  table  given  on 
the  following  page  comprising  certain  selected  cases  considered  as  normal. 
Finding  that  in  the  great  majority  of  cases  the  dimensions  of  the  right 
and  left  eyes  did  not  sensibly  differ  in  the  same  subject,  he  used  but  one 
eye  of  each. 

The  table  shows  that  the  eyeball  may  properly  be  described  as  an 
ellipsoid,  flattened  slightly  from  above  downward  and  from  side  to  side. 
For  practical  purposes  the  antero-posterior,  or  longest,  diameter  may  be 
considered  as  about  twenty-four  millimetres  (nineteen-twentieths  of  an 
inch),  while  the  vertical,  or  shortest,  is  about  one  millimetre  less,  and  the 
transverse  is  midway  between  the  other  two. 


THE  ANATOMY  OF  THE  EYEBALL. 


113 


DIMENSIONS  OF   THE   EYEBALL. 

FEMALES. 


6 

3 

S 

C3 

ij 

41 

41 

sn 

3-E  . 

|| 

H 

a 

2.S 

zM 

«• 

O^  gj 

Q 

T?  ** 

3 

a> 

a 

o-g 

:=S 

8>S  s 

e  K  § 

O 

II 

Q 

I 

P 

P 

•°  2^. 

Io6 

£36 

1 

S* 

Pi  9 

|a 

s 

I 

8 
S 

if! 

^  a  3 

Ill 

8  a>  o 
BC« 

5«2 

III 

6 

I 

a 

2 

9 

[^  c3  O 

K  ^5  CO 

31^5; 

Q 

^ 

H 

^ 

1-1 

H 

Q 

Q 

1 

Eight 

18 

23.0 

23.2 

23.0 

23.4 

234 

26 

33 

2 

Eight 

25 

23.4 

22.8 

22.5 

23.3 

23.3 

25 

32 

3 

Left 

28 

24.0 

233 

23.3 

23.5 

23.8 

26 

33 

4 

Eight 

30 

23.5 

22.6 

22.6 

24.1 

23.8 

26 

34 

5 

Left 

35 

23.9 

23.1 

23.1 

23.7 

23.7 

28 

33 

6 

Left 

40 

25.0 

23.6 

23.6 

24.3 

23.7 

29 

34 

7 

Eight 

50 

24.3 

238 

24.0 

24.6 

25.1 

27 

33 

8 

Left 

66 

26.4 

27.1 

23.4 

25.7 

25.3 

32 

37 

9 

Left 

69 

23.6 

23.5 

23.0 

25.4 

25.3 

28 

33 

10 

Left 

72 

22.9 

22.8 

22.3 

23.5 

23.6 

27 

34 

11 

Left 

74 

23.4 

23.3 

22.6 

23.8 

23.3 

28 

32 

12 

Left 

81 

23.2 

22.5 

22.5 

23.1 

23.4 

25 

31 

Average  Dimensions. 

23.9 

23.4 

23.0 

23.8 

23.8 

27.2 

33.2 

MALES. 


1 

Eight 

20 

24.8 

23.3 

23.8 

23.7 

23.9 

28 

33 

2 

Left 

22 

23.6 

22.8 

22.5 

23.5 

23.5 

26 

33 

3 

Left 

25 

24.2 

22.4 

22.2 

23.5 

23.6 

27 

34 

4 

Eight  ! 

26 

24.3 

23.4 

23.4 

23.7 

23.5 

27 

33 

5 

Eight 

31 

24.7 

25.9 

22.8 

24.4 

24.8 

30 

37 

6 

Left2 

35 

26.3 

25.4 

25.2 

.       . 

31 

39 

7 

Left3 

45 

25.2 

24.6 

24.0 

24.8 

25.0 

29 

37 

8 

Eight 

50 

24.4 

23.9 

25.8 

23.9 

24.5 

27 

35 

9 

Left 

59 

25.0 

23.8 

23.4 

24.3 

24.3 

27 

36 

10 

Left 

63 

240 

24.0 

24.0 

25.5 

24.7 

28 

35 

11 

Left* 

67 

24.9 

24.9 

24.0 

28 

34 

12 

Left 

70 

,  24.3 

23.1 

24.5 

24.6 

24.0 

25 

32 

13 

Left 

75 

24.8 

23.9 

23.8 

24.6 

245 

27 

35 

14 

Left 

79 

24.7 

23.6 

23.6 

24.2 

24.5 

27 

35 

Average  Dimensions. 

24.6 

23.9 

23.6 

24.1 

24.2 

27.5 

34.5 

Average  for  both  Sexes. 

24.2 

23.6 

23.2 

23.9 

23.9 

27.3 

33.8 

There  is  still  a  small  amount  of  asymmetry  not  accounted  for.  On 
looking  carefully  at  the  eyes  of  a  living  person,  it  will  be  seen  that  the 
pupil  is  placed  a  little  nearer  the  nasal  side.  The  nasal  hemisphere  is,  in 
fact,  somewhat  smaller  than  the  outer  or  temporal  hemisphere,  and  planes 
passed  through  the  equator  of  the  lens  in  either  eye  do  not  exactly  coin- 
cide, but  tend  to  converge  at  a  very  obtuse  angle  towards  the  median  line, 


1  Observed  three  hours  after  death. 
3  Observed  four  hours  after  death. 
VOL.  I.— 8 


2  Observed  two  hours  after  death. 
*  Observed  one  hour  after  death. 


114 


THE  ANATOMY  OF  THE  EYEBALL. 


and  in  some  individuals  below  the  horizontal  plane.     The  longest  equa- 
torial diameters  are  not  horizontal,  but  oblique. 

Individual  peculiarities  in  the  dimensions  of  the  eyeball  are  not  infre- 
quent, and  may  lead  to  anomalies  of  vision.     Normally  the  retina  is  so 


FIG.  5. 


An  emmetropic  or  normal  eye.    (Oliver.) — Parallel  rays  06  are  focussed  upon  the  retina,  while  diver- 
gent rays,  as  those  proceeding  from  the  point  B,  come  to  a  focus  behind  the  retina. 

placed  that  the  rays  of  light  coming  from  distant  objects,  and  therefore 
practically  parallel,  focus  directly  upon  it  when  they  are  refracted  by  the 
transparent  media  at  the  front  of  the  eyeball,  and  consequently  form  a  dis- 
tinct image.  (See  Fig.  5.) 


A  myopic  or  short-sighted  eye.    (Oliver.)— Parallel  rays  converge  before  reaching  the  retina, 
must  be  divergent  in  order  to  focus  there. 


Rays 


This  normal  condition  is  called  emmetropia.1  If  the  axis  of  the  eye  is 
longer  than  usual,  the  rays  coming  from  distant  objects  converge  before 
reaching  the  retina,  and  the  image  appears  blurred,  while  divergent  rays 
from  objects  near  at  hand  are  properly  focussed  and  produce  a  distinct 
image.  (See  the  line  a"b"  in  Fig.  2,  also  Fig.  6.)  This  is  short-sighted- 


Fio.  7. 


A  hypermetropic  or  far-sighted  eye.    (Oliver.)— Parallel  rays  converge  behind  the  retina.    Rays  must 
be  convergent  in  order  to  focus  there. 

ness,  or  myopia?  If  the  axis  of  the  eye  is  shorter  than  usual,  the  distant 
rays  focus  behind  the  retina,  and  a  similar  blurring  of  the  image  occurs, 
which  can  be  remedied  only  by  the  use  of  a  convex  lens  to  make  the  rays 
convergent.  (See  the  line  a'b'  in  Fig.  2,  also  Fig.  7.)  This  is  one  form 
of  far-sightedness,  known  as  hypermetropia?  Similar  conditions  are  caused 
by  anomalies  in  the  refracting  media  of  the  eye. 

1  From  l^/i£T/jof,  proportional,  and  wi/>,  WTO?,  eye. 

2  From  fivo-ty,  /miTrof,  short-sighted. 

3  From  vntpfterpog,  out  of  proportion,  and  ut[>,  undf,  eye. 


THE  ANATOMY  OF  THE  EYEBALL.  115 

Females  usually  have  slightly  smaller  eyes  than  males,  as  is  shown  by 
the  table ;  yet  this  is  by  no  means  universal.  When  in  popular  language 
the  size  of  the  eyes  is  mentioned,  it  is  usually  their  prominence  that  is 
noted  rather  than  the  actual  dimensions  of  the  organs,  as  the  position  of 
the  eyes  in  the  orbits  causes  a  change  in  their  apparent  size  according  as 
they  retreat  or  are  set  forward. 

The  axial  length  of  the  eyeball  at  birth  is  stated  by  Jager  to  be  17.53 
millimetres.  The  diameters  are  then  approximately  equal,  the  spherical 
form  being  more  nearly  realized,  and  the  child  is  usually  hypermetropic. 
Increase  in  size  is  slow  during  the  first  year,  after  that  pretty  regular  up 
to  puberty,  when  full  dimensions  are  attained.  The  cornea  grows  faster 
proportionally,  attaining  its  adult  dimensions  about  the  third  year. 

Comparing  the  eyes  of  different  animals,  it  is  found  that  their  size  does 
not  depend  upon  the  gross  bulk  of  the  animal,  but  rather  upon  the  neces- 
sity which  it  has  for  acute  and  instant  vision.  In  proportion  as  the  globe 
of  the  eye  is  larger  the  refracting  media  (cornea  and  lens)  are  farther 
distant  from  the  retina,  the  image  produced  is  larger,  and  its  details  are 
more  readily  and  quickly  made  out.  Hence  we  find  that  animals  requiring 
to  move  quickly  have  larger  eyes  in  proportion  to  the  size  of  the  body, — the 
eye  of  an  antelope,  for  example,  being  larger  than  that  of  an  elephant.  The 
eye  of  the  chimney-swallow  is  one-thirtieth  the  volume  of  the  body,  while 
the  viper's  eye  is  but  one-thousandth.  The  eyes  of  all  birds  are  relatively 
larger  than  those  of  mammals. 

In  animals  of  the  same  general  habits  the  smaller  ones  have  the  larger 
eyes.  The  wild  cat,  for  example,  has  larger  eyes  proportionally  than  the 
lynx,  and  the  lynx  larger  than  the  lion.1  Nocturnal  animals  usually  have 
large  eyes,  and  this  is  also  the  case  with  many  fishes.  It  has  been  suggested 
that  the  larger  size  of  the  retinal  image  in  these  animals  compensates  to 
some  extent  for  the  small  amount  of  light  to  which  they  are  accustomed. 

The  average  weight  of  the  eyeball  may  be  considered  as  about  seven 
grammes.2  At  birth  it  weighs  about  3.8  grammes,  being  much  larger  and 
heavier  in  comparison  with  the  entire  body  than  is  the  eye  of  the  adult. 

The  weight  of  the  two  eyes  as  compared  with  the  rest  of  the  body  is  at 
birth  as  1  : 419,  while  at  adult  age  it  is  but  as  1 : 4832.  While  the  body 
increases  in  volume  21-fold  from  birth  to  adult  age,  the  eye  grows  about 
1.8-fold.3  Like  the  brain  and  the  ear,  the  eye  attains  its  structure  and 

1  See  on  this  subject  Emmert  and  Fischer,  in  Corresp.-Blatt  f.  Schweiz.  Aerzte,  1887,  p. 
276.     Also  Nuel,  in  Diet,  encycl.  des  sciences  medicates,  article  CEil. 

2  This  is  the  average    given    by    Merkel    (Grafe    and    Samisch's    Handbuch    der 
gesammten   Augenheilkunde)    and   Scbafer   (Quain's   Anatomy,    10th   edition).      Other 
authorities  state  the  weight  as  follows :  Krause,  104  to  128  grains  (6.74  to  8.79  grammes) ; 
Henle,  6.3  to  8  grammes  ;  Huschke,  6.6  to  8.2  grammes;  Sappey,  Debierre,  and  Gerlach, 
7  to  8  grammes  ;  Testut,  7  to  7.5  grammes  ;  Macalister,  7.2  grammes.     It  seems  probable 
that  in  this,  as  in  the  measurements,  allowance  has  not  always  been  made  for  the  rapidity 
of  evaporation  of  the  fluids  of  the  eye  after  death. 

J  Vierordt.  Daten  und  Tabellen. 


116 


THE  ANATOMY  OF  THE  EYEBALL. 


proportions  much  earlier  than  the  rest  of  the  body,  an  arrangement  which 
seems  to  be  required  by  the  needs  of  the  organism.  Its  volume  is  about 
6.5  cubic  centimetres,1  and  its  specific  gravity  must  therefore  be  about 
1.077.2 

RELATIONS. 

The  eye  is  situated  in  the  bony  cavity  of  the  orbit,  in  which  it  is  held 
by  appropriate  fascia ;  about  one-third  of  its  anterior  surface  is  free,  or,  to 
be  more  exact,  covered  only  by  a  thin  transparent  pellicle  derived  from  the 
epidermis,  called  the  conjunctiva,  and  protected  by  movable  curtains  of 
integument,  the  eyelids ;  the  posterior  two-thirds  are  separated  from  the 
orbital  fat  by  an  aponeurosis  called  the  bulbar  fascia,  within  which  the  eye 
rolls  like  a  ball  within  a  socket,  its  attachments  being  so  slight  that  when 
the  conjunctiva  is  severed  and  the  attached  muscles,  vessels,  and  nerves  are 
cut,  the  globe  may  be  removed  like  a  nut  from  its  shell.  We  have  then  to 
consider  the  relations  of  the  eyeball  to  (1)  the  orbit,  (2)  the  eyelids,  (3)  the 
conjunctiva,  and  (4)  the  bulbar  fascia. 

Shaped  like  an  irregular,  prostrate  cone  or  pyramid  with  rounded  angles, 
the  orbit  is  longer  on  the  temporal  side  and  on  the  floor  than  on  the  nasal 
side  and  roof;  its  apex  is  situated  at  or  a  little  below  the  optic  foramen,  and 
its  base  at  the  facial  surface  is  bounded  by  the  edges  of  the  orbital  arches. 

Its  average  dimensions  have  been  carefully  calculated  by  Emmert 3  from 
a  series  of  selected  skulls  free  from  abnormalities,  as  follows  : 


Adult  Males, 
20  to  67  Years. 

Adult  Females, 
23  to  67  Years. 

Children, 
10  to  17  Years. 

Width           

Millimetres. 
41  6 

Millimetres. 
39.8 

Millimetres. 
34.3 

Height  

34 

33.6 

29.2 

Depth     

39.8 

39.4 

34.75 

Length  of  external  wall    .    .           .... 

46.4 

46 

39.4 

Length  of  internal  wall     

41.4 

40.3 

36 

Distance  hetween  outer  edges  of  orbits   .    . 
Distance  hetween  axes  of  orbits   

99.7 
60 

96 

58.3 

80.8 
48.1 

Angle  which  facial  openings  of  orbit  \ 
make  with  each  other                       /  '    ' 
Angle  hetween  orbital  axes   

147° 

434° 

146.5° 

44.7° 

144.6° 
42.4° 

Angle  between  external  orbital  walls  .    .    . 

89.9° 

89.9° 

87.4° 

The  dimensions  of  the  orbit  have  some  ethnological  importance.     In 

1  This  is  the  estimation  given  by  Schafer.     Henle  gives  it  as  6  cubic  centimetres  ; 
Vierordt,  6.6  cubic  centimetres ;  Macalister,  a  little  over  6  cubic  centimetres. 

2  The  following  statements  concerning  the  specific  gravity  of  the  eyeball  may  be  cited  : 
Huschke  (Bau   des   menschlichen   Korpers,  vol.  v.),  1.022  to  1.0302;    Fricke  (cited  by 
Vierordt  in  Daten  und  Tabellen  fur  Mediciner),  1.212  to  1.0302;  Davy  (Transactions  of 
the  Med.-Chir.  Soc.  of  Edinburgh,  vol.  Hi.),  1.091;    Macalister  (Text-Book  of  Human 
Anatomy),  1.025.     The  latter  author  seems  to  have  overlooked  the  fact  that  when  the 
weight  and  volume  of  a  body  are  given  the  specific  gravity  can  be  deduced.     From  his 
figures  of  weight  and  volume  the  specific  gravity  would  be  more  than  1.100. 

*  Emmert  (Emil),  Auge  und  Schadel,  Berlin,  1880.     These  measurements  are,  on  the 
whole,  the  most  extensive,  and  very  carefully  taken.     Authors  vary  as  to  the  dimensions 


THE  ANATOMY  OF  THE  EYEBALL. 


117 


anthropoid  apes  the  height  is  greater  than  the  breadth.  In  man  at  birth 
the  two  are  nearly  or  quite  equal,  and  this  primitive  condition  is  retained 
among  the  Polynesians  and  the  Chinese,  also  to  some  extent  in  females  of 
the  Indo-European  races,  who  generally  have  orbits  higher  in  proportion 
to  their  width  than  males  have.  Some  savage  races,  however,  such  as  the 
Kaffirs  and  the  Tasmanians,  have  orbits  very  low  in  proportion  to  their 
width. 

The  axes  of  the  two  orbits  are  not  parallel,  but,  as  stated  above,  are 
inclined  at  an  angle  of  from  42°  to  450,1  and  are  depressed  below  the 
horizontal  plane  from  15°  to  20°.  The  ocular  and  orbital  axes  cannot, 
therefore,  coincide.  The  average  distance  between  the  anterior  poles  of  the 
ocular  axis  is  from  fifty-eight  to  sixty  millimetres.  (Merkel.) 

The  eye  is  so  situated  in  the  orbital  cavity  that  a  line  drawn  from  the 
upper  to  the  lower  orbital  margin  just  touches  the  cornea  (Fig.  8) ;  the 
back  part  of  the  ball  is  therefore  from  sixteen  to  eighteen  millimetres  from 
the  optic  foramen.  It  is  not  exactly  in  the  axis  of  the  cavity,  being  one  or 
two  millimetres  nearer  to  the  temporal  side  than  to  the  nasal,  and  a  little 
nearer  to  the  upper  than  to  the  lower  orbital  margin.  (Fig.  9.)  A  section 
through  the  orbit  farther  back  (Fig.  10)  shows  that  at  the  equator  it  lies 
at  about  the  same  distance  from  the  upper  as  from  the  lower  wall. 

The  capacity  of  the  adult  orbit  is  about  thirty  (twenty-five  to  thirty- 


of  the  orbit,  as  will  be  seen  from  the  following  table, 
methods  of  measurement. 


This  is  mainly  due  to  different 


Breadth. 

Height. 

Axial 
Length. 

Merkel  (Topographische  Anatomic)  

Millimetres. 
f  c?40.5 

Millimetres. 
35 

Millimetres. 
43 

Arlt  (Archiv  fur  Ophthalmologie,  1857)  

l?40 
36 

34.5 
30 

40.5 
42 

Richet  (Anatomie  chirurgicale)  ...            

40-46 

40 

45-50 

Luschka  (Anatomie  des  Menschen)   

50 

40 

Vierordt  (Daten  und  Tabellen)   

36 

33 

47 

Benedikt  (Eulenberg's  Real-Encyclopadie)  ....... 

39 

/c?88 

Gerlacb  (Specielle  Anatomie  des  Menschen)    

42 

\  $34 
35 

43 

(Javat  (Annales  d'oculistique,  1873)  

42 

36 

De  Wecker  (Traite  des  maladies  des  yeux)       

39 

35 

40 

35 

40-50 

Broca  (Topinard's  Anthropologie)  found  that  in  different  races  of  men  the  following 
were  the  average  distances  from  the  optic  foramen  to  the  superior  border  of  the  orbit : 


Millimetres. 

Esquimaux 57.7 

Usbecks 57.5 

Australians 56.2 

Chinese  ...        55.6 

New  Caledonians     .......  55.6 

Papuans 55.8 

» 46°.     (Testut.) 


Millimetres. 

Spanish  Basques 47 

French  Basques 50.2 

Dutch 49.8 

Arabs 50.3 

Parisians,  twelfth  century     .    .    .   49.6 
Contemporary  Parisians     ....   50.9 


118 


THE  ANATOMY  OF  THE  EYEBALL. 


three)  cubic  centimetres,  according  to  Gayat,1  that  of  a  child  of  ten  years 
being  twenty-two  cubic  centimetres.  The  eyeball,  therefore,  occupies  about 
one-fifth  of  the  cavity. 

FIG.  8. 


Rectus  superior. 
Levator  palpebrae  superioris. 

Supra-orbital  nerve. 


Frontalis. 


Orbicularis 
palpebrarum. 


Septum  orbitale. 


Rectus  inferior. 


Fascia  bulbi. 


Sagittal  section  of  the  orbit  and  eyeball.    (After  Merkel.) 


Position  of  eyeball  in  orbit.  (Merkel.)— Lp,  levator  palpebrse ;  Us,  rectus  superior ;  Rm,  reetus  me- 
dialis ;  Ei,  rectus  inferior ;  Rl,  rectus  lateralis ;  Os,  obliquus  superior ;  2V,  its  trochlea  Oi,  obliquus 
inferior. 

Where  can  the  eyeball  best  be  reached  for  examination  or  operation  ? 
It  is  obvious  that  above  it  is  quite  well  protected  by  the  overhanging  edge 


1  Gayat  (J.).     Essais  de  mensuration  de  1'orbite.     Annales  d'oculistique,  Bruxelles, 
1873,  Ixx.  5-20. 


THE  ANATOMY  OF  THE  EYEBALL. 


119 


of  the  orbit  and  the  superciliary  arch.  On  the  inner  side  also  it  is  not  easily 
accessible,  owing  to  the  projection  of  the  bridge  of  the  nose.  Below,  the 
orbital  rim  is  smooth  and  projects  but  little,  so  that  the  depths  of  the  orbit 
are  readily  reached.  But  the  eyeball  itself  is  more  readily  accessible  on 


Oi      V' 

Frontal  section  of  the  orbit  and  eyeball  just  anterior  to  the'equator.  (Merkel. )— Am,  maxillary  sinus ; 
Cg,  crista  galli ;  Gl,  lacrymal  gland ;  Lp,  levator  palpebrse ;  Oi,  obliquus  inferior ;  Os,  obliquus  supe- 
rior ;  Ri,  rectus  inferior ;  Rl,  rectus  lateralis ;  Rm,  rectus  inedialis ;  Rs,  rectus  superior ;  T,  temporal 
muscle;  V,  V,  veins. 


Cavity  of  eyeball. 


Optic  nerve. 


Eyeball.' 


Malar  bone. 
Sphenoid  bone. 


Optic  commissure. 

The  eyes  and  optic  nerves  seen  from  above  after  removal  of  the  roof  of  the  orbit.— The  left  eye  ifl  shown 
in  section.    AO,  axis  of  orbit ;  AE,  axis  of  eye. 


the  outer  side.  If  a  line  is  drawn  touching  the  orbital  rim  on  either  side 
it  will  cut  the  ball  quite  back  of  the  cornea.  (Fig.  11.)  This  is  due 
partly  to  the  direction  of  the  orbital  axis  and  partly  to  the  fact  that  the  rim 
retreats  here  to  some  extent.1  By  suitably  moving  the  eye  the  ball  can  be 

1  It  may  be  interesting  to  note  that  in  most  mammals  the  external  wall  of  the  orbit  is 
but  little  developed.  The  malar  bone  does  not  send  a  process  up  to  the  frontal,  and  the 
orbit  communicates  widely  with  the  temporal  fossa. 


120  THE  ANATOMY  OF  THE  EYEBALL. 

explored  here  almost  as  far  back  as  the  equator,  while  above  and  below 
only  a  small  strip  of  the  sclera  can  be  felt. 

In  considering  the  relations  just  described  it  should,  however,  be 
remembered  that  the  situation  of  the  eyeball  in  the  orbit  is  subject  to  con- 
siderable variations,  both  physiological  and  pathological.  A  greater  or  less 
amount  of  orbital  fat  may  shove  it  forward,  producing  a  staring  expression, 
or  cause  it  to  sink  deeper  within  the  cavity,  as  in  the  hollow-eyed  aspect 
of  severe  emaciation.  Cohn  l  found  variations  of  as  much  as  ten  milli- 
metres behind  the  edge  of  the  orbit  and  twelve  millimetres  in  front  of  the 
same  in  healthy  individuals.  The  forward  projection  may  indeed  reach 
twenty-four  millimetres  in  unusual  cases.  The  condition  of  protrusion  is 
known  as  exophthalmos?  the  reverse  as  enopkthalmos?  Protrusion  may  be 
produced  also  by  various  other  causes,  such  as  paralysis  of  the  recti  muscles 
(also  seen  when  their  tendons  are  cut)  or  the  distention  of  the  intra-orbital 
vessels.  The  sudden  staring  during  surprise  or  terror  is  probably  due  to 
the  latter  cause,  as  is  also  the  protrusion  seen  in  some  females  during  men- 
struation. Enophthalmos  may  arise  from  the  sudden  draining  of  fluid 
from  the  tissues  in  Asiatic  cholera,  from  atrophy  of  any  of  the  orbital 
contents,  from  any  injury  or  disorder  that  enlarges  the  orbital  cavity,  or 
from  paralysis  of  the  sympathetic  nerve.  In  the  latter  case  it  has  been 
suggested  that  the  disturbance  is  caused  by  the  effect  of  this  paralysis  upon 
the  orbital  muscle  of  Miiller,4  a  thin  sheet  of  non-striated  muscular  fibre 
that  lies  in  the  membrane  that  stretches  over  the  spheno-maxillary  fissure. 
Losing  its  tonicity  by  a  paralysis  of  its  nerve-supply, — the  sympathetic, — 
it  permits  a  bulging  of  the  orbital  contents  and  consequent  retraction  of  the 
eyeball. 

The  eyelids6  vary  their  relation  to  the  eyeball  according  as  they  are 
closed  or  open.  When  open  (Fig.  12)  they  form  a  considerable  aperture, 
the  palpebral  opening,6  curved  above  and  below  by  the  edges  of  the  two 
lids,  which  are  united  internally  and  externally  to  form  the  angles  of  the 
eye.7  The  external  angle  of  the  eye  is  said  by  Testut  to  be  ten  to  twelve 
millimetres  from  the  cornea,  five  to  six  millimetres  from  the  orbital  arch, 
and  ten  millimetres  below  the  fronto-malar  suture.  The  internal  angle  is 
five  to  seven  millimetres  from  the  globe.  The  opening  exposes  nearly  one- 

1  Klinische  Monatsblatt,  1867. 

1  From  ffifytfa/ljuof ,  having  prominent  eyes. 

3  From  ev,  within,  -f-  o^tfafywJf,  eye  :  having  eyes  deeply  set. 

*  Named  in  honor  of  H.  Miiller,  an  ophthalmologist  of  Wurzburg,  born  1820,  died  1864. 

5  Syn. :  palpebrce. 

6  Syn. :  rima  palpebrarum,  the  palpebral  slit,  is  used  to  indicate  the  slit  between  the 
eyelids,  whether  closed  or  open.    Fissura  palpebrarum,  the  palpebral  fissure,  may  be  used 
in  the  same  sense.     Rictus  palpebrarum,  or  gape  of  the  eyelids,  is  used  to  indicate  the  space 
between  the  open  lids. 

7  Syn.  :    corners  of  the  eye ;  palpebral  commissures  ;    anguli  oculi,  or  canthi  oculi,  or 
commissurce  palpebrales  externce  and  internee,  or  laterales  and  mediates,  or  temporales  and 
nasales,  or  minores  and  majores. 


THE  AXATOMY  OF  THE  EYEBALL. 


121 


FIG.  12. 


fifth  of  the  surface  of  the  ball, — nearly  the  whole  of  the  cornea  and  a  con- 
siderable portion  of  the  sclera.  Its  longitudinal  axis  is  not  quite  horizon- 
tal, the  outer  angle  being  ordinarily  slightly  inclined  upward  and  situated 
about  four  millimetres  above  a  horizontal  line.  This  inclination  is  greater 
in  the  Mongolian  and  other  Altaic  or  Mongoloid  races,  as  the  Samoyedes, 
Finns,  and  Esquimaux.  Mondiere l  found  that  an  average  of  three  hundred 
measurements  of  these  peoples  gave  an  upwardly  sloping  angle  of  nearly 
five  degrees.  The  palpebral  opening  is  somewhat  less  than  thirty  milli- 
metres in  length  (including  five  to  seven  millimetres  for  the  little  recess  at 
the  inner  angle  known  as  the  lacrymal  bay 2),  being  less  in  females.  It  is 
curved  more  above  than  below,  and 
when  this  curvature  is  excessive,  as 
it  sometimes  is  in  persons  with 
prominent  eyes,  the  "almond  eye" 
so  much  praised  'by  Eastern  poets 
is  produced.  It  may  be  noted  that 
the  height  of  the  opening  is  usually 
somewhat  less  among  Orientals  than 
with  Europeans.  While  subject  to 
a  considerable  degree  of  variation, 
the  opening  may  be  stated  as  being 
from  twelve  to  fifteen  millimetres 
high  when  opened  widely  without 
raising  the  eyebrows  or  wrinkling 
the  skin  of  the  forehead.  In  chil- 
dren the  length  of  the  palpebral 
fissure  is  unusually  great,  and  this, 
in  conjunction  with  the  greater 

elasticity  of  the  skin  of  the  lids,  is  the  cause  of  their  widely  opened  eyes. 
There  appears  to  be  a  slight  decrease  in  the  length  of  the  fissure  in  old 
age.3 

With  eyes  looking  straight  ahead,  the  edge  of  the  lower  lid  touches  the 
bottom  of  the  cornea  or  falls  slightly  below  it,  and  the  upper  lid  usually 
impinges  upon  the  cornea  about  one  or  two  millimetres.  When  looking 
upward,  the  upper  lid  rises  and  the  palpebral  opening  dilates  so  that  a  strip 
of  the  sclera  is  visible  below.  When  looking  downward,  the  upper  lid  sinks 
to  the  upper  border  of  the  pupil,  the  lower  remaining  at  the  central  margin. 
When  the  eyes  are  fully  closed,  the  upper  lid  descends  to  the  lower  edge 
of  the  cornea,  the  inner  angle  remains  fixed  by  the  palpebral  ligament,  but 
the  outer  angle  descends  about  five  millimetres.  During  sleep  or  uncon- 
sciousness the  eyes  turn  slightly  upward  and  inward. 

1  Memoires  dc  la  Societe  cTAnthropologie,  1875,  ii.  451. 

2  Syn.  :  locus  lacrymalis. 

s  Fuchs  (Ernst).  Zur  Physiologic  und  Pathologie  des  Lidschlusses.  Archiv  fur  Oph- 
thalmologie,  xxxi.,  1885,  Abth.  II.  97. 


The  palpebral  opening.  (Merkel.)— The  contour 
of  the  underlying  bones  is  shown  by  the  unbroken 
line,  A ;  that  of  the  eyeball  by  the  broken  line  of 
short  spaces,  B;  that  of  the  conjunctival  sac  by  the 
broken  line  of  long  spaces,  C. 


122  THE  ANATOMY  OF  THE  EYEBALL. 

The  conjunctiva1  is  a  layer  of  mucous  membrane  of  a  lymphoid  charac- 
ter, continuous  with  the  epidermis  at  the  edges  of  the  lids,  lining  their 
deeper  surfaces  (palpebral  conjunctiva1),  being  thence  reflected  upon  the 
eyeball  (ocular  conjunctiva3),  and  entirely  covering  its  anterior  third.  The 
place  of  reflection  is  called  the  fornix*  of  the  conjunctiva.  It  will  be  seen 
that  when,  by  closing  the  lids,  the  edges  of  the  palpebral  conjunctiva  are 
brought  together,  the  entire  membrane  forms  a  closed  sac, — the  conjunct! val 
sac,* — applied  to  the  front  of  the  eye,  reminding  one  of  the  arrangement  of 
the  lining  of  the  serous  cavities  of  the  body.  When  the  lids  are  parted 
this  sac  is  opened,  and  any  small  foreign  bodies  that  impinge  against  the 
anterior  surface  of  the  eye  are  likely  to  lodge  in  it.  Its  extent  and  rela- 
tions to  the  surrounding  structures  should,  therefore,  be  noted. 

When  the  eyes  are  open  the  fornix  is  about  thirteen  millimetres  from 
the  edge  of  the  upper  lid,  while  it  is  but  nine  millimetres  from  the  lower 
lid.  On  the  sides  also  the  sac  varies  in  depth,  forming  at  the  lateral  angle 
a  shallow  pocket  five  millimetres  deep,  but  at  the  medial  angle  becoming 
almost  obliterated  by  the  seinilunar  fold,  under  which  it  passes  for  only  two 
millimetres.  The  fornix  is  five  millimetres  from  the  orbital  rim  above,  six 
millimetres  below,  and  four  millimetres  at  the  lateral  angle.  (Gerlach.)  Its 
distance  from  the  cornea  is  stated  by  Testut  to  be  ten  millimetres  above, 
eight  millimetres  below,  fourteen  millimetres  at  the  lateral  angle,  and  seven 
millimetres  at  the  medial  angle.6 

The  ocular  conjunctiva  is  distinguished  from  the  palpebral  by  its  less 
vascular  condition  and  paler  tint.  It  is  divided  into  a  scleral 7  portion  and 
a  corneal  portion,8  which  differ  somewhat  in  structure,  the  scleral  being  com- 
posed of  stratified  pavement  epithelium  with  a  regular  submucosa,  while 
the  corneal  has  an  epithelial  layer  only,  the  submucous  tissue  blending 
with  the  corneal  tissue  proper,  or  being  reduced  to  a  very  delicate  struc- 
tureless layer.  Near  the  cornea  it  is  closely  adherent  to  the  sclera,  often 
in  adults  forming  a  slightly  thickened  ring  containing  numerous  vascular 
papillae  and  known  'as  the  limbus  conjunctivas.9  Fatty  deposits 10  are  fre- 
quently seen  on  the  medial  side  of  the  limbus,  especially  in  old  age. 

1  From  L.  conjunctivus,  -a,  -um,  serving  to  connect,  because  it  connects  the  eyelids  with 
the  ball.  Syn. :  menibrana  conjunctiva  ;  Sindehaut,  G. 

A  more  complete  description  of  this  membrane  is  given,  under  the  anatomy  of  the 
appendages  of  the  eye,  on  pp.  89,  90. 

1  Syn.  :  tarsal  conjunctiva  ;  conjunctiva  palpebrarutn. 

3  Syn. :  conjunctiva  oculi  or  bulbi. 

*  From  L.  fornix,  fornicis,  an  arch  or  vault.  Gerlach  improperly  calls  the  conjunc- 
tival  sac  the  fornix.  Syn.  :  fold  of  transition  ;  conjunctival  cul-de-sac;  fornix  conjunctiva:. 

5  Syn.  :  saccus  or  sinus  conjunctives. 

6  Merkel  gives  the  distance  above  as  eight  millimetres ;  below,  ten  millimetres.     It 
doubtless  varies  considerably  with  the  prominence  of  the  eyes. 

7  Syn. :  conjunctiva  sclerotica  or  bulbi ;  tunica  adnata  oculi. 

8  Syn. :  conjunctiva  cornece. 

9  Syn.  :  annulus  conjunctivas;  limbus  cornea. 

10  Pinguecula. 


THE  ANATOMY  OF  THE  EYEBALL.  123 

The  external  part  of  the  scleral  conjunctiva  is  united  to  the  ball  by 
connective  tissue  partly  condensed,  a  continuation  of  the  bulbar  fascia,  and 
in  part  quite  lax,  continuous  with  the  adipose  capsule  of  the  eye.  The 
laxity  of  the  union  of  the  conjunctiva  with  its  subjacent  tissues,  both  at  the 
fornix  and  upon  the  sclera,  secures  the  necessary  freedom  of  movement  to 
the  ball.  In  whatever  direction  the  eye  may  be  moved,  the  conjunctiva  on 
the  opposite  side  is  stretched.  At  the  nasal  side,  where  the  fornix  lies  so 
near  the  ball,  movement  is  provided  for  by  an  accessory  fold  (the  semi- 
lunar  fold1),  a  vestige  of  the  third  eyelid  found  in  some  lower  animals. 
The  laxity  of  the  tissues  beneath  the  conjunctiva  makes  it  very  easy  to 
raise  it  whenever  it  is  necessary  to  perform  any  operations  within  the  orbit. 
It  also  explains  the  frequency  of  ecchymosis  in  this  situation.  Sometimes, 
indeed,  a  fracture  of  the  base  of  the  skull  or  other  internal  injury  causing 
the  rupture  of  vessels  may  become  known  by  an  ecchymosis  of  the  scleral 
conjunctiva,  the  blood  gradually  infiltrating  the  looser  tissues  of  the  orbit 
and  appearing  at  last  upon  the  ball.  If  desirable  to  fix  the  eyeball  for 
the  performance  of  any  operation,  the  conjunctiva  should  be  seized  near  the 
cornea,  where  it  is  more  firmly  attached. 

The  part  of  the  orbit  not  occupied  by  the  eyeball  is  filled  with  loose 
connective  tissue  enclosing  in  its  meshes  masses  of  fat,  and  therefore  often 
called  the  adipose  body  of  the  orbit.2  This  fills  in  all  the  interstices  between 
the  muscles,  nerves,  and  vessels  that  pass  forward  to  the  ball,  making  an 
excellent  padding,  in  which  all  these  structures  can  lie  without  being  dis- 
turbed by  shocks  or  displaced  by  the  ocular  movements.  About  the  ball 
this  connective  tissue  is  condensed  to  a  firm  aponeurosis  that  effectually 
confines  the  loose  tissue,  leaving  the  eye  comparatively  free. 

The  researches  of  Schwalbe3  have  demonstrated  that  this  bulbar  fascia, 
or  capsule  of  Tenon*  is  in  reality  the  lining  membrane  of  a  lymph-lacuna, 
the  interfascial  space,5  that  surrounds  the  ball,  and  communicates  on  the  one 
hand  with  the  intra-ocular  space  between  the  chorioid  and  the  sclera  (peri- 
chorioideal  space),  and  on  the  other  with  the  perineural  space  about  the 
optic  nerve,  and  thence  through  the  interstices  of  the  dural  sheath  with  the 
subdural  and  subarachnoid  spaces  of  the  cerebral  meninges.  (See  Fig.  13.) 

The  bulbar  fascia  passes  forward  and  is  attached  to  the  deeper  surface 
of  the  conjunctiva  near  the  edge  of  the  cornea,  and  is  there  reflected  upon 
the  sclera,  adhering  closely  to  it  and  clothing  the  posterior  two-thirds  of 
the  surface  of  the  globe  with  an  extremely  delicate  layer. 

1  Syn. :  semilunar  plica;    nictitating  membrane;  plica  semilunaris ;  palpebra  tertia; 
membrana  nictitans. 

2  Syn.  :  corpus  adiposum  orbitce ;  capzula  adiposa  bulbi;  adipose  capsule  of  the  eye. 

3  Archiv  fur  mikroskop.  Anat.,  vi.  41,  1870. 

*  Named  for  Jacques-Rene  Tenon,  a  surgeon  of  Paris,  born  1724.  died  1816. 

Syn. :  aponeurosis  orbito-ocularis  (Richet)  ;  orbital  aponeurosis  ;  orbito-ocular  aponeu- 
rosis or  fascia;  fascia  bulbi  or  Tenoni;  tunica  vaginalis  oculi  (Hyrtl)  or  bulbi;  vaginal 
tunic  (Leidy)  ;  capsula  Jibrosa  bulbi ;  Bonnet's  capsule. 

5  Tenon's  space  (Schwalbe) ;  supra-scleral  space ;  spatium  interfasciale. 


124 


THE  ANATOMY  OF  THE  EYEBALL. 


Like  a  serous  membrane,  the  fascia  has,  therefore,  a  parietal  layer  against 
the  orbital  fat,  constituting  the  capsule  of  Tenon  in  the  more  restricted 
and  usual  sense,  and  a  visceral  layer  ensheathing  the  eyeball,  and  some- 
times called  the  episclera.  These  layers  are  not,  however,  totally  separated 
from  each  other,  as  are  those  of  a  serous  membrane,  but  are  united  by  fine 
trabeculae l  that  cross  the  periscleral  space  and  are  sufficiently  lax  to  allow 
the  eye  to  move  with  perfect  freedom.  The  entire  internal  surface  of  the 
capsule,  together  with  the  trabeculae,  is  lined  with  endothelium.  It  is,  there- 

FIG.  13. 


Sagittal  section  of  the  eye,  showing  the  bulbar  fascia  and  its  attachments.— a,  prolongation  of 
bulbar  fascia ;  b,  levator  palpebrse ;  c,  rectus  superior ;  d,  perineural  space ;  e,  e',  septum  orbitale ;  /,  tendon 
of  levator;  g,  tarsus  superior;  h,  cornea;  i,  tarsus  Inferior;  k,  inferior  rectus  muscle;  I,  inferior  oblique 
muscle ;  m,  prolongation  of  bulbar  fascia.  $ 

fore,  anatomically  similar  to  the  lymph-sacs  found  elsewhere  in  the  body, 
as  the  subarachnoidal  sacs,  and  the  perivascular  sheaths  of  the  vessels  of 
the  pia  mater  and  of  the  mesentery. 

The  structures  attached  to  the  eyeball  pass  through  this  fascia  and 
receive  investments  from  it.  Behind,  about  the  optic  nerve,  it  becomes 
looser  in  texture,  and  permits  the  passage  of  fluid  between  that  nerve  and 
the  ciliary  vessels  and  nerves,  along  an  area  one  centimetre  in  diameter. 

The  interfascial  space  contains  a  small  amount  of  fluid  derived  from 
the  vessels  that  supply  its  walls,  and  in  this,  as  in  other  respects,  shows 
considerable  analogy  to  the  synovial  cavity  of  an  arthrodial  joint. 

1  Adventitia  oculi.     (Lockwood.) 


THE   ANATOMY    OF   THE   EYEBALL.  125 

THE  EXTERNAL  OR  FIBROUS  COAT.1 

The  essential  characteristics  of  this  envelope  depend  upon  its  structure, 
which  is  of  firmly  condensed  fibrous  tissue.  Hence  it  is  strong,  compara- 
tively inextensible,  and  has  a  capsular  character  resembling  in  some  respects 
the  tunica  albuginea  of  the  testis.  It  is  much  the  thickest  of  the  three 
coats,  and  serves  as  a  protecting  envelope  to  the  inner,  more  delicate  struc- 
tures, its  office  being  like  that  of  the  dura  mater  of  the  central  nervous 
system,  with  which,  indeed,  it  is  continuous  through  the  fibrous  sheath  of 
the  optic  nerve.  In  view  of  this  special  character,  it  is  not  strange  that  in 
lower  vertebrates  portions  of  this  capsule  become  cartilaginous,  or  even 
ossified.  In  fishes  and  amphibia  it  becomes  cartilaginous  only,  while  in 
reptiles  and  birds  it  is  cartilaginous  behind  and  often  protected  in  front  by 
a  ring  of  platelets  of  bone  called  sclerotals.  In  mammals  it  is,  however, 
almost  invariably  fibrous,  the  only  exception  being  the  lowly  monotremes, 
which  in  this  as  in  so  many  other  particulars  show  their  affinity  with  birds 
and  reptiles.  In  most  mammals  the  eye  is  efficiently  protected  by  the  walls 
of  the  orbit,  and  it  is  perhaps  for  this  reason  that  no  ossification  of  the 
capsule,  occurs.  Calcareous  deposits  are  sometimes  found  in  the  external 
coat,  but  the  normal  interstitial  tissue  never  ossifies,  this  occurring  only  in 
exudations  from  the  chorioid  arising  under  pathological  conditions.2 

Unlike  the  other  coats,  which  are  incomplete  at  some  part  of  the  cir- 
cumference, the  fibrous  coat  forms  a  complete  investment,  with  the  excep- 
tion of  such  orifices  as  are  necessary  for  the  passage  of  the  vessels  and 
nerves  that  supply  the  interior. 

The  normal  tension  of  the  capsule  is  said  to  be  equal  to  that  produced 
by  a  column  of  mercury  twenty-six  millimetres  high,  and  is  quite  sufficient 
to  make  the  ball  firm  and  resistant  to  the  touch.  Under  pathological  con- 
ditions it  may  vary  considerably,  reaching  as  high  as  seventy  millimetres 
of  mercury.  When  unusual  pressure  is  induced  by  inflammatory  processes, 
such  as  engorgement  of  vessels,  effusions,  etc.,  it  reacts  upon  the  contents 
of  the  eyeball  because  of  the  unyielding  and  inextensible  character  of  the 
external  coat,  injuring  and  finally  destroying  them.  This  character  appears 
to  be  less  marked  in  childhood  than  in  later  years,  possibly  because  the  coat 
is  then  thinner :  hence  a  very  considerable  expansion  of  the  ball  may  occur 
in  hydrophthalmos.  It  may  also  occur  in  the  adult  when  the  tissues  are 
softened  by  inflammation,  but  this  is  due  to  degenerative  changes  rather 
than  to  a  true  elasticity. 

Like  most  structures  of  condensed  connective  tissue,  the  capsule  is 
scantily  supplied  with  blood-vessels  and  has  no  proper  lymphatics,  the 
place  of  the  latter  being  supplied  by  lymph-lacunse. 

1  Syn. :  tunica  externa  or  fibrosa ;  capsula  fibrosa  (H.  Meyer) ;  dura  oculi ;  pachymeninx 
opMhalmencephali.     The  two  latter  terms  relate  to  the  conception  that  the  external  coat  is 
an  extension  of  the  dura  mater  of  the  brain. 

2  Grossmann  (L.).     De  1' ossification  dans  1'oeil.     Arch,  d'ophth.,  Paris,  1889,  ix.  137. 


126  THE   ANATOMY   OF   THE    EYEBALL. 

THE   SCLERA. 

The  sclera,1  or  posterior  portion  of  the  capsule,  is  white  and  opaque. 
It  is,  as  Bowman  has  pointed  out,  an  excellent  example  of  change  produced 
in  the  optical  properties  and  appearance  of  a  substance  by  structural  pecu- 
liarities. Although  it  has  essentially  the  same  fibrous  constitution  as  the 
transparent  cornea,  yet,  as  its  fibres  are  arranged  irregularly,  some  running 
in  a  meridional  direction  and  others  equatorially,  and  by  no  means  lying  in 
concentric  lamellae,  it  almost  wholly  intercepts  the  transmission  of  light. 
It  has  not  even  the  silvery  sheen  of  aponeurotic  tissues  caused  by  the 
parallel  but  wavy  course  of  contiguous  fibres,  but  is  a  dead  white,  owing 
to  the  dispersion  of  the  rays  by  its  felt-like  web,  the  glistening  reflection 
that  occurs  from  its  anterior  exposed  portion  being  due  rather  to  the  moist 
conjunctiva  than  to  the  properties  of  the  cornea  itself. 

While  the  tissue  is  mainly  of  the  white,  fibrous  variety,  there  is  yet  a 
considerable  number  of  yellow,  elastic  fibres  scattered  through  it,  especially 
at  the  anterior  part  of  the  globe  and  at  the  canals  for  the  passage  of  vessels 
and  nerves.  On  the  inner  surface  these  elastic  fibres  become  more  numer- 
ous and  connect  with  the  elastic  net-work  of  the  chorioid.  Like  the  fibres 
of  that  membrane,  they  have  among  them  a  considerable  number  of  pig- 
men  ted  cells,  so  that  when  the  sclera  is  separated  from  the  underlying  coat, 
which  is  easily  done  because  of  an  intervening  lymph-space,  its  concave  sur- 
face is  seen  to  be  brownish  fading  to  a  dirty  white  in  front.  It  is  therefore 
known  as  the  lamina  fusca2  (Briicke),  although  it  is  by  no  means  a  separate 
sheet,  but  rather  an  intrusion,  more  or  less  extensive,  of  the  peculiar  pig- 
ment of  the  chorioid  into  the  deeper  portion  of  the  sclera.  (See  Fig.  14.) 
This  pigment,  together  with  that  of  the  darker  chorioid,  may  sometimes 
show  faintly  through  the  sclera,  imparting  to  it  a  bluish  tint  like  that  of 
skimmed  milk  or  of  some  kinds  of  porcelain.  This  is  usual  in  children,  in 
whom  the  sclera  is  thin,  and  may  also  be  observed  in  dark  races  and  in  bru- 
nettes of  the  white  race,  in  whom  the  pigment  is  more  copious.  It  is  not 
infrequently  seen  during  congestion  of  the  internal  coats  of  the  eye.  The 
pigmented  cells  are  sometimes  scattered  in  irregular  masses  throughout  the 
entire  substance  of  the  cornea,  even  on  its  outer  surface,  especially  near  the 
exits  of  the  anterior  ciliary  veins.  This  is  quite  common  among  negroes. 
In  old  age  the  sclera  becomes  of  a  dull  yellowish  hue,  due  to  the  infiltra- 
tion of  fat.  This  is  especially  marked  in  the  neighborhood  of  the  cornea. 
If  abrasions  of  the  conjunctiva  exist,  the  sclera  may  be  darkened  by  the 
incautious  use  of  eye-washes  containing  nitrate  of  silver. 

1  From  OK^.rip6f,  hard.      Syn  :  tunica  sclerotica ;   sderotica ;   sclerotic  coat ;  sclerotic  ; 
tunica  alba  or  albuginea;  white  of  the  eye ;  cornea  opaca. 

According  to  Hyrtl  (Onomatologia  anatomica),  sclera  is  the  older  term  and  more 
accurate  etymologically.  Oribasius  calls  the  outer  coat  aKtypd  fiffviy^  (dura  membrana),  and 
Galen  Tra%v  KCU  aK^r/pbv  aKEiraafia  bfydakfiov  (crassum  et  durum  involucrum  oculi).  Sclerotica 
is  a  barbarism  that  first  appears  in  the  Latin  translations  of  Khazes  and  Avicenna. 

2  From  li.fuscus,  -a,  -um,  dark,  dusky,  swarthy.     Syn.  :  tunica  arachnoidea  oculi. 


THE  ANATOMY  OF  THE  EYEBALL. 


127 


The  statement  of  Schafer  in  the  last  edition  of  Quain's  Anatomy,  that 
the  maintenance  of  the  form  of  the  eye  chiefly  depends  on  the  sclera,  is 
inexact.  In  some  of  the  lower  vertebrates,  it  is  true,  the  sclera  is  rigid 
and  maintains  its  form  after  the  contents  of  the  globe  have  been  evacuated, 
but  in  man  it  is  the  pressure  of  those  contents  that  controls  the  form  ;  when 
they  are  copious  the  globe  is  full  and  round,  when  scanty  it  is  flaccid  or 
collapsed.  Behind,  where  the  sclera  supports  the  retina,  and  where  con- 
stancy of  curvature  is  important  in  order  that  the  optical  image  may  be 
accurately  projected,  the  greatest  thickness  is  found,  reaching  nearly  one 
millimetre  in  the  neighborhood  of  the  optic  nerve.  From  this  region  for- 
ward it  grows  rapidly  thinner  until,  just  behind  the  insertion  of  the  recti 
muscles,  its  thickness  is  no  more  than  four-tenths  of  a  millimetre.  Here 


FIG.  14. 


Bulbar  fascia. 
I 


Vessel  surround- 
ed    by     lym-     .== 
phatic  sheath 


Lamina  fusca. 


Lamina  vascu- 

losa. 


Basilar  lamina. 


Choriocapillaris. 


Transverse  section  of  sclera  and  chorioid.    (Altered  from  Testut.) 


it  is  liable  to  bulge  when  intra-ocular  pressure  is  increased,  and  here  also 
perforation  and  evacuation  of  pus  occur  when  deep-seated  inflammations  are 
neglected.  In  front  of  the  insertion  of  the  tendons  it  receives  a  reinforce- 
ment due  to  a  blending  with  their  tissue,  being  six-tenths  of  a  millimetre 
thick.  Yet  this  appears  to  be  the  weakest  part  of  the  capsule,  for  it  is 
here,  near  the  corneal  margin,  that  the  eye  usually  ruptures  from  external 
violence.  Because  of  the  inextensible  character  of  the  fibrous  capsule,  a 
rupture  is  almost  invariably  by  contre-coup, — that  is  to  say,  opposite  the 
side  on  which  the  blow  is  received. 

An  idea  of  the  density  of  the  scleral  tissue  may  be  obtained  by  com- 
paring its  weight  with  that  of  the  entire  eye.  The  most  recent  estimates 
on  this  subject  are  those  of  Testut,  who  found  that  an  average  taken  from 
both  eyes  of  five  adults  showed  that  the  sclera  is  about  one-sixth  the  total 


128 


THE  ANATOMY  OF  THE  EYEBALL. 


weight  of  the  eye.1  Among  previous  observers,  Sappey  states  it  as  one-ninth, 
while  Huschke  gives  it  as  one-fourth,  which  seems  unreasonably  large. 

The  external  surface  of  the  sclera  is,  as  has  already  been  mentioned, 
clothed  with  episcleral  tissue  connected  with  the  orbital  aponeurosis.  An- 
teriorly this  is  continuous  with  the  subconjunctival  connective  tissue.  Within 
this  tissue  there  runs  a  net-work  of  vessels  connecting  with  those  of  the 
conjunctiva,  but  closely  attached  to  the  ball  and  not  affected  by  the  move- 
ments of  that  membrane.  The  injected  appearance  of  this  net-work  may 
be  an  important  sign  in  inflammation  of  the  sclera. 

The  four  recti  muscles  of  the  eye  passing  forward  from  the  apex  of  the 
orbit  are  inserted  by  flattened  tendons  on  the  anterior  portion  of  the  ex- 
ternal surface  of  the  sclera,  near  the  corneal  margin.  The  distances  from 
the  cornea  of  the  insertions  of  the  several  tendons  can  for  practical  pur- 
poses be  remembered  by  the  round  numbers  given  by  Tillaux,  as  follows : 2 

Rectus  medialis 6  millimetres. 

Rectus  inferior 6  millimetres. 

Rectus  lateralis 7  millimetres. 

Rectus  superior 8  millimetres. 

The  proximity  of  the  line  of  insertion  to  the  cornea  has  some  effect 
on  the  action  of  the  muscle,  for  it  is  evident  that  the  nearer  it  is  set  the 
farther  it  can  roll  the  eye,  and  therefore  the  more  effect  it  can  have  upon 
changing  the  direction  of  its  axis.  The  tendons  of  the  medial  and  inferior 
recti,  being  the  nearest,  can  have  a  greater  effect  than  the  others,  and  it  is 
interesting  to  note  that  it  is  precisely  these  muscles  that  are  most  frequently 
operated  upon  for  strabismus. 

It  will  be  seen  upon  an  inspection  of  Figs.  15  and  16  that  the  inser- 
tions follow  a  somewhat  spiral  course,  commencing  with  the  rectus  medialis 
and  passing  downward  and  outward  to  the 'rectus  superior.  They  are  not, 
however,  continuous  with  each  other,  and  the  idea  sometimes  advanced  that 
there  is  a  common  sheet  into  which  they  all  pass  at  their  insertion  has  no 
foundation  in  fact. 

The  mid-points  of  the  insertions  of  the  rectus  lateralis  and  medialis 

1  Accurately,  as  1  :  6.15. 

2  The  actual  distances  are  given  by  different  authorities  as  follows  : 


Rectus 
Medialis. 

Rectus 
Inferior. 

Rectus 
Lateralis. 

Rectus 
Superior. 

Fuchs     .           

Millimetres. 
5.5 

Millimetres. 
6.5 

Millimetres. 
6.9 

Millimetres. 

7.7 

Testut    

5.8 

6.5 

7.1 

8 

Gerlach  ... 

5.4 

6.9 

7.2 

7.5 

Sappey  . 

5.5 

6.7 

7.2 

8.5 

Krause  

6.91 

7.07 

7.85 

7.54 

Macalister     

7 

7 

7.5 

7.5 

The  measurements  of  Fuchs,  being  taken  from  a  larger  series  of  subjects,  are  perhaps 
most  widely  accepted.  See  also  page  97.  It  should  be  noted  that  Merkel  accepts  Fuchs's 
measurements  as  more  accurate  than  his  own.  See  his  Topographische  Anatomic,  i.  293. 


FIG.  15. 


Insertion  of  the  ocular  muscles  upon  the  Sclera  of  the  right  eye.  (Drawn  from  the  determinations 
of  Fuchs.)— A,  view  from  above ;  B,  from  nasal  side  ;  C,  from  below ;  D,  from  temporal  side  ;  a,  rectus 
superior;  b,  rectus  inferior;  c,  rectus  medialis;  d,  rectus  lateral  is;  e,  obliquus  superior;/,  obliquus 
inferior ;  EE,  equator  ;  ocx,  axis. 


FIG.  16. 


Insertion  of  the  ocular  muscles  upon  the  sclera  of  the  left  eye  as  seen  from  the  front.  (Drawn  from 
the  determinations  of  Fuchs.)—  a,  rectus  superior;  b,  rectus  inferior;  c,  rectus  medialis;  d,  rectus 
lateralis. 


THE  ANATOMY  OF  THE  EYEBALL.  129 

fall  almost  exactly  in  the  horizontal  meridian,  while  those  of  the  rectus 
superior  and  inferior  deviate  somewhat  from  the  vertical  meridian,  that  of 
the  latter  muscle  lying  over  one  millimetre  to  the  nasal  side.  (Gerlach.) 

The  lines  of  insertion  are  usually  slightly  curved,  with  the  convexity 
towards  the  cornea.  The  ends  of  the  lines  of  the  rectus  superior  are  some- 
times bent  abruptly  backward.  Occasionally  the  lines  are  not  regularly 
curved,  but  wavy  in  their  course. 

The  insertion  is  by  no  means  a  merely  superficial  one.  The  fibres  of 
the  tendon  interpenetrate  and  firmly  unite  with  the  connective-tissue  net- 
work of  the  sclera,  spreading  out  in  a  fan-like  manner,  and  thereby  causing 
the  thickening  already  referred  to. 

The  oblique  muscles  are  inserted  well  back  in  the  posterior  hemisphere 
of  the  eyeball,  from  six  to  eight  millimetres  from  each  other.     (See  Fig.  17.) 
The  lines  of  their  insertion  are  directed  diagonally 
across  the  meridians,  that  of  the  superior  oblique  FIG.  17. 

making  an  angle  of  forty-five  degrees  with  the  ver- 
tical meridian,  while  that  of  the  inferior  oblique 
makes  with  the  horizontal  meridian  one  of  about 
nineteen  degrees.  Their  insertions  are  not,  there- 
fore, parallel,  as  is  sometimes  stated.  That  of  the 
superior  oblique  is  mainly  in  the  superior  external 
quadrant  of  the  hemisphere ;  its  distance  from  the  insertion  of  the  ocular 
sheath  of  the  optic  nerve  is  from  seven  to  ten  milli-  muscies  upon  the  sciera  of 

.     '  .  .    . .  .  the  right  eye,  as  seen  from 

metres,  and  its  anterior  end  lies  in  about  the  same  the  rear.  (Drawn  from  the 
meridian  as  the  external  end  of  the  rectus  superior,  determinations  of  _  Fuchs.)- 

r  '     xx,   vertical    meridian;   yy, 

from  one-half  to  eight  millimetres  from  it  (average,    horizontal  meridian. 
4.6  millimetres),  and  about  the  same  distance  behind 

the  equator  as  the  latter  is  in  front.  A  sheet  of  firm  connective  tissue 
often  unites  the  two  tendons. 

Fuchs1  states  that  there  are  two  types  of  insertion  of  the  superior 
oblique :  one  where  the  line  has  an  equatorial  direction  and  crosses  the 
equatorial  diameter,  another  where  it  is  more  nearly  meridional  and  en- 
tirely in  the  external  quadrant.  In  the  first  type  the  line  is  long,  with  a 
strong  forward  concavity,  in  the  second  it  is  shorter  and  flatter,  beginning 
farther  forward.  The  first  is  usual  in  emmetropic  and  hypermetropic  eyes, 
the  second  in  myopic.  The  emmetropic  form  is  probably  the  primitive  one, 
the  myopic  being  a  modification  induced  in  early  life  by  straining  the  muscle 
for  near  vision,  this  causing  it  to  shift  its  insertion  to  a  more  favorable  posi- 
tion by  the  well-known  process  of  inducing  atrophy  by  tension  on  one  side 
while  new  fibres  develop  on  the  other.  It  should  be  noted,  however,  that 
the  second  variety  may  be  found  in  eyes  with  perfectly  normal  vision. 

The  length  of  the  line  of  insertion  of  the  superior  oblique  varies  con- 
siderably, as  might  be  supposed  from  the  above.  Fuchs  found  that  in 

1  Fuchs  (Ernst).     Beitriige  zur  normalen  Anatomic  des  Augapfels.    Archiv  fur  Oph- 
thalmologie,  1884. 
VOL.  I.— 9 


130 


THE  ANATOMY  OF  THE  EYEBALL. 


thirty-one  emmetropic  eyes  it  averaged  10.7  millimetres  (from  7.5  to  12.7), 
and  that  it  was  shorter  in  myopic  eyes,  in  twenty  of  which  it  averaged  9.6 
millimetres  (from  6.8  to  14). 

The  inferior  oblique  is  peculiar  in  having  no  tendon  of  insertion,  its 
striated  fibres  passing  close  up  to  the  sclera  and  even  penetrating  its  sub- 
stance. The  line  of  insertion  is  very  variable,  as  it  usually  lies  in  the 
inferior  external  quadrant  of  the  posterior  hemisphere  and  is  affected  by 
the  antero-posterior  extension  that  takes  place  in  this  region  in  myopic 
eyes.  It  is  quite  near  the  posterior  pole,  its  distance  from  it  averaging  2.2 
millimetres  in  emmetropic  and  4.1  millimetres  in  myopic  eyes.  The  an- 
terior end  of  the  line  is  usually  in  the  same  meridian  with  the  lower  end 
of  the  insertion  of  the  rectus  lateralis,  the  two  being  about  the  same  dis- 
tance from  the  equator  and  about  9.5  millimetres  (from  6.8  to  11.2  milli- 
metres) apart.  This  separation  is  greater  in  myopic  eyes.  The  insertion 
may  lie  partly  or  wholly  above  the  horizontal  meridian  and  be  placed  at 
varying  degrees  of  obliquity.  The  line  is  usually  a  flat  curve,  with  its  con- 
vexity upward  and  a  little  forward.  Further  details  are  given  on  page  98. 
Regarding  the  sclera  as  a  segment  of  a  spheroidal  body,  the  surface 

appears  incomplete  in  two  places  (Fig.  1 8)  : 
one  in  front,  where  the  cornea  is  set, 
known  as  the  corneal  interval ; l  the  other 
behind,  where  the  optic  nerve  enters  the 
optic  foramen  or  canal.2  At  the  corneo- 
scleral  junction  the  irregular  fibres  of  the 
sclera  are  intimately  blended  with  the 
lamellated  fibres  of  the  cornea,  and  they 
are  so  arranged  that  the  opaque  sclera 
overlaps  the  transparent  cornea  externally, 
so  that  the  edge  of  the  foramen  is  bevelled 
at  the  expense  of  its  interior  surface.  This 
bevelling  is  not  uniform,  being  greater  above  and  below  than  at  the  sides, 
so  that  the  scleral  limit  appears  anteriorly  slightly  elliptical  with  major  axis 
horizontal,  while  within  it  is  circular.  The  axes  of  the  ellipse  are  1 1  and 
11.6  millimetres;  the  diameter  of  the  circle  is  11.9  millimetres.  (Merkel.3) 

1  Syn. :   anterior  scleral  foramen ;  foramen  sclerce  anterius ;  foramen  corneas ;  rima 
cornealis.     (Anatomische  Gesellschaft,  1895.) 

2  Syn. :  foramen  opticum  sclerce  or  sclerotica. 

3  The  following  measurements  are  given  by  other  authorities : 


FIG.  18. 


11  to  11.6 


Intervals  in  the  sclera.    (Testut.)— A,  cor 
neal  interval ;  B,  optic  foramen. 


Ellipse. 

Circle. 

Minor  Axis. 

Major  Axis. 

Diameter. 

Schwalbe  (following  Helmholtz  and  Knapp)     .... 
Testut     

11 
11 

10 

11.9 
12 

12 

13 
12 
13 

Vierordt      

Sappev    • 

THE  ANATOMY  OF  THE  EYEBALL. 


131 


Occasionally   the   innermost   edge  of  the   bevel   overlaps   the   cornea 
slightly,  thus  making  a  regular  setting  or  groove  for  it  like  the  metal  rim 


FIG.  19. 


FIG.  20. 


The  junction  of  the  sclera  and  the  cornea.— a,  corneal  conjunctiva;  6,  scleral  sinus;  c,  pectinate  liga- 
ment ;  d,  scleral  conjunctiva ;  e,  radial  ciliary  muscle ;  /,  circular  muscle ;  g,  ciliary  process ;  It,  lens. 

that  holds  a  watch-crystal,  and  justifying  the  name  of  Comealfalz  (corneal 
groove  or  setting)  that  the  German  anatomists  give  to  this  junction. 

The  relations  of  the  corneo-scleral  junc- 
tion to  the  structures  within  the  eye  should 
be  carefully  noted,  as  operative  incisions 
are  often  made  in  this  neighborhood.  On 
examining  Fig.  19  it  will  be  seen  that  the 
iris  is  attached  just  at  the  posterior  edge  of 
the  bevel,  so  that  an  incision  at  or  even  a 
little  behind  the  anterior  edge,  which  ap- 
pears externally  as  the  corneal  margin, 
must  necessarily  enter  the  anterior  chamber 
or  space  in  front  of  the  iris.  To  enter  the 
posterior  chamber  or  space  between  the  iris 
and  the  lens,  the  incision  or  puncture  must 
be  from  three  to  four  millimetres  behind 
the  external  corneal  edge. 

The   optic   foramen   or   canal   of   the 
sclera  is  situated  about  three  millimetres 
from  the  posterior  pole  on  the  nasal  side 
and  about  one  millimetre  below  the  horizontal  meridian.     (See  Fig.  20.) 
By  macerating  a  specimen  it  is  easy  to  show  that  it  is  not  a  simple  single 


Posterior  view  of  right  eye,  showing 
entrance  of  optic  nerve.  (Testut)— A, 
nasal  side;  B,  temporal  side.  1,  verti- 
cal meridian ;  2,  horizontal  meridian ; 
3.  optic  nerve;  4,  4,  ciliary  vessels  and 
nerves ;  4',  4',  long  ciliary  arteries ;  5,  5', 
superior  vorticose  veins;  6,  6',  inferior 
vorticose  veins. 


132 


THE  ANATOMY  OF  THE  EYEBALL. 


canal,  but  a  series  of  minute  orifices  through  which  pass  the  single  fibres 
of  the  optic  nerve.  On  reaching  the  sclera  the  external  or  dural  sheath 
of  the  nerve  becomes  continuous  with  it,  and  the  connective  tissue  that 
surrounds  and  interpenetrates  the  different  bundles  of  fibres  (periueurium, 
endoueurium)  becomes  condensed  to  a  perforated  sheet  that  stretches  across 
the  canal  and  is  continuous  with  the  sclera  on  either  side.  This  is  the 
cribrose  lamina,1  so  called  from  its  numerous  orifices  for  the  axis-cylinders 
of  the  nerve-fibres.  (See  Fig.  21.)  The  analogy  of  this  structure  with  the 
cribriform  plate  of  the  ethmoid,  through  which  pass  the  filaments  of  the 


Section  through  the  optic  nerve  entrance. — a,  retina ;  6,  chorioid ;  c,  sclera ;  d,  lamina  cribrosa ; 
e,  depression ;  /,  blood-vessels. 

olfactory  nerve,  and  with  the  tractus  spiralis  foraminosus  of  the  temporal 
bone,  through  which  pass  fibres  of  the  auditory  nerve,  is  very  striking. 

As  the  fibres  of  the  optic  nerve  pass  forward  to  go  through  the  lamina 
cribrosa  they  lose  their  medullary  sheaths.  Hence  the  diameter  of  the 
nerve  gradually  decreases,  and  the  optic  canal  of  the  sclera  is  funnel-shaped, 
its  entrance  being  from  3  to  3.5  millimetres  in  diameter,  its  exit  from  1  to 
1.5  millimetres.  All  around  the  interior  opening  the  sclera  projects  with 
a  crest-like  edge,  which  may  be  called  the  pecten  sclerae?  or  scleral  rim. 
Hyrtl  points  out  that  the  intimate  connection  between  the  sheath  of  the 
optic  nerve  and  the  sclera  is  the  cause  of  the  phenomena  of  subjective  light 
sensations  and  weakness  of  vision  which  occur  in  rheumatic  scleritis  and 
other  rheumatic  inflammations  of  the  eye. 

1  From  L.  cribrosus,  -a,  -um,  riddled,  perforated  like  a  sieve.    Syn. :  lamina  cribrosa; 
cribrum.     (Langer.) 

2  From  L.  pecten,  a  comb  or  crest :  the  Skeralkamm  of  Gerlach. 


THE  ANATOMY  OF  THE  EYEBALL.  133 

Besides  these  large  openings  in  the  sclera,  there  are  other  smaller  ones 
for  the  arteries,  veins,  and  nerves  that  pass  through  it  to  supply  structures 
in  the  interior  of  the  eyeball.  Of  these  there  are  three  sets,  situated 
respectively  posteriorly,  anteriorly,  and  midway  between  the  other  two. 

The  foramina  of  the  posterior  set  are  for  the  posterior  ciliary  arteries. 
These  take  a  very  oblique  course  through  the  sclera.  Twigs  from  the  short 
posterior  ciliary  arteries  form  a  vascular  circle  around  the  optic  nerve,  known 
as  the  circulus  vasculosus  nervi  optid,  or  circlet  of  Zinn.1 

The  foramina  of  the  anterior  set  are  for  the  anterior  ciliary  arteries 
and  veins.  Those  for  the  veins  are  so  small  as  to  be  barely  visible,  and  are 
quite  near  the  corneal  edge ;  those  for  the  arteries — five  to  eight  in  number 
— are  larger,  and  are  situated  farther  back,  some  two  millimetres  from  the 
cornea.  These  vessels  anastomose  by  episcleral  branches  .with  the  conjunc- 
tiva! arteries,  and  then  penetrate  the  sclera  to  supply  the  iris  and  other 
structures  in  the  vicinity.  Hence  they  are  injected  in  iritis,  and  one  of 
the  most  significant  signs  of  that  affection  is  a  pericorneal  blush  about 
two  millimetres  from  the  corneal  margin,  from  which  it  is  separated  by  a 
grayish  line  corresponding  to  the  scleral  bevel,  comparatively  free  from 
blood-vessels. 

The  foramina  of  the  middle  set  are  for  the  vorticose  veins,2  the  principal 
venous  channels  of  discharge  from  the  eyeball.  Fuchs  has  carefully  in- 
vestigated these  important  members  of  the  circulatory  apparatus  of  the  eye. 
He  finds  their  typical  arrangement  to  be  as  follows.  Four  veins  reach  the 
ball,  grouped  in  two  pairs,  an  upper  and  a  lower.  The  two  veins  of  the 
upper  pair  lie  on  either  side  of  the  vertical  meridian,  but  are  not  sym- 
metrically placed  with  regard  to  it,  the  outer  vein  being  somewhat  the 
nearer  to  the  meridian.  They  penetrate  the  sclera  from  7  to  8  millimetres 
behind  the  equator,  the  external  one  being  somewhat  farther  back  than  the 
other.  The  two  veins  of  the  lower  pair  have  a  similar  relation  to  the 
lower  half  of  the  vertical  meridian,  but  the  veins  penetrate  the  sclera  some- 
what farther  forward,  from  5.5  to  6  millimetres  from  the  equator.  All  the 
veins  pass  through  the  sclera  very  obliquely,  their  courses  trending  forward 
and  diverging  from  the  vertical  meridian.  They  emerge  from  the  inner 
surface  of  the  sclera  from  2.5  to  3.5  millimetres  behind  the  equator.  Within 
the  sclera  each  vein  is  surrounded  by  a  perivascular  lymph-space,  across 
which  pass  fine  trabeculas  of  connective  tissue  that  attach  the  vein  to  the 
wall  of  the  canal.  The  other  blood-vessels  and  nerves  that  pass  through 
the  sclera  are  also  surrounded  by  lymph-spaces.  All  these  open  on  one 
side  into  the  periscleral  space,  on  the  other  into  the  perichorioideal  space. 
Many  variations  of  the  vorticose  veins  occur.  The  number  of  the  veins 
may  be  from  five  to  seven,  the  internal  ones  being  more  frequently  doubled 

1  Named  for  J.  G.  Zinn,  a  naturalist  of  Gottingen,  born   1727,  died  1759.     Syn. : 
circulus  arteriosus  nervi  optici  or  Zinnii ;  Skleralgefdsskranz,  G.  ;  Zmnscher  or  Hallerscher 
Kronz,  G. 

2  Svn.  :  vence  vorticosce  or  stellatce. 


134  THE  ANATOMY  OF  THE  EYEBALL. 

than  the  others.  When  .one  of  the  veins  divides  before  penetrating  the 
solera,  its  branches  may  pass  for  some  distance  along  the  surface  before 
entering,  reaching  sometimes  nearly  to  the  insertions  of  the  recti  muscles. 

Reference  has  been  made  (see  page  104)  to  the  effect  which  the  oblique 
muscles  of  the  eye  have  upon  the  vorticose  veins.  The  veins  on  the  outer 
side  above  and  below  may  be  compressed  by  the  tendons  of  these  muscles, 
those  on  the  inner  side  being  unaffected.  The  most  favorable  position  for 
compression  is  that  in  which  the  eyes  are  adjusted  for  looking  at  near 
objects.  Fuchs  has  suggested  that  this  reduction  of  two  of  the  principal 
venous  exits  of  the  eye  may  cause  a  stasis  of  blood  with  consequent  in- 
crease of  the  internal  pressure,  which,  when  the  eyes  are  young  and  the 
tissues  flexible,  may  result  in  a  lengthening  of  the  autero-posterior  axis, 
with  consequent  myopia.  It  is  well  known  that  children  who  are  forced  to 
use  their  eyes  much  in  reading  and  other  close  occupations  at  an  early  age 
are  likely  to  become  near-sighted. 

The  internal  surface  of  the  sclera,  bounded  by  the  lamina  fusca,  is,  like 
the  external,  separated  from  the  neighboring  tissues  by  a  lymph-space,  the 
perichorioideal  space  (Fig.  14),  across  which  pass  trabeculse  much  thicker 
and  stronger  than  those  of  the  episclera.  When  the  sclera  is  stripped  away 
from  the  chorioid,  its  internal  surface  appears  ragged  and  tattered  because 
of  the  breaking  and  fraying  of  the  trabeculse  that  pass  to  the  supra-cho- 
rioideal  lamina  of  the  chorioid,  a  tissue  closely  resembling  the  lamina  fusca 
of  the  sclera.  The  pigmented  character  of  the  lamina  fusca  has  already 
been  mentioned.  Like  other  lymphatic  spaces,  the  perichorioideal  space  is 
lined  with  endothelium,  which  is  continued  upon  the  trabeculse.  It  com- 
municates with  the  periscleral  space  by  means  of  perivascular  and  peri- 
neural  lymph-channels,  as  before  indicated.  Along  the  walls  of  the  peri- 
chorioideal space,  making  grooves  in  the  lamina  fusca,  run  the  filaments  of 
the  ciliary  nerves,  which,  having  pierced  the  sclera  near  the  posterior  pole, 
are  passing  forward  to  be  distributed  to  the  cornea,  the  iris,  and  the  ciliary 
muscle.  Hence  these  nerves  are  confined  between  the  chorioid  and  the 
inextensible  sclera,  and  in  case  of  any  increase  of  intra-ocular  tension  they 
become  compressed,  often  with  serious  consequences.  In  acute  glaucoma 
— a  disorder  accompanied  by  increased  pressure — the  sensitiveness  of  the 
cornea  and  the  reaction  of  the  iris  may  be  wholly  abolished  by  compression 
of  these  nerves.  The  pain  in  these  cases  may  be  very  severe  and  of  a  dull, 
sickening  character  like  that  in  orchitis,  which,  it  will  be  remembered,  is 
also  due  to  the  compression  of  nerve-filaments  against  an  inextensible 
fibrous  envelope,  the  tunica  albuginea  testis.  The  long  ciliary  arteries  also 
run  along  the  walls  of  the  space,  and  may  be  affected  by  compression. 

In  structure  the  sclera  is  a  firm,  dense,  fibrous  membrane.  It  cannot 
be  separated  into  lamellae,  as  its  fibrous  bundles  are  intimately  interwoven, 
crossing  mainly  at  right  angles,  running  in  meridional  and  equatorial  direc- 
tions, those  having  the  former  course  being  seen  mostly  on  the  surface  and 
behind,  those  of  the  latter  around  the  cornea.  Advantage  is  taken  of  this 


THE   ANATOMY   OF   THE   EYEBALL.  135 

in  incising  the  sclera,  a  meridional  cut  being  usually  preferred  if  at  some 
distance  from  the  cornea,  as  it  is  less  likely  to  gape.  The  tendons  of  the 
recti  muscles  reinforce  the  sclera  with  meridionally  directed  fibres,  while 
those  of  the  oblique  muscles  become  equatorial.  In  both  cases  the  fibres 
penetrate  obliquely  through  the  entire  thickness  of  the  tunic.  Near  the 
posterior  pole  (region  of  the  fovea  centralis)  there  is  found  a  strand  of 
fibres  that  penetrates  obliquely  all  the  layers.  This  is  called  the funiculus 
sclerse  by  Hannover,  who  held  it  to  be  a  cicatrix  showing  where  the  cho- 
rioideal  fissure  of  foetal  life  closed  up.  Schwalbe  concludes,  after  careful 
examination,  that  this  is  not  the  true  interpretation  of  the  structure,  but 
that  it  is  merely  a  strand  of  connective  tissue  that  accompanies  the  posterior 
ciliary  arteries.  Ammon1  has  described  in  the  sclera,  as  well  as  in  the 
deeper  tunics  of  the  eye,  some  traces  of  the  primitive  chorioideal  fissure, 
and  has  named  them  the  raphe  sclerse.  Whenever  the  sclera  is  cut,  the 
cicatricial  tissue  that  forms  at  the  wound  is  of  an  irregular  character,  and 
may  readily  be  distinguished  from  the  other  portions  of  the  membrane. 

Between  the  interwoven  bundles  lie  lacunar  spaces,  like  those  of 
aponeurotic  membranes,  constituting  an  intricate  and  extensive  system  of 
lymphatic  canaliculi  that  communicate  on  the  one  hand  with  the  peri- 
chorioideal  space,  on  the  other  with  the  periscleral  space.  Some  of  these 
lacunae  are  lined  with  endothelium. 

It  is  mostly  by  this  lymphatic  circulation  that  the  sclera  is  nourished, 
for  it  is  very  scantily  supplied  with  blood-vessels.  A  few  branches  from 
the  ciliary  arteries  penetrate  its  substance,  forming  a  large-meshed  capillary 
net-work.  Each  artery  is  usually  accompanied  by  two  veins.  As  a  conse- 
quence of  this  scanty  blood-supply,  the  sclera  is  but  little  disposed  to  inflam- 
mation, operations  upon  it  are  usually  attended  with  favorable  results,  and 
it  bears  sutures  very  well. 

Near  the  corneal  margin  there  runs  in  the  deeper  portion  of  the  sclera  a 
passage  concerning  which  much  controversy  has  arisen.  This  is  the  scleral 
sinus  or  canal  of  Schlemm,  which  may  be  described  as  an  overflow  channel 
surrounding  the  cornea.  It  is  connected  with  the  venous  system,  but  is 
also  described  as  a  lymph-space.  It  appears  to  be  empty  under  ordinary 
conditions.  When  intra-ocular  pressure  is  increased,  it  affords  some  relief 
by  acting  as  a  conduit  of  discharge.  Its  minute  description  and  connec- 
tions will  be  given  later. 

The  nerves  of  the  sclera  are  derived  from  the  ciliary  nerves.  Within 
the  substance  of  the  tunic  they  first  lose  their  medullary  sheaths,  then 
break  up  into  fine  fibrillse  that  appear  to  end,  like  the  nerves  of  the  cornea, 
in  pointed  filaments  between  the  bundles  of  the  connective-tissue  fibres. 
The  investigation  of  the  scleral  nerve  endings  by  modern  methods  offers  a 
promising  field. 

In  its  chemical  characters  the  sclera  resembles  other  fibrous  tissues  in 

1  Prager  Vierteljahrsschrift,  1860,  i.  140. 


136  THE  ANATOMY  OF  THE  EYEBALL. 

that  it  yields  gelatin  upon  boiling.     Wagner  found  that  the  fresh  sclera 
of  the  pig  yielded  65  per  cent,  of  water  and  .867  per  cent,  of  ash. 

THE    CORNEA. 

As  object-glass  of  the  ocular  camera,  the  cornea l  is  one  of  the  most 
important  portions  of  the  apparatus.  Being  necessarily  placed  at  the  front, 
and  exposed  whenever  the  eyelids  are  parted,  it  is  more  frequently  injured 
than  any  other  part  of  the  eye.  In  occupations  in  which  small  flying 
particles  abound,  such  as  grinding,  stone-cutting,  and  some  forms  of  iron- 
working,  fragments  often  impinge  upon  it  or  bury  themselves  in  its  sub- 
stance, requiring  for  their  removal  operative  interference. 

Perhaps  the  most  striking  property  of  the  cornea  is  its  almost  perfect 
transparency.  This  is  so  nearly  complete  that  under  ordinary  conditions 
we  are  quite  unconscious  of  any  obstacle  to  the  passage  of  light.  When 
viewed  obliquely  or  under  a  strong  light,  the  cornea  may,  however,  be  seen, 
as  some  reflection  then  occurs  from  the  individual  fibres.  It  seems  astonish- 
ing that  a  membrane  of  so  complex  a  constitution  should  be  transparent, 
composed  as  it  is  of  a  stroma  of  connective-tissue  fibres  in  several  layers, 
lined  in  front  and  behind  with  epithelium  resting  on  a  basement  membrane, 
having  also  within  its  substance  two  kinds  of  cells,  a  complicated  system 
of  lymph-passages,  and  an  intricate,  close-meshed  plexus  of  nerves. 

Evidently  all  these  structures  must  be  transparent.  But  it  is  well 
known  that  a  mixture  of  transparent  substances  intercepts  the  light,  the 
rays  being  thrown  out  of  their  direct  course  and  diffused  by  passing  from 
one  to  another.  Thus,  while  a  sheet  of  pure  ice  is  transparent,  it  becomes 
opaque  when  broken  into  fine  fragments,  being  then  intermingled  with  air, 
which  has  a  different  refractive  power.  By  adding  water  to  the  fragments, 
transparency  is  partially  restored,  the  index  of  refraction  of  water  more 
closely  approaching  that  of  ice. 

The  elements  of  the  cornea  must,  therefore,  have  about  the  same  re- 
fractive power.  Are  they  also  aided  by  the  interposition  of  some  fluid 
medium  ?  It  was  formerly  thought  that  this  was  the  case,  and  that  trans- 
parency was  insured  by  an  infiltration  of  the  aqueous  humor  from  the  an- 
terior chamber  of  the  eye.  This  is,  however,  so  far  from  being  true,  that 
when  an  infiltration  does  occur  by  reason  of  a  lesion  of  the  epithelial  lining 
of  the  chamber,  or,  after  death,  by  a  change  in  the  osmotic  action  of  the 
membrane,  the  cornea  speedily  becomes  clouded.  Besides,  when  desiccated 
so  that  no  fluid  remains  between  its  elements,  it  still  retains  its  transparency. 

The  interposition  of  a  fluid  medium  is,  therefore,  unnecessary.  Why, 
then,  are  not  the  rays  diffused  as  they  are  by  the  fragments  of  broken  ice  ? 
If  those  fragments  could  be  replaced  in  their  relative  situations  with  no  film 

1  From  L.  corneus,  -a,  -ttm,  horny.  Syn. :  cornea  pellucida.  Galen's  term  for  the 
cornea  was  Keparosidrjq  X'ITUVI  horn-like  tunic,  from  «£paf,  -a-of.  From  this  is  derived  the 
term  keratitis  for  inflammation  of  the  cornea.  Hyrtl  points  out  that  the  term  should 
properly  be  keratoiditis. 


THE  ANATOMY  OF  THE  EYEBALL.  137 

of  extraneous  matter  intervening,  it  is  evident  that  the  transparency  of  the 
mass  would  be  fully  restored. 

It  is  easily  shown  that  the  transparency  of  the  cornea  depends  on  such 
a  close  and  intimate  contact  of  its  elements,  for  any  action  that  tends  to 
disarrange  or  displace  these  elements  produces  an  opacity.  If,  for  example, 
a  perfectly  fresh  eye  be  compressed  between  the  thumb  and  the  finger,  the 
cornea  immediately  becomes  clouded,  returning  to  its  natural  condition  when 
the  pressure  is  removed.  This  also  occurs  when  the  membrane  is  compressed 
between  plates  of  glass  or  when  it  is  stretched  by  any  means.  In  acute 
inflammations  of  the  eye  the  pressure  caused  by  engorgement  of  the  vessels 
is  likely  to  produce  some  opacity.  At  birth  the  cornea  is  hazy,  and  it  also 
becomes  clouded  immediately  after  death,  owing  to  a  lessening  of  intra-ocular 
tension.  Gerlach  holds  that  the  slight  variations  of  tension  that  occur 
during  perfect  health  have  an  effect  upon  the  transparency  of  the  cornea, 
that  the  brilliant  sparkle  noted  in  emotional  excitement  and  during  fever 
is  due  to  the  increased  tension  effecting  a  more  accurate  readjustment  of 
the  elements,  and  that  a  decrease  of  tension  causes  the  dimness  of  the  eyes 
under  depressing  emotions  and  during  failure  of  bodily  powers. 

Slight  variations  in  transparency  due  to  other  causes  may  also  occur. 
The  conjunctiva  that  lines  the  outer  face  of  the  cornea  is  an  epithelial  mem- 
brane whose  cells  desquamate  and  are  shed  off  as  in  other  cuticular  struc- 
tures. The  desquamated  cells,  scattered  like  dust  upon  the  exposed  surface, 
temporarily  dim  the  sight  until  washed  away  by  the  lacrymal  secretion. 
Collections  of  such  cells  may  appear  in  the  field  of  vision  as  floating  specks, 
or  muscse  volitantes.  The  slight  dimness  which  is  noticed  upon  opening 
the  eyes  after  sleep,  and  which  causes  an  inclination  to  rub  them,  is  due 
to  such  a  collection. 

The  index  of  refraction  of  the  cornea  is  stated  by  Krause  as  1.3523, 
that  of  distilled  water  being  1.3358.1  ' 

Some  interesting  experiments  have  been  made  to  determine  the  be- 
havior of  the  cornea  with  regard  to  the  rays  of  the  invisible  portions  of 
the  spectrum.  Its  power  of  absorption  of  the  infra-red  or  heat  rays  is  a 
little  superior  to  that  of  water,  but  not  notably  so.  The  retina,  therefore, 
appears  to  be  remarkably  insensitive  to  heat  rays,  as  there  is  no  obstacle  that 
prevents  their  reaching  it.  The  chemical  or  ultra-violet  rays  also  appear 
to  pass  through  the  cornea  without  sensible  diminution.  There  is  some 
absorption  of  blue  rays,  as  light  that  has  passed  through  the  cornea  colors 
an  alcoholic  solution  of  guaiacum  a  yellowish  green. 

1  Krause  (W.).  Die  Brechungsindices  der  durchsichtige  Medien  des  menschlichen 
Auges,  1855. 

Other  authorities  have  given  the  index  as  follows  :  Charles  Chossat,  1.33  (Bull.  Soc. 
philomath,  de  Paris,  1818,  p.  95) ;  Aubert,  1.377  (Grafe  undSamisch,  Handb.  der  gesamm- 
ten  Augenheilkunde,  ii.  p.  409) ;  L.  Matthiessen,  1.3754  (Archiv  f.  die  gesammte  Physi- 
ologic, xix.,  1879,  p.  492);  A.  Macalister,  1.3825  (Text-Book  of  Human  Anatomy,  1889, 
P- 


138  THE  ANATOMY  OF  THE  EYEBALL. 

The  cornea  is  set  in  the  bevel  of  the  corneal  interval,  already  described. 
Its  tissue  does  not  suddenly  cease  at  its  circumference,  but  intimately  inter- 
blends  with  the  sclera.  (See  Fig.  22.)  It  appears  to  project  from  the 
latter,  the  amount  of  such  projection  being  2.7  millimetres  (Merkel1), 
measured  from  a  chord  drawn  at  its  base. 

The  curvature  of  its  anterior  or  exposed  surface  is  not  quite  regular, 
being  that  of  an  irregular  ellipsoid  whose  radius  in  the  horizontal  meridian 
is  7.8  millimetres,  while  in  the  vertical  meridian  it  is  but  7.7  millimetres. 
(Donders.)  The  variations  may,  however,  be  much  greater  than  these.  In 
consequence  of  this  asymmetry,  rays  proceeding  from  any  point  of  an  object 
are  not  all  accurately  focussed  to  a  corresponding  point  of  the  retina,  and  a 
defect  of  vision  known  as  astigmatism  results.  This  occurs  to  a  slight 
degree  in  every  eye.  Leroy 2  examined  the  eyes  of  fifteen  cuirassiers  of 
about  the  same  height,  physiological  habits,  and  education,  all  possessing 

FIG.  22. 


Fibres  of  the  sclera  blending  with  those  of  the  cornea.    (Bowman.) — xx,  line  of  junction. 

vision  nearly  or  quite  perfect,  taking  five  points,  one  at  the  centre,  the 
others  nineteen  degrees  from  the  centre,  above,  below,  to  the  right,  and  to 
the  left.  At  each  of  these  he  measured  the  curvature  of  the  horizontal 
and  vertical  meridians.  He  found  that  the  curvature  diminished  from  the 
centre  of  the  cornea  to  its  periphery,  and  was  less  on  the  temporal  side  than 
elsewhere.  Considering  the  temporal  flattening  as  unity,  the  flattenings 
above  and  below  would  be  two  and  the  nasal  flattening  four.  The  max- 
imum flattening  is  found  in  that  part  of  the  cornea  nearest  to  the  insertion 
of  the  rectus  medialis,  which  is  much  stronger  than  the  other  muscles  of 
the  eye.  He  holds  that  the  unequal  action  of  the  eye  muscles  is  the  main 
factor  in  producing  the  asymmetry  of  the  cornea. 

The  curvature  of  the  posterior  surface  appears  to  be  more  regular  and 
to  approach  that  of  a  sphere  having  a  radius  of  six  millimetres.  :(Merkel.3) 
Careful  observations  with  the  ophthalmometer  show  that  the  contraction 

1  Macalister,  op.  cit.,  gives  this  as  2.6  millimetres. 

2  Leroy  (C.  J.  A.).    Sur  la  forme  de  la  cornee  humaine  normale.    Comptes-rendus  de 
1'Acad.  des  Sciences,  Paris,  1888,  cvii.  696,  697. 

3  Macalister,  6.7  millimetres. 


THE  ANATOMY  OF  THE  EYEBALL.  139 

of  the  muscles  of  the  eye  in  noway  affects  the  curvature  of  the  cornea,  and 
that  accommodation  for  near  and  remote  vision  cannot  be  affected  by  it. 

Since  the  outer  and  inner  surfaces  are  not  similarly  curved,  it  follows 
that  they  cannot  be  parallel,  and  the  cornea  must  vary  in  thickness.  It  is 
somewhat  thicker  than  the  sclera,  averaging  .9  millimetre  at  the  periphery. 
(Merkel.1)  Abscesses  are  more  likely  to  perforate  the  membrane  at  its 
thinner  portion. 

The  cornea  attains  its  permanent  dimensions  very  early,  and  varies  but 
little  after  the  third  year.  It  appears,  therefore,  that  accurate  sight  is  of 
so  much  importance  to  the  young  animal  that  the  development  of  that 
organ  is  hastened  much  beyond  that  of  the  rest  of  the  body.  It  has  been 
very  aptly  said  by  Priestley  Smith 2  that  as  the  brain  develops  faster  than 
the  general  mass  of  the  body,  so  the  eye  develops  faster  than  the  brain  and 
the  cornea  faster  than  the  rest  of  the  eye.  Petit  states  that  the  absolute 
thickness  of  the  cornea  is  greater  at  birth  than  at  any  other  time  of  life. 
In  young  children  it  is  frequently  thicker  at  the  centre,  and  to  this  has 
been  ascribed  the  short-sightedness  common  among  infants.  The  cornea  is 
of  slightly  smaller  diameter  in  females  (.1  millimetre).  Its  size  has  no 
relation  to  its  refractive  powers,  it  being  no  larger  in  myopic  than  in  emme- 
tropic  eyes.  It  appears  to  attain  its  full  growth  so  early  that  it  is  not 
affected  by  any  subsequent  alterations  in  the  posterior  hemisphere.  It  may 
undergo  a  slight  diminution  in  size  in  old  age. 

In  elderly  persons  there  is  usually  seen  a  narrow  gray  or  yellowish- 
white  crescentic  line,  either  at  the  lower  or  the  upper  border  of  the  cornea, 
concentric  with  the  limbus.  This  is  the  arcus  senilis,3  and  is  due  to  a  finely 
granular  infiltration  of  hyaline  substance.  It  usually  appears  first  at  the 
upper  portion.4  Somewhat  later  a  similar  arch  is  formed  opposite  to  the 
first  at  the  lower  border,  and  these  finally  unite,  forming  a  complete  ring, 
wider  above  and  below  than  on  the  sides.  (See  Fig.  23.)  Its  outer  edge 
is  usually  sharp,  a  clear  space  existing  between  it  and  the  limbus,  while  on 
the  inner  side  it  fades  gradually  away.  Sometimes,  however,  there  is  an 
outside  line  that  appears  as  if  the  sclera  were  impinging  unusually  on  the 
cornea,  then  a  clear  space,  followed  by  a  second  ring.5  It  never  interferes 
with  vision,  although  it  may  extend  some  distance  towards  the  centre.  It 


1  The  following  measurements  are  given  by  others  : 

Central. 

Peripheral. 

Testut  

.    .            .8 

1 

.    .            .8 

1.1 

Macalister   

.    .             .9 

1.12 

Gerlach    . 

.    1.1-1.2 

1.2-1.3 

1  Smith  (Priestley).    The  Size  of  the  Cornea  in  Relation  to  Age,  Sex,  Refraction,  etc. 
Lancet,  1889,  ii.  1062. 

3  Syn.  :  gerontoxon  (Gr.  yepwv,  old  man,  and  r6i-oi>,  bow,  arch) ;   macula  arcuata  or 
cornece  ;  marasmus  senilis  cornece ;  annulus  senilis. 

4  Canton  (E.).     On  the  arcus  senilis,  London,  1863. 

6  Testelin  (A.).     Diet,  encycl.  des  sciences  medicales,  Paris,  1867,  vi.  4. 


140 


THE  ANATOMY  OF  THE  EYEBALL. 


FiQ.  23. 


is  occasionally  seen  in  young  persons,  even  at  the  ages  of  six,  eight,  and 
ten  years,1  but  is  rare  before  fifty,  and  frequently  absent  until  sixty  years 
of  age.  It  is  more  frequent  and  earlier  in  men  than  in  women,  and  its 
early  development  appears  to  be  hereditary.  Canton  figures  five  members 
of  a  single  family,  aged  respectively  eighteen,  twenty,  twenty-five,  fifty- 
three,  and  fifty-six,  all  possessing  the 
arch  in  some  form,  the  two  last — the 
father  and  mother — having  complete 
circles.  In  warm  climates  it  is  developed 
earlier  than  in  cold  latitudes :  at  least  it 
is  more  frequently  seen  in  negroes  of 
the  north  coast  of  Africa.2  It  is  usually 
simultaneously  and  symmetrically  de- 
veloped in  both  eyes.  It  is  commonly 
said  that  it  is  due  to  fatty  degeneration 
or  infiltration  of  the  cornea,  but  Fuchs 
has  shown  that  this  cannot  be  the  case.3 
He  found  that  the  infiltrated  material 
never  has  any  relation  to  the  cells  of 
the  corneal  tissue,  but  is  free  upon  the 
surface  of  the  connective-tissue  fibres. 
Neither  ether  nor  chloroform  has  any 
effect  upon  it,  so  it  is  certainly  not  of  a 
fatty  nature.  He  believes  it  to  arise 
from  a  hyaline  degeneration  of  certain 
fibres.  The  arcus  senilis  appears,  there- 
fore, to  be  a  normal  phenomenon  oc- 
curring in  perfectly  healthy  subjects, 
due  to  the  decrease  of  nutrition  con- 
comitant with  advancing  years,  and  has 
no  relation,  as  was  formerly  supposed, 
to  fatty  degeneration  of  the  heart. 

Gruber 4  seeks  to  explain  its  occur- 
rence in  this  restricted  area  by  the  pecu- 
liarities of  circulation  in  the  cornea. 
The  peripheral  zone  he  thinks  is  nour- 
ished mainly  by  transudation  of  nutri- 
tive materials  from  the  circumcorneal  plexus,  and  as  age  advances  and  the 
circulation  is  less  active  this  nutrition  is  more  feeble  and  degeneration  ensues. 

1  Woodman  (W.  B.).    St.  Andrew's  Med.  Grad.  Assoc.  Trans.,  Lond.,  1872-73, vi.  144. 

2  Furnari.     Voyage  medicale  dans  1'Afrique  septentrionale,  Paris,  1845. 

8  Fuchs  (E.).  Zur  Anatomie  der  Pinguecula.  Archiv  fur  Ophthalmologie,  Leipzig, 
1891,  xxxvii.,  Abth.  iii.,  154,  155. 

4  Gruber  (K.).  Die  Entstehung  der  Greisenbogens  der  Hornhaut,  Wien.  med. 
"Wochenschr.,  Jahrg.  xxiv.,  No.  24. 


0 


Examples  of  the  arcus  senilis.  (Canton.) 
—A,  upper  arch ;  B,  upper  and  lower  arches ; 
C,  complete  ring. 


THE  ANATOMY  OF  THE  EYEBALL.  141 

He  thinks  the  middle  of  the  cornea  depends  for  nutrition  upon  the  vital  force 
of  the  cells  themselves,  causing  an  intercellular  flow  of  lymph.  This  force 
remains  about  equal  during  life  :  hence  these  cells  do  not  degenerate. 

The  average  weight  of  the  cornea  is,  according  to  Huschke,  about  one 
hundred  and  eighty  milligrammes,  or  one-fortieth  of  the  weight  of  the  entire 
eye.  Its  specific  gravity  is  stated  by  Davy  to  be  1.076. 

The  cornea  does  not,  like  the  sclera,  yield  gelatin  on  boiling,  but  rather 
a  special  form  of  chondrine,  called  by  Michel  and  Wagner  corneo-chon- 
drine.  It  also  contains  globuline  and  albuminoid  substance,  and  72.75  per 
cent,  of  water.  The  ash  yielded  is  but  0.66  per  cent. 

Like  the  sclera,  the  proper  substance  of  the  cornea  is  composed  of  closely 
woven  bundles  of  white  fibrous  tissue,  arranged,  however,  in  more  dis- 
tinctly separated  lamella.  Owing  to  its  peculiar  situation  at  the  front  of 
the  globe,  and  to  the  fact  .that  it  here  constitutes  the  entire  thickness  of  the 
capsule,  it  comprises  certain  additional  structures,  the  fibrous  tissue  being 
lined  on  either  side  by  a  clear  structureless  layer  on  which  rests  an  epithe- 
lium. (See  Fig.  24.)  There  are,  then,  from  without  inward,  five  well- 
marked  layers,  as  follows  : 


FIG.  24. 


Epithelium. 
Anterior  limiting  layer. 


Cornea  proper. 


Posterior  limiti  ng  layer.    -         ^^  ^^^__  Endothelium. 

Transverse  section  of  the  cornea,  slightly  magnified.    (Gerlach.) 

1.  The  external  epithelium,  a  continuation  of  the  conjunctival  epithe- 
lium already  mentioned. 

2.  The  anterior  limiting  layer. 

3.  The  cornea  proper. 

4.  The  posterior  limiting  layer. 

5.  The  internal  epithelium,  or  endothelium  lining  the  anterior  chamber. 

These  layers  do  not  have  the  homologies  that  their  situation  and  ap- 
pearances indicate.  The  anterior  and  posterior  limiting  layers  are  by  no 
means  of  the  same  nature,  nor  is  the  conjimctival  epithelium  strictly  com- 
parable with  that  lining  the  anterior  chamber. 

Attempts  have  been  made  to  classify  the  layers  according  to  their  em- 
bryological  history.  Kessler l  holds  that  the  first  trace  of  the  cornea  is  a 
structureless  sheet  of  epithelial  origin  developed  between  the  epidermis  and 
the  lens,  and  that  into  this  connective-tissue  elements  extend  as  a  secondary 
phenomenon,  the  anterior  and  posterior  limiting  layers  being  the  remains 
of  this  foetal  condition.  This  view,  however,  is  erroneous.  Kolliker* 

1  Kessler  (Leonhard).     Entwickelung  des  Auges  der  Wirbelthiere,  Leipzig,  1877. 

2  Kdlliker  (Albert).     Entwickelungsgeschichte,  2d  edition. 


142  THE  ANATOMY  OF  THE  EYEBALL. 

failed  to  find  either  membrane  in  a  foetus  of  ten  days,  and  even  at  birth 
the  posterior  one  cannot  be  discovered.  Indeed,  the  posterior  limiting 
layer  increases  in  thickness  with  age,  which  does  not  seem  likely  to  be  the 
case  with  the  relic  of  a  foetal  structure.  The  original  structureless  sheet 
appears  to  be  a  mesenchymic  blastema  derived,  like  other  similar  elements, 
from  the  mesoderm. 

Waldeyer,1  basing  his  conclusions  upon  the  observations  of  Manz  and 
Lorent,  holds  that  the  corneal  tissue  is  derived  from  three  sources :  an 
anterior  or  conjunctival  portion,  comprising  the  anterior  epithelium,  the 
anterior  limiting  layer,  and  a  small  portion  of  the  cornea  proper ;  a  middle 
or  scleral  portion,  comprising  the  remainder  of  the  cornea  proper ;  and, 
finally,  a  posterior  or  chorioideal  portion,  consisting  of  the  endothelial  lining, 
the  posterior  limiting  layer,  and  some  parts  of  the  middle  coat  that  inter- 
pose between  the  cornea  proper  and  the  posterior  limiting  layer. 

Some  objections  have  been  made  to  this  view.  While  there  is  no  doubt 
that  the  epithelium  of  the  anterior  surface  is  a  continuation  of  the  coujunc- 
tival  epithelium,  it  is  doubtful  if  the  anterior  limiting  layer  can  be  con- 
sidered as  a  continuation  of  the  subepithelial  layer  of  the  conjunctiva.  In 
some  fishes,  especially  in  Petromyzon  (Langerhans,  "VV.  Miiller),  nearly  the 
entire  cornea  is  conjunctival,  the  scleral  part  being  not  represented  at  all, 
and  the  posterior  limiting  layer  lying  directly  upon  the  anterior  epithelium. 
Again,  Kolliker  thinks  that  Waldeyer's  observers  have  mistaken  for  the 
posterior  layer  of  the  cornea  the  pupillary  membrane,  a  thin,  very  vascular 
sheet  lying  on  the  surface  of  the  lens  and  first  sharply  distinguished  from 
the  cornea  when  the  anterior  chamber  is  formed.  It  is,  however,  well 
known  that  the  endothelium  lining  the  posterior  surface  of  the  cornea 
is  a  continuation  of  that  lining  the  anterior  surface  of  the  iris,  which  is 
considered  of  chorioideal  origin ;  and  the  posterior  limiting  layer  is,  as 
will  be  hereafter  shown,  a  product  derived  from  the  endothelial  cells. 

According  to  the  most  recent  views  on  the  subject,  the  following  classifi- 
cation, which  is  essentially  that  of  Schwalbe,2  gives  the  correct  morpho- 
logical relations  of  the  layers. 

Classification  of  the  Layers  of  the  Cornea. 

A.  Cutaneous  Portion. 

I.  Conjunctival  Cornea.3 

1.  External  epithelium. 

B.  Capsular  Portion. 

II.  Scleral  Cornea.* 

2.  Anterior  limiting  layer. 

3.  Cornea  proper. 

1  In  Grafe  und  Samisch's  Handbuch  der  gesammten  Augenheilkunde,  vol.  i. 

1  Schwalbe  (G.),  Sinnesorgane,  1887. 

8  Syn. :  pars  conjunctivalis,  or  cutanea  cornece. 

*  Syn. :  pars  scleralis  comece. 


THE  ANATOMY  OF  THE  EYEBALL. 


143 


III.   Chorioideal  Cornea.1 

4.  Posterior  limiting  layer. 

5.  Internal  endothelium. 

The  cornea  proper 2  constitutes  the  main  substance  of  the  capsule,  giving 
it  strength  and  character.  It  forms  a  direct  continuation  of  the  sclera,  and 
is  composed  of  from  sixty  to  sixty-five  layers  of  flattened  bundles  of  white 
fibrous  tissue.  It  has  long  been  known  to  be  lamellated,  even  Avicenna 
(A.D.  980  to  1036)  having  been  aware  of  the  fact.  (Merkel.)  This  lamellar 
arrangement  should  be  remembered  when  operating,  as  the  point  of  the 
instrument  may  engage  between  the  layers  and  be  diverted  from  its  course 
if  the  knife  is  not  properly  held  and  the  cut  firmly  made.  It  requires,  as 
Hyrtl  expresses  it,  some  boldness  to  pierce  properly  the  cornea. 

The  arrangement  in  lamellae  may  be  demonstrated  by  dissection,  as  one 
thin  flap  after  another  may  be  raised  or  torn  up,  not,  however,  without 
leaving  a  rough  surface  beneath.  It  is  also  displayed  in  cases  where  inflam- 
matory action  has  partially  disintegrated  the  cornea,  as  at  the  border  of  an 
ulcer,  the  edges  of  the  lamellae  then  turning  up  like  the  leaves  of  a  dog's- 
eared  book.  In  some  of  the  lower  animals  the  lamellaB  are  more  distinct 
than  in  man :  in  the  frog,  for  example,  they  run  from  side  to  side  almost 
without  interruption.  In  the  human  cornea  the  bundles  composing  the 
lamellae  are  not  all  parallel,  some  of  them  passing  into  other  levels  at  very 
oblique  angles.  The  breaking 

of  these    occasions   the   rough  FlG-  25- 

surface  already  mentioned  as 
seen  when  the  lamellae  are  for- 
cibly separated. 

A  section  shows  that  the 
bundles  of  contiguous  layers  are 
arranged  in  a  crib-like  manner, 
being  nearly  at  right  angles,  so 
that  when  the  fibres  of  one  show 
longitudinally  those  of  the  other 
are  cut  transversely.  (See  Fig. 
25.)  The  arrangement  is  not 
quite  rectangular,  there  being  a 
deviation  to  the  right  of  some 
six  degrees  or  more,  so  that  the 

bundles  of  the  first  and  third  layers  are  not  parallel,  nor  those  of  the  second 
and  fourth,  etc.  When  the  objective  of  a  microscope  used  for  examining 
the  structure  is  slowly  raised  or  lowered  so  as  to  focus  in  turn  the  successive 
layers,  the  bundles  appear  to  revolve  like  the  spokes  of  a  wheel.  The 

1  Syn. :  pars  uvealis,  or  chorioidealis  corner. 

2  Syn.  :  substantia  propria  corneas ;  substantia  fibrosa  cornece ;  stroma  of  the  cornea ; 
fibrous  layer  of  the  cornea  ;  lamellated  tissue  of  the  cornea  (Bowman) ;  lamellated  cornea ; 
mesocornea  (Leidy). 


- 


,.,.V- 


Section  of  the  cornea  of  an  ox,  highly  magnified,  showing 
the  arrangement  of  the  lamellae.    (Ranvier.) 


144  THE  ANATOMY  OF  THE  EYEBALL. 

more  superficial  lamellae  differ  somewhat  from  the  others,  being  slightly 
thinner  and  more  interwoven.  They  are  also  interpenetrated  with  the 
arcuate  fibres 1  derived  from  the  anterior  limiting  membrane. 

It  is  well  known  that  white  fibrous  tissue  has  the  property  of  double 
refraction.  Viewed  through  crossed  Nicol  prisms,  it  appears  light  upon  a 
dark  field  if  the  axis  of  the  fibres  makes  an  angle  of  forty-five  degrees  with 
the  plane  of  polarization.  When  a  section  of  the  cornea  is  placed  in  this 
way  it  shows  a  series  of  alternate  light  and  dark  bands  corresponding  to 
the  lamellae  that  are  cut  longitudinally  and  transversely.  An  entire  cornea 
mounted  with  its  convex  side  uppermost  shows  a  dark  cross  on  a  bright 
field  when  viewed  by  polarized  light.  (His.) 

The  lymph-passages  of  the  cornea  have  been  for  many  years  an  object 
of  investigation.  The  diverse  views  that  have  been  held  may  be  classified 
partially  as  follows : 

1.  They  do  not  exist,  the  appearances  cited  in  favor  of  them  being 
artificially  produced.     (Sappey.) 

2.  They  exist,  but  are  almost  wholly  filled  with  cellular  elements. 
(Recklinghausen. ) 

3.  They  not  only  exist,  but  are  completely  lined  with  endothelial  cells. 
(Hoyer.) 

4.  They  are  spaces  partially  lined,  partially  interlamellar  and  inter- 
fascicular.     (Schwalbe,  Gutmann.) 

Among  the  earliest  to  investigate  this  subject  was  Bowman,  who  found 
that  upon  injecting  mercury  under  a  gentle  pressure  into  the  edge  of  the 
cornea,  certain  tubular  passages  appear  which  he  called  the  corneal  tubes. 
(See  Fig.  26.)  These  generally  run  parallel  to  each  other  and  to  the  lamellae, 

but  may  lie  obliquely  or 

FIG.  26.  diverge  in  various  direc- 

tions, like  the  cracks  in  a 
shattered  pane  of  glass. 
They  are  sometimes  mo- 
niliform,  and  always  have 
pointed  extremities.  The 
whole  cornea  may  be  filled 
with  such  tubes.  They 

The  "corneal  tubes."    (Bowman.)  do  not  communicate  with 

the   lymphatics   or   other 

vessels,  and  when  the  fluid  that  fills  them  escapes  it  is  usually  into  the 
anterior  chamber.  When  too  strongly  urged,  it  parts  the  contiguous  la- 
mellae, forming  flat,  irregular  patches.  An  injection  of  atmospheric  air 
answers  quite  as  well  as  one  of  mercury.  As  these  tubes  are  easily  seen  by 
the  naked  eye,  it  seems  hardly  probable  that  they  relate  to  the  minute  struc- 
ture of  the  cornea.  In  fact,  sections  show  that  they  have  no  proper  wall, 

1  Syn. :  fibrce  arcuatce  (Schwalbe) ;  fibres  suturales  (Ranvier). 


THE  ANATOMY  OP  THE  EYEBALL.  145 

and  that  they  are  probably  caused  by  a  mechanical  separation  of  the  fas- 
ciculi in  the  various  lamellae. 

The  question,  therefore,  arises  whether  there  are  in  the  cornea  aoy  other 
structures  that  indicate  lymph-passages.  Toynbee,  in  1841,  discovered  in 
the  cornea  certain  appearances  which  he  supposed  to  be  cells.  These  were 
likened  by  Virchow  to  the  osteoblasts  or  bone-corpuscles  which  occupy  the 
lacunae  of  osseous  tissue,  and  were  accordingly  named  the  corueal  corpuscles. 
When  His  and  von  Recklinghausen  invented  silver  staining,1  this  view 
appeared  to  be  confirmed.  The  cornea  was  the'  first  object  upon  which  the 
new  method  was  used.  The  application  of  the  solid  stick  produced  a  dark 
ground  upon  which  light  spots  of  a 

peculiar  pattern  appeared,  these  spots  FIG.  27. 

corresponding  apparently  with  the  cor- 
neal  corpuscles.  This  was  the  so- 
called  "  negative"  picture.  (See  Fig. 
27.)  A  longer  exposure  with  dilute 
solutions  produced  a  pattern  the  re-  /j^  ***$^  •*  '<  ' 

verse  of  this,  the  spaces  being  deeply  -  ," 

stained    while   the   ground    remained  <"•• 

comparatively  light.      This  was   the      £^£wV-  . 

"positive"  picture.     (See  Fig.  28.) 

In  opposition  to  Virchow's  theory    f|     ^iS^^  *^  ^SJfiBS»S^frSail 
that  these  appearances  are  produced    ;i  3  <r*9-'-  >'")        /  r^-: 

solely  by  the  corneal  corpuscles,  von     C^ ''"'"- '^4«to  -   'cX^ 

Recklinghausen    held    that    they   are 

-,  .     .    .  ,,  Cornea  of  a  frog  stained  so  as  to  show  the 

Caused     by    an     intricate     System     of  "negative"  picture.    (Ranvier.) 

lymph- lacunae  (Saftliicken),  connected 

with  each  other  by  delicate  canaliculi  (Saftcanalchen).  He  was  led  to  this 
view  by  observing  that  wandering  lymph-cells  pass  with  great  ease  through- 
out the  substance  of  the  cornea.  If  a  negative  silver-stained  cornea  be 
placed  within  the  dorsal  lymph-sac  of  a  living  frog  and  allowed  to  remain 
a  few  days,  it  is  found  that  leucocytes  in  great  numbers  penetrate  it  and  are 
seen  within  the  white  passages.  He  therefore  distinguished  in  the  cellular 
elements  of  the  cornea  two  classes, — the  fixed  cells  that  lie  in  the  lymph- 
lacunae,  and  the  wandering  cells  that  pass  from  place  to  place  along  the 
canaliculi.  As  a  confirmation  of  this  theory,  C.  F.  Miiller  found  that  by 
injecting  the  cornea  very  carefully,  approaching  more  nearly  the  physio- 

1  The  first  employment  of  nitrate  of  silver  was  apparently  made  by  Finzler,  who 
worked  under  the  direction  of  Coccius.  (See  his  dissertation,  "  De  argenti  nitrici  usu  et 
effectu,  praesertim  in  oculorum  morbis  sanandis,"  Leipzig,  1854.)  He  noted  the  corneal 
corpuscles,  but  believed  them  to  be  produced  by  the  corrosive  action  of  the  salt.  It  appears 
that  His  used  the  solid  pencil  as  early  as  1857  for  developing  th'e  structure  of  the  cornea  ; 
but  the  earliest  publication  of  any  exact  use  of  the  substance  for  histological  purposes  was 
made  by  von  Kecklinojhausen  in  an  article  in  the  Archiv  fur  pathologische  Anatomic  und 
Physiologic,  I860,  xix.  451,  entitled  "  Eine  Methode  mikroskppische  Hdhle  und  solide 
Gebilde  von  einander  zu  scheiden."  He  used  solutions. 

VOL.  I.— 10 


146 


THE  ANATOMY  OF  THE  EYEBALL. 


logical  conditions  than  Bowman  did,  it  is  possible  to  fill  the  lymph-lacunae 
without  causing  such  a  separation  of  the  fibres  as  leads  to  the  formation  of 
the  corneal  tubes.  An  intricate  net-work  of  connecting  passages  is  thus 
made  out  differing  totally  from  the  tubes.  More  recently,  Gutmann,1  by 
using  asphalt  dissolved  in  chloroform,  has  been  able  to  show  this,  even  in 
the  cornea  of  the  ox,  which  was  thought  not  to  contain  them.  This  net- 
work agrees  in  its  general  plan  with  that  developed  by  silver  staining,  but 
the  passages  are  somewhat  wider,  owing  probably  to  a  slight  distention 
from  the  injection.  They  increase  slightly  in  size  from  the  superficial  to 
the  deep  layers,  being  largest  near  the  posterior  limiting  membrane. 

FIG.  28. 


Cornea  of  a  frog  stained  so  as  to  show  the  "  positive"  picture.    (Ranvier.) 

Other  methods  of  demonstration  have  also  been  used,  so  that  at  present 
it  can  hardly  be  said  that  the  existence  of  lymph-passages  is  open  to  doubt. 

It  must,  however,  be  admitted  that  if  we  seek  for  distinct  vessels  pro- 
vided with  a  definite  wall,  they  are  not  found  in  the  cornea.  The  passages 
appear  to  be  rather  interfascicular  spaces  than  true  vessels,  the  adhesion 
of  the  fascicles  to  each  other,  at  no  time  very  great,  being  here  wanting 
entirely.  The  fixed  cells  of  the  cornea,  which  are  believed  to  be  adherent 
either  to  one  side  or  the  other  of  the  canals,  offer  the  only  trace  of  endothelial 
lining  such  as  usually  exists  in  the  lymphatics  of  connective  tissue,  as,  for 
instance,  in  the  sclera.  This  lining  appears,  however,  to  be  incomplete. 

It  is  instructive  to  compare  the  intimate  structure  of  the  cornea  with 
that  of  bone.  In  both  there  are  laminae,  between  which  lie  spaces  or 
lacunae  interconnected  by  delicate  canaliculi.  Even  the  perforating  fibres 
of  bone  appear  to  be  represented  by  the  sutural  fibres  of  the  cornea.  It 
should  be  remarked,  however,  that  these  resemblances  may  be  merely  acci- 
dental. The  osteoblasts  of  bone  appear  to  have  no  analogue  in  the  cornea, 
as  the  corneal  corpuscles  to  which  they  were  formerly  compared  are  now 

1  Gutmann  (G.).  Ueber  der  Lymphbahnen  der  Cornea.  Archiv  fur  mikroskopische 
Anatomie,  1888,  xxxii.  593. 


THE  ANATOMY  OF  THE  EYEBALL.  147 

believed  to  be  a  composite  product  made  up  of  the  fixed  corneal  cells  and  a 
certain  amount  of  infiltrated  material. 

At  the  edge  of  the  cornea  its  lymphatic  net-work  communicates  with 
that  of  the  sclera,  the  difference  in  calibre  of  the  passages  being  such  that 
fluid  more  readily  passes  into  than  away  from  the  cornea.  Through  the 
anterior  limiting  membrane  the  lymph  penetrates  along  the  nerves  by 
special  passages — the  perineural  canals — formed  in  the  connective  tissue  of 
their  sheaths,  thus  reaching  the  system  of  lacunae  that  exists  in  the  anterior 
epithelium,  particularly  between  the  "  prickle-cells"  of  the  deeper  layers. 
From  this  it  passes  into  the  well-marked  lymph  system  of  the  scleral  con- 
junctiva. Fluid  also  reaches  the  net- work  by  osmosis  from  the  anterior 
chamber.  The  interchange  here  is  active,  as  shown  by  the  rapidity  with 
which  diffusion  takes  place  when  special  fluids  are  injected  into  the  chamber. 
The  quantity  of  fluid  in  the  chamber  remains  about  the  same,  and  but  little 
exudation  appears  on  the  anterior  surface,  even  when  the  mtra-ocular 
pressure  is  notably  increased. 

The  external  epithelium  of  the  cornea1  resembles  in  many  respects  the 
epidermis  of  the  general  surface  of  the  body,  being  stratified  in  from  six  to 
eight  layers,  and  rapidly  renewed  from  the  basement  layer.  It  is  somewhat 
thicker  at  the  periphery  than  at  the  centre.  Leber  thinks  that  it  has  an 
important  office  in  preventing  the  diffusion  of  tears  into  the  general  sub- 
stance of  the  cornea.  It  is  quite  soft,  and  easily  removed  by  knife  or 
needle.  In  reptiles,  that  shed  their  skin  entire,  this  epithelium  is  cast  off 
with  the  external  cuticle. 

The  anterior  limiting  layer2  is  clear,  homogeneous,  and  anhistous, — that 
is  to  say,  without  apparent  structure.  It  offers  such  resistance  that  par- 
ticles of  steel  or  other  angular  fragments  often  stick  just  within  the  surface 
of  the  cornea.  It  differs  greatly  in  different  animals,  being  absent  in  some, 
such  as  the  horse,  goat,  dog,  and  cat  (His),  but  in  man,  ruminants,  and 
birds  being  well  developed.  At  five  years  of  age  it  is  about  equal  in  thick- 
ness to  the  posterior  limiting  layer;  after  that  the  latter  gains  upon  it. 
It  passes  insensibly  into  the  cornea  proper,  from  which  it  is  separated 
with  difficulty.  If  any  part  is  torn  away,  it  rolls  up  so  that  the  attached 
or  deeper  surface  lies  inward  in  the  roll.  If  destroyed  by  inflammatory 
processes,  it  is  never  renewed.  It  thins  away  and  is  lost  from  one  to  one 

1  Syn. :    conjunctiva  cornece;    corneal  conjunctiva;    corneal  epithelium;   ectocornea. 
(Leidy.) 

2  Syn.  :  Bowman's  membrane  (for  Sir  William  Bowman,  Professor  of  Physiology  in 
King's  College,  London,  born  1816,  died  1892,  being  described  in  his  Lectures  on  the 
Parts  concerned  in  the  Operations  on  the  Eye,  London,  1849) ;    anterior  elastic  lamina 
(Bowman) ;  lamina  elastica  anterior ;  membrane  of  Reichert  (for  Karl  Reichert,  1811-1883, 
who  discovered  it  at  about  the  same  time  as  Bowman) ;  vordere  Grenzschicht,  G.  (Reichert) ; 
anterior  homogeneous  lamina ;  anterior  or  external  basement  membrane ;  aussere  or  vor- 
dere Basalmembran,  G.  (Henle) ;   subepitheliale  Schicht,  G.   (Arnold) ;   stratum  nervosum 
(Cohnheim,  under  the  erroneous  impression  that  it  is  mainly  composed  of  an  intricate 
plexus  of  nerve-filaments). 


148  THE  ANATOMY  OF  THE  EYEBALL. 

and  a  half  millimetres  from  the  edge  of  the  cornea,  where  the  marginal 
looped  plexus  of  blood-vessels  begins. 

Bowman,  the  discoverer  of  this  structure,  describes  its  union  with  the 
subjacent  tissue  as  follows : 

"  The  manner  in  which  the  anterior  elastic  lamina  is  united  to  the  larnella& 
which  it  serves  to  cover  is  very  interesting.  It  must  be  borne  in  mind 
that  the  anterior  surface  of  the  cornea  is  convex,  and  that  the  maintenance 
of  its  exact  curvature  is  of  primary  importance  to  vision,  as  it  is  there  that 
the  first  inflexion  of  the  rays  of  light  falling  in  the  eye  takes  place ;  and, 
further,  that  the  conjunctival  epithelium,  being  a  soft  and  fragile  substance, 
must  take  the  figure  of  the  surface  on  which  it  rests :  hence,  probably,  the 
arrangement  I  am  about  to  mention.  The  anterior  elastic  lamina,  a  firm, 
resisting,  uniform  layer,  placed  in  front  of  the  more  soft  and  porous  lamel- 
lated  tissue,  is  tied  down  to  the  anterior  lamellae,  at  innumerable  points,  by 
filaments  of  similar  texture  to  itself,1  which  it  sends  in  among  them.  These, 
as  they  penetrate  the  lamellae,  divide  and  expand  in  such  a  manner  as  to 
take  firm  hold  of  them,  and  are  thus  gradually  spent  among  the  four  or 
five  lamellae  which  lie  nearest  the  surface.  It  is  singular,  too,  that  these 
filaments  are  not  set  vertically,  but  everywhere  in  a  slanting  direction 
among  the  lamellae,  so  that  in  a  vertical  section  they  appear  to  cross  one 
another  at  right  angles.  This  arrangement  might,  I  imagine,  be  shown  on 
mechanical  principles  to  be  the  best  possible  for  the  maintenance  of  the 
convexity  of  the  front  of  the  cornea." 

The  exact  nature  of  this  layer  has  long  been  a  matter  of  discussion,  and 
cannot  yet  be  said  to  be  entirely  settled.  As  it  is  unaffected  by  dilute  acids, 
Bowman  supposed  it  to  be  similar  to  the  elastic  fibres  of  connective  tissue, 
and  therefore  called  it  the  anterior  elastic  lamina.  The  name  is  an  un- 
fortunate one,  as  the  membrane,  although  elastic,  is  by  no  means  identical 
in  its  reactions  with  elastic  tissue.  The  latter  may  be  colored  by  cosine, 
which  leaves  the  former  unaffected,  and  picrocarminate  of  ammonium  stains- 
the  two  quite  differently.  In  boiling  water  and  in  liquor  potassas  the  anterior 
limiting  layer  swells  up  (Henle2),  and  permanganate  of  potassium  breaks  it 
up  into  fibrillae  that  resemble  those  of  the  cornea  proper  (Rollett s).  These 
reactions  also  distinguish  it  from  elastic  tissue. 

Kessler's  view  that  both  the  anterior  and  posterior  limiting  layers  are 
the  remains  of  the  original  blastema  in  which  the  cornea  was  developed  has 
already  been  adverted  to.  The  two  differ  so  much  in  their  chemical  re- 
actions that  it  seems  impossible  that  they  should  have  the  same  intimate 
constitution.  For  example,  the  anterior  layer  is  not  affected  by  a  dilute 
solution  of  osmic  acid,  which  stains  the  posterior  layer  brown ;  the  former 
is  colored  bright  red  by  picrocarminate  of  ammonium,  which  stains  the 

1  In  an  annexed  figure  he  styles  these  fibres  "  fibrous  cordage." 

*  Handbuch  der  systematischen  Anatomic  des  Menschen,  1-873,  ii.  629. 

8  Strieker's  Handbuch  der  Gewebelehre. 


THE  ANATOMY  OF  THE  EYEBALL.  149 

latter  orange ;  haematoxylin  stains  the  anterior  layer  very  slowly,  but  at 
once  imparts  to  the  posterior  one  a  characteristic  violet  hue. 

The  view  most  generally  accepted  is  that  the  anterior  layer  is  merely  a 
condensed  and  somewhat  modified  limiting  layer  of  the  cornea  proper,  and 
Rollett's  success  in  reducing  it  to  fibrillse  is  usually  cited  as  conclusive  evi- 
dence of  this.  It  is  difficult,  however,  to  reconcile  this  view  with  the  fact 
that  acetic  acid  or  dilute  mineral  acids  do  not  affect  this  layer  at  all,  while 
they  at  once  cause  ordinary  white  fibrous  tissue,  like  that  of  the  cornea 
proper,  to  swell  up. 

Ranvier 1  has  shown  that  the  cornea  of  the  ray  possesses  an  unusually 
thick  anterior  limiting  layer,  from  which  delicate  prolongations  (sutural 
fibres 2)  extend  through  the  entire  thickness  of  the  cornea  proper.  These 
have  the  same  reactions  as  the  membrane  itself,  picrocarminate  of  am- 
monium tinging  the  entire  system  a  bright  rose  color,  while  the  corneal 
lamellae  remain  unstained.  In  the  human  cornea  traces  of  a  similar  system 
are  found,  the  sutural  fibres  being  represented  by  the  oblique  filaments  so 
graphically  described  by  Bowman. 

Searching  for  an  analogue  for  this  tissue,  he  finds  it  in  the  circular 
and  spiral  fibres  found  in  connective-tissue  bundles,  particularly  those  of 
tendons  and  of  the  arteries  at  the  base  of  the  brain.  When  these  bundles 
are  treated  with  acetic  acid  they  swell  up,  but  the  fibres  referred  to  remain 
unaffected  and  appear  as  circular  or  spiral  constricting  bands,  giving  the 
bundles  a  moniliform  aspect.  They  are  not  found  at  equal  distances  or 
distributed  according  to  any  fixed  law.  In  some  cases  it  is  thought  that 
the  substance  of  which  these  bands  are  composed  was  primitively  a  com- 
plete investment  of  the  bundle  and  was  broken  into  shreds  by  the  swelling ; 
in  others,  that  they  are  the  filamentous  processes  of  stellate  connective-tissue 
cells.  The  reactions  of  these  bands  appear  to  be  identical  with  those  of 
the  anterior  limiting  layer.  Ranvier  therefore  concludes  that  the  cornea  is 
to  be  considered  as  a  flattened  band  of  tendon-like  connective  tissue  with 
a  special  envelope  (the  anterior  limiting  layer)  and  encircling  bands  (the 
sutural  fibres).  At  the  edge  of  the  cornea  he  was  able  to  trace  the  transition 
from  the  modified  to  the  usual  form. 

The  posterior  limiting  layer,3  often  called  the  membrane  of  Descemet,  is 

1  Ranvier  (L.).  Le9ons  d'anatomie  generate,  annee  1878-79  ;  Terminaisons  nerveuses 
sensitives'  Cornee.  Paris,  1881. 

7  The  "  fibrous  cordage"  of  Bowman  ;  fibrce .arcuatce  (Schwalbe). 

8  Syn. :  membrane  of  Descemet,  membrana  Descemetiana  or  Descemeti  (for  Jean  Des- 
cemet, a  physician  of  Paris,  born  1732,  died  1810) ;  membrane  of  Demours,  membrana 
Demoursiana  or  Demoursi  (for  Pierre  Demours,  an  oculist  of  Paris,  born  1702,  died  1795) ; 
membrane  of  Duddell,  membrana  Duddelliana  (for  Benedict  Duddell,  an  oculist  of  London, 
1729) ;  membrane  of  the  aqueous  humor  or  membrana  humoris  aquei  (Descemet) ;  mem- 
brana pro  humore  aqueo ;  capsula  aquea  cariilaginosa  or  praeaquosa ;  lame  cartilagineu.se 
(Demours) ;  posterior  elastic  lamina  (Bowman)  ;  lamina  elastica  posterior ;  innere  Basal- 
membran  (Henle) ;  internal  basement  membrane;  vitreous  lamella  of  the  cornea;  ento- 
cornea  (Leidy). 

A  long  controversy  occurred  with  regard  to  the  priority  of  discovery  of  this  mem- 


150  THE  ANATOMY  OF  THE  EYEBALL. 

distinguished  from  the  anterior  limiting  layer  by  several  notable  peculiari- 
ties. Its  different  behavior  with  staining  fluids  has  already  been  adverted 
to.  It  has  a  considerably  greater  resistance  to  alkalies  and  acids,  as  well  as 
to  boiling  water.  Indeed,  it  may  be  isolated  by  submitting  the  cornea  to 
strong  alkaline  or  acid  solutions,  which  dissolve  the  other  constituents  while 
leaving  the  membrane  of  Descemet  unaffected.  Anatomical  differences 
also  are  not  wanting.  It  is  thickest  at  the  circumference,  while  the 
anterior  limiting  layer  thins  away  towards  the  edge.  Although  closely 
united  to  the  cornea  proper,  it  may  yet  be  detached  by  maceration,  and  then 
has  a  tendency  to  roll  up,  with  the  surface  formerly  adherent  inward  in  the 
roll.  It  is  more  easily  digested  by  trypsin  than  is  elastic  connective  tissue. 
In  ordinary  preparations  it  appears  as  a  perfectly  homogeneous  glass-like 
sheet  without  trace  of  cells  or  other  structure.  Some  observers  (Schweigger- 
Seidel)  report  that  it  shows  a  fibrillary  striation  after  boiling  in  a  ten  per 
cent,  solutiou  of  sodium  chloride.  When  cut  it  crackles  under  the  knife, 
and  the  edge  of  a  fracture  may  be  straight,  curved,  conchoidal,  or  stepped 
and  irregular.  An  attempt  to  disassociate  it  causes  it  to  break  up  in  every 
direction,  like  glass,  following  no  definite  planes.  From  its  general  be- 
havior, Schweigger-Seidel  believed  it  to  be  composed  of  thin  platelets. 

In  early  foetal  life  this  membrane  is  absent,  the  endot helium  lining  the 
anterior  chamber  resting  immediately  upon  the  cornea  proper.  About  the 
second  or  third  month  it  appears  as  a  very  narrow  stripe,  presenting  the 
same  glassy  appearance  as  in  -the  adult.  It  increases  in  thickness  with  age, 
and  in  the  latter  years  of  life  shows  on  its  posterior  surface  numerous 
papillary  eminences,  isolated  or  arranged  in  groups.  These  are  apparently 
due  to  a  progressive  but  irregular  growth.  In  view  of  these  evidences  of 
gradual  development,  Ranvier  thinks  that  the  membrane  is  a  product  of 
the  endothelial  cells  that  coat  it. 

Internal  Endothelium.1 — The  layer  of  cells  that  lines  the  anterior  cham- 
ber is  often  called  an  epithelium,  but,  as  it  is  undoubtedly  a  form  of  connec- 
tive tissue  having  an  epithelioid  character,  it  seems  better  to  apply  to  it  the 

brane.  The  first  published  mention  of  it  by  Descemet  occurs  in  his  dissertation  "  An  sola 
lens  crystallina  cataractse  sedes?"  Paris,  1758.  Demours's  first  publication  was  in  his 
Lettre  a  M.  Petit,  Paris,  1767  ;  also  in  the  Memoires  de  1'Academie,  1768,  page  177. 

Some  time  prior  to  this,  however,  the  membrane  was  seen  and  mentioned  by  Duddell, 
as  appears  from  the  following  passage  from  his  Treatise  on  the  Diseases  of  the  Horny 
Coat  of  the  Eye,  London,  1729  :  "  I  have  found  by  the  Dissection  of  Eyes  Fibres  both  in 
the  outward  and  inward  Chamber ;  one  was  adherent  to  the  Arachnoides  [capsule  of  the 
lens],  and  another  to  the  Cornea.  I  found  a  Film  in  a  Horse's  Eye,  of  a  yellowish  Colour, 
which  swimming  in  the  Aqueous  Humour  of  the  outward  Chamber  cover'd  half  the  Pupil. 
I  made  an  Incision  in  the  lower  Part  of  the  Horny-coat  with  a  Lancet,  and  putting  a 
blunt  Needle  into  the  Orifice,  I  drew  Part  of  it  out ;  the  other  End  was  adherent,  a  little 
above  and  sideways  from  the  Incision,  and  broke  off  almost  by  the  Adherency :  And 
examining  the  Film,  I  found  that  it  was  a  little  Pelicle,  that  had  separated  from  the 
Cornea,  excepting  only  where  it  stuck." 

1  Syn. :  endothelium  of  the  membrane  of  Descemet ;  internal  epithelium  of  the 
cornea  ;  epithelium  of  the  membrane  of  Descemet ;  epithelium  humoris  aquei. 


THE  ANATOMY  OF  THE  EYEBALL. 


151 


name  endothelium,  invented  by  His  for  sack  tissues.  It  is  composed  of 
transparent,  flattened,  polygonal  cells  that  have  some  peculiarities  that  will 
be  mentioned  under  the  section  on  histology. 

At  the  circumference  of  the  cornea  the  posterior  limiting  membrane  is 
thickened  into  a  ring-like  margin,   the  annular  ligament.1      From  this 

FIG.  29. 


Meridional  section  showing  the  connection  between  the  cornea  and  the  middle  coat  of  the  eye. 
(Testut.) — a,  posterior  limiting  layer;  6,  annular  ligament ;  c,  scleral  fibres;  d,  ciliary  fibres;  e,  posterior 
fibres  forming  the  pectinate  ligament ;  /,  Fontana's  space ;  g.  scleral  sinus ;  h,  meridional  fibres  of  the 
ciliary  muscle;  i,  annular  fibres  ofc same;  Jk,  ciliary  process:  I,  iris;  m,  cornea;  n,  angle  of  the  iris; 
o,  sclera ;  p,  scleral  vein. 

FIG.  30. 


A  preparation  made  parallel  to  the  surface  of  the  annular  ligament.  (Gerlach.)— a,  meridional 
fibres  of  the  ciliary  muscle;  6,  circular  fibres  of  the  annular  ligament;  c,  longitudinal  fibres  of  the 
annular  ligament  forming  a  reticulum  in  Fontana's  space;  d,  posterior  limiting  layer,  covered  with 
the  endothelial  cells  of  the  anterior  chamber. 

separate  bundles  of  fibres,  lying  in  three  distinct  planes,  pass  to  the  sclera, 
to  the  tendon  of  the  ciliary  muscle  (hereafter  to  be  mentioned),  and  to  the 
iris.  (See  Figs.  29  and  30.)  The  strands  of  the  latter  group  are  known 
as  the  pectinate  ligament.2  The  narrow  region  between  the  cornea  and  the 

1  Syn.  :  ligameniitm  annulare  ;  annulus  tendinosus. 

2  Syn.  :   ligamentum  pectinatum  iridis  (Hueck)  ;  processus  peripherici ;  pillars  of  the 
iris. 

The  name  in  the  text  is  derived  from  L.  pecten,  -mis,  a  comb.     In  hoofed  animals,  in 


152 


THE  ANATOMY  OF  THE  EYEBALL. 


FIG.  31. 


iris,  near  this  ligament,  is  often  called  the  an<?fe  o/  *Ae  I'm,1  and  the  region 
occupied  by  the  bundles  of  the  ligament  the  space  of  the  angle  of  the 
iris.2  The  intervals  between  them  are  free  from  interstitial  material,  and 
communicate  with  the  anterior  chamber,  being  in  fact  a  continuation  of  it. 
They  are  much  better  marked  in  the  ox  and  horse  than  in  man. 

It  will  be  perceived  that  the  arrangement  just  described  is  such  as  to 
connect  the  membrane  of  Descemet  rather  with  the  middle  coat  of  the  eye 
than  with  the  outer  one,  thus  supplementing  what  has  already  been  de- 
duced from  embryological  evidence, — namely,  that  the  membrane  is  derived 
from  the  middle  coat.  An  examination  of  the  strands  of  the  pectinate 
ligament  shows  that  each  is  coated  by  a  continuation  of  the  membrane  of 

Descemet,  and  by  an  endothelial  layer 
that  grows  thinner  as  it  recedes  from  the 
cornea,  the  membrane  also  undergoing  re- 
duction. (See  Fig.  31.) 

The  intervals  of  the  space  of  the  angle 
of  the  iris  communicate  with  the  lymphatic 
spaces  of  the  iris,  and  by  this  way  fluid 
may  be  carried  directly  from  the  anterior 
chamber  into  the  lymphatic  circulation. 

Just  without  this  space,  within  the 
precincts  of  the  sclera,  where  the  internal 
surface  of  that  membrane  bends  inward 
before  uniting  with  the  tissue  proper  of 
the  cornea,3  there  is  found  the  curious 
passage  or  system  of  passages  already 
referred  to  as  the  scleral  sinus,  or  canal 
of  Schlemm* 

This  has  in  cross-section  so  much  the 
appearance  of  one  of  the  intervals  of  Fon- 
tana's  space  that  it  has  been  thought  to  be 
of  the  same  nature  and  therefore  to  belong  to  the  lymphatic  system.    It  is 

whom  the  structure  was  first  observed,  the  arrangement  in  distinct  bundles  passing  from 
the  cornea  to  the  iris  is  much  more  marked,  and  these  appear,  when  viewed  from  the  side 
of  the  chamber,  like  the  teeth  of  a  comb. 

1  Syn. :  angle  of  the  anterior  chamber ;  irido-corneal  angle  ;  Iriswinkel,  G.  ;  angulus 
iridis. 

2  Syn.  :  space  of  Fontana,  named  for  Felice  Fontana,  professor  at  Pisa  and  afterwards 
at  Florence,  born  1720,  died  1805.     Fontana,  misled  by  the  unusual  development  of  this 
trabecular  tissue  in  the  eyes  of  kine,  and  probably  making  some  error  in  manipulation, 
described  it  as  a  canal.     The  term  "  Fontana's  spaces"  is  often  applied  to  the  numerous 
small  intervals  between  the  trabeculae.     This  seems  to  be  inadmissible. 

8  The  internal  scleral  sulcus  of  Schwalbe. 

4  Named  for  Friedrich  Schlemm,  an  anatomist  of  Berlin,  born  1795,  died  1858. 

Syn.  :  sinus  sclerce  (Kochon-Duvigneaud) ;  canalis  or  sinus  Schlemmii ;  canalis  Lauthi ; 
plexus  ciliaris  (Leber)  ;  circulus  venosus  Schlemmii,  or  circulus  Schlemmii  (Leber,  1895)  ; 
sinus  venosus  sclerce,,  or  sinus  venosus  Schlemmii  (Gutmann  and  Waldeyer,  1895)  ;  sinus 


Meridional  section  of  rabbit's  cornea, 
showing  strands  of  the  pectinate  liga- 
ment. (Ranvier.) — The  axis  of  the  strands 
is  formed  of  an  extension  of  the  tissue 
of  the  cornea  proper ;  the  cortex  of  them 
is  an  extension  of  the  posterior  limiting 
layer,  and  upon  this  extends  an  epithe- 
lial investment  continuous  with  the  en- 
dothelium  of  the  anterior  chamber. 


THE  ANATOMY  OF  THE  EYEBALL.  153 

lined  with  endothelium,  and  has  a  thin  wall  closely  united  with  the  scleral 
tissue.  It  is  not  in  all  places  a  simple  canal,  but  divides  at  short  distances 
into  several  branches,  which  again  reunite.  This  has  led  some  to  designate 
it  as  a  plexus.  Where  single,  it  is  oval  or  triangular  in  section,  having  a 
lono-  diameter  of  0.32  millimetre  and  a  short  one  of  0.048  millimetre. 

O 

(Schwalbe.)  There  pass  from  it  to  the  anterior  ciliary  veins  small  veinlets 
that  receive  branches  from  the  ciliary  muscle,  and  this  has  led  to  the  theory 
that  it  acts  as  a  reservoir  for  the  blood  expressed  from  that  muscle  during 
contraction. 

Rochon-Duvigneaud l  has  called  attention  to  the  fact  that  this  passage 
greatly  resembles  in  structure  the  sinuses  of  the  dura  mater.  Like  them, 
it  has  an  endothelial  lining  resting  immediately  upon  the  circumjacent  tissue, 
and  not,  like  the  vorticose  and  other  veins,  upon  a  wall  of  its  own,  sepa- 
rated from  the  tissue  in  which  it  lies  by  an  adventitia.  There  are  also 
trabeculae  and  fibrous  crests  within  it  that  serve  to  subdivide  it  more  or 
less  completely.  These  peculiarities  were  noted  by  some  of  the  earlier 
observers,  who  gave  it  the  name  of  sinus.2 

The  exact  nature  of  this  sinus  has  been  for  many  years  a  subject  of 
dispute.  As  it  is  usually  empty  after  death,  it  has  been  held  to  be  a  lymph- 
channel.  This  is  supported  by  the  evidence  of  some  who  showed  that  it 
may  be  filled  either  wholly  or  partially  by  colored  injections  made  into  the 
anterior  chamber  after  death  at  about  normal  pressure.  Schwalbe3  suc- 
ceeded in  doing  this  with  soluble  Berlin  blue,  alkanet  turpentine,  and 
asphalt  dissolved  in  chloroform ;  Heisrath 4  with  sulphindigotate  of  sodium, 
eosiue  carmine,  and  defibrinated  blood;  Calori5  with  cinnabar  and  basic 
acetate  of  lead ;  Gutmann,6  quite  recently,  with  Japanese  ink,  Berlin  blue, 
and  defibrinated  blood.  The  annexed  figure  (Fig.  32),  taken  from  Gut- 
maun's  article,  shows  fine  granules  of  the  ink  lying  within  the  lumen  of 

venosus  iridis  (Henle)  ;  sinus  venosus  cornece ;  circulus  venosus  iridis ;  sinus  circularis  iridis  ; 
circular  sinus  (Leidy). 

First  discovered  by  Albinus,  as  is  shown  by  a  catalogue  of  his  preparations,  published 
in  1775.  First  described  by  Schlemm,  in  Rust's  Handbuch  der  Chirurgie,  Berlin  und  Wien, 
1830,  iii.  333.  He  states  that  he  found  it  in  the  eye  of  a  person  who  had  been  hung,  and 
cautions  against  confounding  it  with  Fontana's  canal. 

1  Rochon-Duvigneaud.     Recherches  anatomiques  sur  Tangle  de  la  chambre  anterieure 
et  sur  le  canal  de  Schlemm.     Arch,  d'ophth.,  Par.  1892,  1893,  xii.,  xiii. 

2  Arnold  (Fr.).    Anatomische  und  physiologische   Untersuchungen  uber  das  Auge, 
1832,  p.  10  et  seq.      Retzius,  Ueber  den  Circulus  venosus  im  Auge.     Archiv  fur  Anatomie 
und  Physiologic,  1834,  pp.  292-295. 

3  Schwalbe.     Lehrbuch  der  Anatomie  der  Sinnesorgane,  1887. 

*  Heisrath.  Ueber  die  Abflusswege  des  Humor  aqueus  mit  besonderer  Beriicksichti- 
gung  des  sogenannten  Fontana'schen  und  Schlemm'schen  Canals.  Arch.  f.  Ophth.,  1880, 
xxvi.  1. 

5  Calori.      Ue'  risultamenti   ottenuti  injettando  i  canali  di   Fontana  e  di  Petit  e  la 
camera  anteriore  dell'  occhio  umano  e  dei  mammiferi  domestic!.     Mem.  Accad.  d.  sc.  d. 

.1st.  di  Bologna,  1874,  3.  s.,  v.  341-351. 

6  Gutmann  (G.).     Ueber  die  Natur  des  Schlemm'schen  Sinus  und  seine  Beziehungen 
zur  vorderen  Augenkammer.     Arch   f.  Ophth.,  1895,  xli.  28. 


154 


THE  ANATOMY  OF  THE  EYEBALL. 


the  sinus.     These  evidently  penetrate  by  means  of  the  space  of  Fontana, 
and  then  by  some  means  through  the  walls  of  the  vessel. 

It  has  been  held  from  these  experiments  that  open  passages  exist  in 
the  walls  by  which  fluids  may  reach  the  sinus.  Schwalbe  cites  similar  con- 
nections between  the  lymphatic  vessels  of  the  Pacchionian  bodies  of  the 
arachnoid  and  the  sinuses  of  the  dura  mater.  He  compares  the  space  of  the 
angle  of  the  iris  to  a  sponge  that  sucks  up  fluid  from  the  anterior  chamber 
and  discharges  it  into  the  sinus.  The  action  is  aided,  as  he  thinks,  by  the 
ciliary  muscle,  some  of  whose  fibres  are  attached  to  the  inner  wall  of  the 

FIG.  32. 


Section  of  a  preparation  made  by  injecting  India  ink  into  the  anterior  chamber.  (Gutmann.) — 
»,  scleral  sinus  in  which  fine  particles  of  ink  are  seen ;  e,  portions  of  the  injection  mass  passing  from 
the  neighboring  tissues  into  the  sinus ;  b,  fusiform  and  acuminate  collections  in  the  spaces  of  the 
neighboring  tissues;  r,  scattered  granular  masses  of  India  ink. 

sinus,  so  that  each  contraction  of  the  muscle  draws  asunder  the  walls  and 
thus  induces  a  suction  upon  the  space  of  the  angle  of  the  iris  and  through 
it  upon  the  anterior  chamber.  The  fact  that  increase  of  intra-ocular  press- 
ure at  once  causes  a  congestion  of  the  anterior  ciliary  veins  seems  to  favor 
this  view.  It  is  true,  however,  that  no  one  has  yet  succeeded  in  showing 
the  actual  channels  by  which  such  a  communication  might  be  effected,  and 
many  experimenters  have  failed  to  obtain  positive  results  from  injections 
made  in  perfectly  fresh  eyes.1 

Leber 2  has  recently  repeated  his  former  experiments  on  this  subject  with 

1  See  Staderini,  Ueber  die  Abflusswege  des  Humor  aqueus,  Archiv  fur  Ophthalmolo- 
gie,  1891,  xxxvii.  3.  Gifford,  Weitere  Versuche  uber  die  Lymphstrome  und  Lymph wege 
des  Auges,  Archiv  fur  Augenheilk.,  1893,  xxvi.  ;  Kochon-Duvigneaud,  loc.  cit. 

1  Leber  (Th.).  Der  Circulus  venosus  Schlemmii  steht  nicht  in  offener  Verbindung  mit 
der  vorderen  Augenkammer.  Arch.  f.  Ophth  ,  1895,  xli.  235. 


THE  ANATOMY  OF  THE  EYEBALL.  155 

a  view  to  reconciling  the  conflicting  evidence.  He  finds  that  the  discordant 
results  obtained  by  different  experimenters  can  be  explained  by  the  different 
conditions  prevailing  and  the  different  methods  pursued.  Freshness  of 
material  is  an  important  element,  as  in  a  very  short  time  after  death  the 
endothelial  cells  contract  so  that  injections  readily  pass  between  them. 
Again,  injections  pass  much  more  readily  into  the  sinus  when  the  anterior 
chamber  has  first  been  emptied  of  its  contents.  In  the  case  of  some  of  the 
liquids  used, — Berlin  blue,  for  example, — it  is  found  that  the  aqueous 
humor  determines  a  precipitate  of  fine  flakes  that  cannot  enter  the  vessels. 
A  slight  amount  of  rough  handling  causes  the  results  to  vary. 

After  exhaustively  examining  the  whole  matter,  he  concludes  that  in- 
jected fluids  do  indeed  pass  from  the  anterior  chamber  to  the  scleral  sinus, 
not  through  open  connections  or  by  stomata,  but  by  intercellular  filtration. 
Dissolved  color  ing- matters  not  precipitated  in  the  anterior  chamber  pass 
without  difficulty,  as  do  also  fine,  suspended  granules,  such  as  those  of 
India  or  Japanese  ink. 

He  also  points  out  that  this  conclusion  is  supported  by  the  ordinary  ex- 
perience of  ophthalmologists.  If  fluids  could  leave  the  anterior  chamber 
by  distinct  openings,  a  constant  intra-ocular  pressure  could  not  be  main- 
tained, nor  would  that  pressure  ever  be  much  greater  than  that  in  the 
veins.  Considerable  difference  in  pressure,  however,  often  exists.  Under 
normal  conditions  no  influx  of  blood  into  the  anterior  chamber  occurs 
when  the  aqueous  humor  is  evacuated,  yet  it  is  difficult  to  understand  how 
this  could  be  prevented  were  there  open  communications  between  the 
chamber  and  any  part  of  the  venous  system.  The  sinus  usually  is  found 
empty  after  death,  because  the  intra-ocular  pressure  does  not  immediately 
cease,  and  this  causes  a  filtration  of  the  aqueous  humor  which  drives  out 
the  blood.  The  anterior  ciliary  veins  share  this  peculiarity.  After  death 
by  diseases  of  the  most  varied  nature  the  sinus  may  be  found  filled  with 
blood,  as  is  shown  by  the  investigations  of  Iwanoff  and  Rollett.1 

Blood-vessels. — In  the  adult  cornea  there  are  no  blood-vessels,  except 
within  a  margin  of  one  to  two  millimetres  near  the  limbus,  where  the 
episcleral  vessels  end  in  a  net- work  of  capillaries  known  as  the  marginal 
looped  plexus.2  During  foatal  life  this  vascular  net- work  extends  between 
the  deeper  layers  of  epithelium  and  the  anterior  limiting  membrane  much 
farther,  and  may  overspread  the  entire  anterior  surface.  The  cornea  of 
some  osseous  fishes  is  vascular  throughout  life.  The  substance  of  the 
cornea  normally  contains  no  vessels.  During  inflammation,  however,  ves- 
sels may  form  in  any  portion,  usually  commencing  as  an  extension  of. 
the  marginal  plexus,  just  beneath  the  external  limiting  membrane,  and 
extending  from  this  into  the  deeper  layers.  In  some  cases  a  newly  formed 


1  Iwanoff  and    Kollett.      Bemerkungen   zur  Anatomie   der  Irisanheftung   und   des 
Annulus  ciliaris.     Arch.  f.  Ophth.,  1869,  xv.  54. 
,  G. 


156 


THE  ANATOMY  OF  THE  EYEBALL. 


FIG.  33. 


tissue  (pannus)  is  deposited  upon  the  cornea.  It  should  be  noted  that 
when  the  vessels  are  superficial  they  are  plainly  visible  and  have  a  vivid 
red  color,  branching  in  an  arborescent  manner ;  but  when  deeply  situated 
they  are  not  easy  to  make  out,  owing  to  the  infiltrated  and  clouded  layers 
covering  them,  and  they  divide  into  branches  that  run  parallel  to  each 
other,  or  nearly  so,  along  the  corneal  laminae.  (Fuchs.) 

Notwithstanding  the  absence  of  blood-vessels,  the  nutrition  of  the 
cornea  is  very  active,  as  is  shown  by  the  rapidity  with  which  wounds  are 
repaired,  a  day  sometimes  sufficing  for  healing  when  the  conditions  are 
favorable.  This  is  probably  due  to  the  extraordinary  copiousness  of  the 
lymph-supply,  which  here  takes  the  place  of  blood.  In  wounds  involving 
loss  of  substance  the  epithelium  is  renewed  by  proliferation  from  that  sur- 
rounding the  wound,  but  the  tissue  of  the  cornea  proper  is  replaced  by 
cicatricial  tissue  which  never  becomes  perfectly  transparent.  The  anterior 
limiting  membrane  is  never  renewed. 

Nerves. — As  might  be  expected  from  the  highly  sensitive  character  of 
the  cornea,  the  nerves  that  supply  it  are  exceedingly  numerous.  From  the 
anterior  ciliary  nerves  trunks  pass  that  unite  near  the  corneo-scleral  junc- 
tion to  form  a  close-meshed  plexus  containing  medullated  and  non-medul- 

lated  fibres.  From  this  plexus, 
known  as  the  plexus  annularis,1 
nerves  pass  to  the  ciliary  muscle, 
to  the  iris,  and  to  the  cornea. 
Those  that  penetrate  the  cornea 
are  of  variable  size,  sixty  to 
eighty  in  number,  forty  to  fifty 
of  which  pass  towards  the  an- 
terior surface,  while  twenty  to 
thirty  are  directed  posteriorly. 
(Dogiel.)  These  nerves  contain 
both  medullated  and  non- 
medullated  fibres,  the  former 
preponderating  in  the  anterior 
set.  The  smallest  of  them  have 
but  two  to  three  nerve-fibres, 
the  largest  have  as  many  as 
twelve.  Soon  after  entering 

they  lose  their  medullary  sheaths,  thus  becoming  transparent,  and  exchange 
•fibres  with  each  other,  forming  a  rich  and  intricate  plexus,  the  deep  stroma 
plexus.2  (2,  Fig.  33.)  This  extends  like  a  net  through  the  anterior  three- 
fourths  of  the  cornea  proper,  the  posterior  fourth  being  supplied  by  simple 
branches  from  it.  Its  strands  are  nerve-fibres,  its  intersections  an  almost 


Nerve-plexuses  of  the  cornea.  (Testut.)— 1,  1,  two 
afferent  nerve-trunks ;  2,  deep  stroma  plexus ;  3,  sub- 
epithelial  plexus;  4,  infra-epithelial  plexus;  a,  cornea 
proper ;  6,  anterior  limiting  membrane ;  c,  anterior  epi- 
thelium. 


1  Syn.  :  orbiculus  gangliosus. 

2  Syn.  :  primary  plexus  ;  fundamental  plexus. 


THE  ANATOMY  OF  THE  EYEBALL.  157 

inextricable  interlacement  of  these  fibres  mingled  with  some  connective- 
tissue  cells.  No  true  ganglionic  cells  are  found  at  these  nodes. 

The  larger  nerve-trunks  are  surrounded  by  special  lymph-canals  like 
those  about  the  vessels,  called  the  perineural  canals.  These  are  composed  of 
two  concentric  endothelial  sheaths  enclosing  between  them  a  space  that 
communicates  with  the  interlamellar  spaces  of  the  cornea.  The  smaller 
branches  appear  not  to  have  these  special  sheaths,  but  to  lie  immediately 
within  the  interlamellar  spaces. 

It  was  formerly  thought  that  this  plexus  was,  like  those  of  the  intes- 
tinal walls,  a  terminal  plexus,  from  which  were  distributed  the  terminal 
nerve-filaments.  This  is,  however,  not  the  case.  From  that  part  nearest 
the  anterior  limiting  membrane  fibres  are  given  off  which  pass  in  a  radial 
direction,  penetrating  that  layer  (perforating  fibres l)  and  then  breaking  up 
into  fine  filaments  which  unite  beneath  the  epithelium  in  a  close-meshed 
plexus,  the  subepithelial  plexus.  (3,  Fig.  33.)  Some  of  the  perforating 
fibres  do  not  enter  this  plexus,  but  terminate  in  the  deeper  layers  of  the 
epithelium  in  rounded  or  conical  end-bulbs.  (Dogiel.)  From  the  sub- 
epithelial  plexus,  again,  numerous  varicose  fibres  pass  between  the  epithelial 
cells  and,  gradually  becoming  more  superficial,  form  an  intra-epithelial 
plexus.  (4,  Fig.  33.)  From  this  arise  the  terminal  fibrillaB,  which  end  in 
rounded  or  knob-like  expansions. 

Around  the  vascular  edge  of  the  cornea  are  found  some  peculiar  end- 
organs,  first  described  by  Ciaccio,2  and  recently  by  Dogiel.3  These  are 
plexiform  glomeruli  of  various  shapes  and  sizes,  formed  by  two  or  three 
nerve-branches  that  divide  and  subdivide.  No  cell-elements  have  been 
observed  between  the  meshes  of  these  glomeruli.  Loop-like  bendings 
with  thickenings  are  also  seen,  especially  in  nerves  that  appear  to  enter 
from  the  circumcorneal  tissue.  Irregular  quadrangular  or  shovel-shaped 
platelets  are  also  found.  These  end-organs  all  appear  to  be  confined  to  the 
exterior  zone  of  the  cornea,  not  occurring  beyond  one-half  to  one  and  a 
half  millimetres  from  the  edge. 

Besides  the  deep  stroma  plexus  and  its  derivatives,  there  is  also  found 
another,  composed  of  finer  fibrils,  formed  of  branches  derived  from  the 
main  trunks,  from  branches  of  the  principal  plexus,  from  the  perforating 
fibres,  and  from  branches  from  the  conjunctiva.  This  forms  the  superficial 
stroma  plexus*  so  called  because  most  apparent  beneath  the  anterior  limiting 
membrane.  It  seems,  however,  from  the  researches  of  Dogiel,  that  it  is  by 
no  means  confined  to  this  situation,  as  it  ramifies  extensively  between  the 
other  layers  of  the  cornea,  so  that  each  layer  may  be  said  to  have  a  plexus 
of  its  own,  except,  indeed,  the  deepest  near  the  posterior  limiting  membrane. 
Fibres  from  this  plexus  run  along  between  the  layers,  frequently  bend- 

1  Syn.  :  fibres  perforantes ;  rami  perforantes. 

2  Ciaccio  (G.  V.)-     Mem.  Accad.  d.  sc.  d.  1st.  di  Bologna,  1873,  ser.  3,  iv.  501. 

3  Dogiel  (A,.  S.).     Die  Nerven  der  Cornea  des  Menschen.     Anat.  Anz.,  1890,  v.  483. 

4  Syn.  :  subbasal  plexus  (Hover) ;  accessory  plexus  (Ranvier). 


158  THE  ANATOMY  OF  THE  EYEBALL. 

ing  at  right  angles  to  communicate  with  those  of  the  neighboring  layers. 
They  terminate  by  free  ends  slightly  thickened  or  delicately  tapering.  It 
was  formerly  thought  that  they  communicated  with  the  corneal  cells  or  with 
the  so-called  corneal  corpuscles,  but  that  has  been  shown  not  to  be  the  case. 
They  often  pass  over  the  surfaces  of  the  corneal  cells.  These  branches  cor- 
respond quite  closely  to  the  corial  branches  of  the  cutaneous  nerves. 

Historically  the  corneal  nerves  are  of  considerable  interest,  as  they  were 
the  first  that  were  shown  to  end  with  free  intercellular  filaments.  This 
method  of  termination  is  now  known  to  be  quite  usual  in  the  skin  and 
mucous  membranes. 

THE  MIDDLE  OB  VASCULAR  COAT.1 

Morphologically  speaking,  this  coat  extends  from  the  optic-nerve  en- 
trance over  the  entire  ball  within  the  outer  coat,  being  divided  into  four 
portions, — 1,  the  chorioid  proper,  extending  as  far  forward  as  the  ora  serrata, 
the  place  where  the  nervous  portion  of  the  retina  ceases ;  2,  a  thickened 
portion,  the  ciliary  body,  affording  attachment  to  the  suspensory  ligament 
of  the  lens  and  containing  within  it  plexuses  of  vessels  and  muscular  fibres  ; 
3,  a  freely  hanging  portion,  the  iris,  that  extends  like  a  diaphragm  across 
the  aqueous  chamber  in  front  of  the  lens ;  4,  the  corneal  portion,  already 
described  as  the  posterior  limiting  membrane  of  the  cornea,  with  its  endo- 
thelium.  This  is  usually  excluded  when  reference  is  made  to  the  middle 


1  Syn. :  tunica  media;  t.  uvea;  t.  uvceformis ;  t.  aciniformis  or  acinalis ;  t.  secundina; 
t.  vasculosa ;  tractus  uvealis  ;  leptomeninx  ophthalmencephali  (considering  this  coat  as  an 
extension  of  the  combined  arachnoid  and  pia  mater  of  the  brain). 

The  term  uvea,  often  found  in  the  older  writers  and  occasionally  employed  by  the 
moderns,  has  varied  somewhat  in  its  application.  The  earlier  anatomists  (Herophilus  and 
others)  compared  the  entire  middle  coat  to  the  inside  of  a  purple  grape-skin,  the  pupil  rep- 
resenting the  place  where  the  stem  had  been  pulled  out,  and  called  it,  therefore,  payoudfc 
XIT&V,  or  grape-like  tunic  (pdf  =  L.  uva,  a  grape).  This  was  translated  into  Latin  as 
tunica  uvea,  although,  as  Hyrtl  points  out,  the  name  has  no  good  etymological  foundation, 
there  being  no  adjective  uveus  derived  from  uva,  a  grape.  The  term  was  in  general  use  for 
the  entire  middle  coat  until  the  seventeenth  century,  and  there  is  a  tendency  among 
modern  writers  to  return  to  it,  as,  for  instance,  Brucke  (Anatomische  Beschreibung  dea 
Augapfels,  1847),  Luschka  (Anatomic  des  Menschen,  1865),  Helmholtz  (Handbuch  der 
physiologischen  Optik,  1867),  Pansch  (Grundriss  der  Anatomie,  1891),  and  many  recent 
writers  on  ophthalmology.  Some  of  the  older  authors,  however,  applied  the  term  to  the 
iris  alone.  Rufus  Ephesius,  for  example,  calls  the  part  of  the  middle  coat  to  which  the 
cornea  is  joined  payoedijq,  the  posterior  part  ^opoetdjfc.  Prom  the  seventeenth  century 
anatomists  began  generally  to  use  the  term  in  this  sense.  (See  Riolanus,  Petit,  Winslow, 
Ruysch.)  Later,  Zinn,  Haller,  and  Albinus  restricted  the  term  to  the  posterior  layer  of 
the  iris,  and  it  is  now  often  so  used.  Haller  uses  the  term  in  both  significations.  Mor- 
phologically, however,  this  is  inaccurate,  as  the  posterior  layer  is  really  a  continuation  of 
the  retina.  Schwalbe  therefore  calls  the  anterior  layer  of  the  iris  its  uveal  portion. 

The  terms  uvceformis.  aciniformis,  and  acinalis,  applied  to  this  coat  by  old  Latin 
authors,  arise  from  the  same  comparison  to  a  grape-skin  (L.  acinus,  a  berry  or  grape). 

Gerardus  Cremonensis,  in  his  translation  of  Avicenna,  applies  to  this  coat  the  term 
secundina,  partly  because  it  is  derived  from  the  secundina  cerebri  (pia  -(-  arachnoid),  partly 
because  it  nourishes  the  eye  as  the  secundina  uteri  (chorion)  do  the  foetus. 


THE  ANATOMY  OF  THE  EYEBALL.  159 

coat.  Many  authors  include  the  first  two  portions  under  the  designation 
chorioid. 

Disregarding,  then,  this  anterior  portion,  the  middle  coat  presents  two 
openings, — one  anterior,  the  pupil,  for  the  admission  of  the  undulations  of 
light ;  one  posterior,  the  optic  foramen  of  the  chorioid,  for  the  admission 
of  the  fibres  of  the  optic  nerve  upon  which  those  undulations  take  effect. 

This  envelope  offers  a  striking  contrast  to  the  external  one.  While  the 
latter  is  firm,  dense,  hard,  and  but  scantily  supplied  with  blood-vessels,  the 
former  is  soft,  easily  torn,  and  extremely  vascular.  This  is  a  natural  con- 
sequence of  its  function,  which  is  necessarily  of  a  nutritive  character ;  lying 
between  the  other  two  coats,  it  serves  to  nourish  both.  Again,  while  the 
outer  coat  is  inextensible  and  immovable  with  respect  to  the  others,  the 
middle  coat,  being  quite  lax,  can  easily  be  stretched  and  moved.  It  is, 
therefore,  a  suitable  seat  for  muscle-fibres  which  play  an  important  part  in 
the  mechanism  of  vision.  In  color,  too,  the  contrast  is  marked.  The 
outer  coat  is  either  white  or  transparent ;  the  middle  one  is  a  dark  reddish 
brown  in  the  posterior  portion  and  of  various  opaque  shades  anteriorly. 

Intra-ocular  tension  is  doubtless  kept  up  by  the  transudation  of  fluid 
from  the  vessels  of  the  middle  coat,  which,  viewed  in  this  light,  form  a 
secreting  organ  of  the  very  simplest  form.  Were  it  not  for  this  provision, 
the  fluids  of  the  eye  would  rapidly  decrease  by  filtration  through  the  coats. 

While  this  coat  contains  nerves  in  great  abundance,  they  are  for  the 
most  part  not  distributed  within  it,  but  pass  to  structures  beyond.  Inflam- 
mation here  does  not  usually  cause  pain,  unless  its  products  distend  the 
eyeball  and  occasion  pressure. 

THE   CHORIOID.1 

Considered  in  its  most  restricted  sense,  this  portion  of  the  middle  coat 
extends  from  the  nerve-entrance  to  the  ora  serrata, — that  is  to  say,  to  about 
seven  millimetres  from  the  edge  of  the  cornea.  It  therefore  covers  about 
the  posterior  two-thirds  of  the  bulb.  Its  thickness  varies  from  0.1  milli- 
metre near  the  nerve  to  0.06  millimetre  near  the  ora  serrata.2  Though 


1  Syn.  :  tunica  chorioidea  or  choroidea ;  chorioidea  or  choroidea ;  chorioid  or  choroid 
coat ;  choroid. 

From  \6piov,  the  chorion,  -\-  tWof,  appearance,  alluding  to  its  vascular  character,  like 
that  of  the  foetal  chorion. 

The  spelling  chorioid  is,  for  etymological  reasons,  to  be  preferred,  although  £o/ooe«J#f 
appears  in  some  of  the  Greek  authors.  (Hyrtl.) 

2  The  following  statements  as  to  the  thickness  of  the  chorioid  are  given  by  different 
authors : 


Merkel  .            

Near  Nerve. 
Millimetre. 
0.1 

Near  Ora  Serrata, 
Millimetre. 
0.06 

Sappev   . 

0.4 

0.3 

Schwalbe  -> 

,  '.    .05-.08 

Gerlach    j 
Vierordt    . 

.02 

0.14-0.02 

160 


THE  ANATOMY  OF  THE  EYEBALL. 


closely  applied  to  the  sclera,  it  may  be  stripped  from  it,  owing  to  the  laxity 
of  its  external  layer.  It  adheres  more  closely  at  the  optic-nerve  entrance 
than  elsewhere,  owing  to  the  connective-tissue  bundles  and  vessels  that 
here  pass  from  the  chorioid  to  the  nerve.  The  laxity  of  the  membrane 
enables  it  to  adjust  itself  to  the  variations  in  volume  that  occur  in  its  very 
numerous  blood-vessels. 

Like  the  inner  surface  of  the  sclera,  the  outer  surface  of  the  chorioid  is 
rough  and  fluffy  from  the  loose  ends  of  connective-tissue  bundles  severed 
by  the  separation  of  the  two  membranes.  The  inner  surface  is,  however, 
smooth  and  easily  separated  from  the  outer  or  pigmented  layer  of  the  retina. 

The  color  of  the  chorioid  and  also  of  the  retina  fades  somewhat  with 
advancing  age,  so  that  the  pupil  of  the  eye  is  less  dark  in  old  people.  The 

Fro.  34. 


Sclera — 


basalis. 


Section  of  the  human  chorioid.    (Bohm  and  von  Davidoff.) 


color  depends  not  only  upon  the  pigment  with  which  its  cells  abound,  but 
also  upon  the  copious  supply  of  blood  it  contains. 

Its  general  appearance  is  that  of  a  striated  surface,  the  striations  being 
caused  by  numerous  vessels  and  nerves  that  pass  from  behind  forward. 
These  structures  are  united  only  by  a  thin  web  of  connective  tissue ;  hence 
fche  consistence  of  the  chorioid  is  slight,  resembling  that  of  its  homologue, 
the  pia  mater  of  the  brain. 

The  essential  characteristic  of  the  chorioid  is  its  vascular  structure,  and 
its  different  layers  may  be  classified  according  to  the  predominant  vessels 
each  contains.  This  will  be  clearly  seen  by  an  inspection  of  Fig.  34.  The 
principal  layer,  the  lamina  vasculosa,  is  almost  wholly  composed  of  large 
arteries  and  veins,  and  interior  to  this  is  the  chorio-capillaris,  made  up  of 
capillaries.  These,  the  essential  features  of  the  chorioid,  are  separated  from 
the  adjacent  coats  of  the  eye  by  non-vascular  layers, — on  the  outer  surface 


THE  ANATOMY  OF  THE  EYEBALL. 


161 


the  suprachorioidea,  on  the  inner  the  lamina  basalis.  Some  authors  divide 
the  outer  portion  of  the  vascular  layer  into  two  laminae,  one  of  larger  ves- 
sels, the  other  (Sattler's  layer)  of  smaller  or  medium-sized  ones.  This 
appears  to  be  an  unnecessary  refinement,  but  it  emphasizes  the  fact  that  in 
the  chorioid  alone,  of  all  regions  of  the  body,  is  seen  a  distinct  gradation 
in  size  of  vessels  in  superimposed  layers.  The  reason  for  this  is  that  the 
tissues  requiring  most  active  nourishment  lie  to  the  inner  side.  The  sclera 

FIG.  35 

Cornea,  f         I  Anterior  chamber. 


Iris. 


Anterior  ciliary 
artery. 


Ciliary 
nerves. 


Vorticose 
vein. 


Long  posterior  cil- 
iary artery  and 
nerve. 


Long  posterior  ciliary  artery 
and  nerve. 

Vessels  and  nerves  of  the  middle  coat.    (Testut.) 

is  comparatively  inert  and  needs  but  little  blood,  while  the  retina  is  very 
active  and  requires  a  great  supply.  The  nutrient  fluids  exude  from  the 
capillaries ;  hence  these  are  developed  on  the  retinal  surface.  This  arrange- 
ment of  the  vessels  explains  why  inflammatory  exudations  from  the  chorioid 
usually  seek  the  inner  surface,  and  are  therefore  so  destructive  in  their  effects. 
The  suprachorioidea  l  is  essentially  a  continuation  of  the  lamina  fusca 

1  Syn.  :  suprachorioidal  lamina  ;  lamina  suprachorloidea  or  suprachoroidea ;  rnembrana 
suprachoroidea  (Montain)  ;  ectochoroidea  (Leidy) ;  tunica  suprachorioidea  or  suprachoroi- 
dea ;  t.  cellulosa ;  t.  arachnoidea ;  rnembrana  villoso-glandulosa, 

The  last  name  was  given  by  B.  A.  Stier,  one  of  the  earliest  to  describe  this  membrane, 
in  a  treatise  "  De  tunica  quadam  oculi  novissime  detecta,"  Halle,  1769,  from  its  villous 
appearance  and  certain  glandular-like  corpuscles  which  he  thought  he  had  discovered  in  it. 
VOL.  I. -11 


162 


THE  ANATOMY  OF  THE  EYEBALL. 


of  the  sclera,  from  which  it  is  separated  when  the  sclera  is  torn  away, 
cleavage  occurring  between  the  laminse  of  connective  tissue  along  the  lines 
of  the  larger  lymph-spaces,  together  constituting  the  perichorioidal  space. 
Throughout  the  laminae  of  this  tissue  numerous  pigmented  cells  are  found 
scattered,  and  these  impart  a  characteristic  reddish-brown  hue  to  the  mem- 
brane, which,  when  bathed  in  fluid  and  showing  through  the  white  web  of 
the  tissue,  appears  much  like  the  inside  of  a  grape-skin.  These  cells  are 
stellate  in  form,  and  are  to  be  distinguished  from  the  pigmented,  epithelial 
cells  of  the  retina,  which  usually  become  detached  with  the  chorioid,  and 
were  formerly  classed  as  belonging  to  it. 

Fio.  36. 


Cli 


0 


Vessels  of  the  chorioid.  (Leber.)— O,  optic-nerve  entrance ;  Cft,  chorioid ;  Oc,  orbiculus  ciliarls  ; 
PC,  ciliary  processes;  J,  iris;  Aa,  anterior  ciliary  arteries;  .46,  short  posterior  ciliary  arteries;  Al,  long 
posterior  ciliary  artery ;  dm,  greater  arterial  circle';  Me,  arteries  of  ciliary  muscle ;  rr,  recurrent  arte- 
ries ;  Vv,  vorticose  veins ;  Va,  anterior  ciliary  veins. 

While  the  suprachorioidea  has  no  vessels  or  nerves  that  specially  belong 
to  it,  nevertheless  it  serves  as  the  passage-way  for  many  of  these  structures 
that  pass  from  behind,  near  the  optic-nerve  entrance,  forward  to  the  ciliary 
body,  the  iris,  and  the  cornea.  (See  Fig.  35.)  The  long  ciliary  arteries,  two 
in  number,  penetrate  the  sclera  almost  exactly  in  the  horizontal  meridian, 
and,  accompanied  by  two  of  the  largest  ciliary  nerves,  pass  directly  through 
the  suprachorioidea.  The  external  is  slightly  above,  the  internal  slightly 
below,  the  horizontal  meridian,  and  their  situation  should  be  remembered 
in  operating  upon  the  sclera.  Exudations  between  the  sclera  and  the 
chorioid  may,  it  is  said,  by  pressure  on  the  nerves,  cause  changes  in  the 
pupil,  though  Ihe  iris  is  not  otherwise  involved.  (Hyrtl.) 


THE   ANATOMY   OF   THE   EYEBALL.  163 

The  lamina  vasculosa,1  which  succeeds  the  suprachorioidea,  is  a  continu- 
ation of  the  connective-tissue  web  of  the  latter,  with  the  addition  of  a 
great  number  of  large  vessels.  The  tissue  is  not  laminated,  but  veins  pre- 
dominate in  the  more  superficial  portion,  especially  in  front,  the  arteries 
lying  semewhat  deeper.  Stellate  pigment-cells  also  occur  here. 

The  principal  arteries  are  the  short  posterior  ciliary,  about  twenty  small 
branches  derived  from  two  trunks  that  arise  from  the  ophthalmic  artery 
above  the  optic  nerve.  These  penetrate  the  sclera  in  the  vicinity  of  the 
nerve-entrance  (Figs.  35  and  36)  and  form  a  very  rich  net-work  that  is 
lost  in  the  chorio-capillaris.  Recurrent  branches  from  the  long  posterior 
ciliary  (Fig.  36,  oo)  and  from  the  anterior  ciliary  trunks  also  supply  the 
anterior  part  of  the  chorioid,  the  main  distribution  of  these  arteries  being 
to  the  ciliary  muscle  and  to  the  iris. 

The  removal  of  blood  from  the  chorioid  is  almost  wholly  effected  by 
four  large  vessels,  called  the  vorticose  veins,2  that  discharge  into  the  oph- 
thalmic vein.  (Fig.  35 ;  Fig.  36,  hh.)  This  name  is  given  them  because 
they  are  formed  of  a  large  number  of  trunks  that  converge  in  long, 
sweeping  curves  to  form  a  single  one,  the  arrangement  resembling  a  vortex 
or  whorl.  Those  of  the  branches  that  come  from  the  chorio-capillaris 
primarily  arise  from  a  sinus,  towards  which  the  capillaries  converge,  form- 
ing a  minute  vortex  not  wholly  dissimilar  to  that  of  the  larger  trunks. 
(Fig.  37.) 

The  vorticose  veins  are  usually  grouped  in  two  pairs,  in  each  of  which 
the  vessels  are  symmetrically  disposed  on  either  side  the  vertical  meridian 
nearly  90°  apart.  Those  of  the  superior  pair  penetrate  the  sclera  at  seven 
to  eight  millimetres  behind  the  equator,  those  of  the  inferior  pair  a  little 
farther  forward.  Their  course  through  the  sclera  is  very  oblique  from 
without  inward,  and  they  also  diverge  from  the  vertical  meridian,  the 
passage  occupying  from  two  to  four  millimetres.  From  this  oblique  course 
it  follows  that  they  are  particularly  exposed  to  compression  from  without. 
Fuchs,3  to  whom  we  owe  the  most  thorough  investigation  of  the  course  of 

1  Syn. :  chorioid  proper,  chorioidea  propria  ;  tunica  vasculosa  Hallen ;  stroma  chori- 
oidea;  mesochoroidea  (Leidy). 

2  Syn. :  ductus  oculi  abducentes  (Hovius,  1716) ;  venae  or  vases  vorticosce  or  verticosce ; 
venae  ciliares  posticcs ;  Wirbel-  or  Wirtelvenen,  G. 

Nicolaus  Stenon,  one  of  the  earliest  investigators  into  the  vascular  system  of  the 
chorioid,  recognized  the  venous  character  of  these  vessels  in  his  work  "  De  musculis  et 
glandulis  observationum  specimen,"  Hafniae,  1664.  Hovius  also,  in  his  "  Tractatus  de  cir- 
culari  humorum  motu  in  oculis,"  Lugdunum  Batav.,  1716,  considered  them  as  veins,  as 
will  be  seen  by  the  name  cited  above.  Frederic  Ruysch,  however,  in  an  "  Epistola  de 
oculorum  tunicis,"  1721,  described  them  as  arteries,  admitting  two  layers  of  arterial  vessels 
in  the  chorioid, — viz.,  the  lamina  vasculosa,  containing  "  ramusculi  disposita  in  orbem," 
and  the  chorio-capillaris,  to  which  his  son  Henry  Kuysch  gave  the  name  of  membrana 
Ruyschiana  "  in  patris  honorem."  It  was  not  until  the  time  of  Haller  that  this  error  was 
corrected. 

3  Fuchs  (Ernst).     Beitrage  zur  normalen  Anatomic  des  Augapfels.     Archiv  fur  Oph- 
thalmologie,  1884. 


164  THE   ANATOMY   OP   THE   EYEBALL. 

these  veins,  thinks  that  at  or  near  their  exit  they  may  be  compressed  by  the 
action  of  the  oblique  muscles,  the  most  favorable  attitude  for  compression 
being  that  in  which  the  eye  is  adjusted  for  near  work.  He  suggests  that 
such  compression  might,  if  long  continued  and  habitual,  cause,  in  young 
people  whose  tissues  are  yet  plastic,  a  distention  of  the  coats  of  the  eye 
resulting  in  an  elongation  of  the  axis  and  consequent  myopia. 

Blood  passes  to  the  vorticose  veins  from  all  parts  of  the  eyeball, 
branches  converging  from  behind  near  the  optic-nerve  entrance  and  from 
the  front  about  the  ciliary  body  and  the  iris.  Those  that  pass  backward 
through  the  orbiculus  ciliaris  are  for  the  most  part  straight,  and  give  to 
this  region  its  characteristic  radiate  appearance.  All  the  intra-ocular  veins 
are  destitute  of  valves.  Variations  in  the  vorticose  veins  are  not  infrequent. 
These  usually  consist  in  an  increase  in  their  number  caused  by  a  doubling  of 
one  or  more  of  their  trunks.  This  occurs  more  frequently  on  the  nasal  side. 

Bundles  of  delicate,  non-striated,  muscular  fibres  are  found  scattered 
throughout  the  chorioid. 

Sattler  has  described  in  the  deeper  portion  of  the  lamina  vasculosa  a 
fine  net-work  of  elastic  fibres  invested  by  a  layer  of  endothelial  cells.  This 
he  believes  to  be  a  vestige  of  the  tapetum,  a  specialized  layer  of  the  chorioid 
found  in  some  animals.  Two  kinds  of  this  structure  are  described, — one, 
the  tapetum  fibrosum,  found  more  especially  in  ruminants  and  pachyderms, 
formed  of  connective-tissue  fibres ;  the  other,  the  tapetum  cellulosum,  found 
in  carnivora,  made  up  of  endothelial  cells.  The  pigment  of  the  retina  being 
wanting  over  certain  definite  areas,  light  falls  upon  this  opaque  layer  and 
is  totally  reflected  at  certain  angles,  causing  a  lustrous  iridescence  that  gives 
rise  to  the  popular  belief  that  the  eyes  of  animals  may  "  shine  in  the  dark." 
The  iridescence  is  caused  by  slight  irregularities  of  the  reflecting  surface, 
being  an  "  interference  phenomenon"  like  that  seen  in  mother  of  pearl. 

The  vascular  net-work  of  the  chvrio-capillaris *  (Fig.  38)  is  extraordi- 
narily rich.  The  vessels  show  a  remarkable  uniformity  of  calibre,  aver- 
aging about  9  n  in  diameter,  and  the  meshes  of  their  net-work  are  very 
close,  being  10-20  M  behind,  near  the  macula  lutea,  15-30  n  at  the  equator, 
and  25-30  M  near  the  ora  serrata.  This  is  the  most  copious  capillary  net- 
work of  the  body,  even  surpassing  that  of  the  alveoli  of  the  lungs.  The 
necessity  for  this  copious  supply  of  blood  is  apparent  when  we  find  that 
the  most  active  part  of  the  retina,  the  rod  and  cone  layer,  has  no  vessels  of 
its  own,  but  is  dependent  upon  this  net- work  for  its  nutrition.  The  reddish 
color  of  this  layer  shows  through  the  retina,  and  is  a  striking  characteristic 
of  the  interior  of  the  eye  when  the  latter  is  viewed  through  the  ophthal- 
moscope. 

1  Syn.  :  membrane  chorio-capillaris ;  lamina  Ruyschii  or  tunica  Ruyschiana  (for  Fred- 
eric Buysch,  professor  of  anatomy  at  Amsterdam,  1638-1731)  ;  membrana  Hovii  (for 
Jacobus  Hovius,  a  Dutch  physician,  known  by  his  "  Dissertatio  de  circular!  humorum 
oculorum  motu,"  published  in  1716  at  Leyden.  Appears  to  be  prior);  entochoroidea 
(Leidy). 


FIG.  37. 


The  origin  of  the  chorioidal  veins.  (Arnold.)— 1,  1,  one  of  the  larger  veins;  2,  2,  communicating 
veins;  3,  3,  "stars  of  Winslow,"  or  vortical  whorls  of  capillaries  by  which  the  latter  arise  from  the 
chorio-capillaris. 


Fro.  38. 


w 


The  chorio-capillaris  X  100.  (Sappey.)— 1,  2,  3,  veins  of  the  lamina  vasculosa;  4,  a  similar  vein 
arising  from  the  chorio-capillaris  by  convergent  branches;  5, 5,  the  capillary  net-work  formed  by  the 
anastomosis  of  similar  convergent  systems. 


THE    ANATOMY    OF   THE    EYEBALL. 


165 


FIQ.  39. 


It  is  in  this  layer  that  are  found,  the  capillary  whorls,  already  men- 
tioned, that  form  the  beginning  of  the  vorticose  veins.  (See  Fig.  37.) 
These  are  known  as  the  stars  of  Winslow,1  and  are  not  dissimilar  to  the 
stars  of  Verheyen  found  under  the  capsule  of  the  kidney.  They  are  not 
as  well  marked  in  man  as  in  animals  that  possess  a  tapetum.  No  pigment, 
no  great  vessels,  and  no  nerves  are  found  in  this  layer. 

The  lamina  basalis 2  appears  to  be  merely  a  condensation  of  the  connec- 
tive-tissue stroma  of  the  preceding  layers,  like  the  basement  membrane  of 
epithelium.  It  is  thin,  clear,  and  transparent,  structureless  or  showing  faint 
fibrillation  on  the  vascular  side,  being  firmly  united  to  the  chorio-capillaris, 
from  which  it  can  be  separated  only  by  maceration  with  strong  alkalies  or 
acids.  When  so  treated,  it  comes  away  in  shreds  that  have  a  tendency  to 
roll  up  as  does  the  limiting  layer  of  the  cornea.  With  age  it  increases  in 
thickness,  and  sometimes  pro- 
duces hemispherical  elevations 
that  press  upon  the  retinal  pig- 
ment and  destroy  it.  (H. 
Miiller.) 

THE   CILIARY   BODY. 

On  looking  at  the  anterior 
half  of  an  eye  bisected  at  its 
equator,  there  can  be  seen  on 
the  inner  surface  of  its  coats, 
about  seven  millimetres  from 
the  iris,  a  delicate,  wavy  line 
forming  a  ring.  This  is  the 
ora  serrata  (Fig.  39,  d,  d), 
already  mentioned,  and  marks 
the  boundary  of  the  nervous 
elements  of  the  retina  as  well 
as  that  of  the  chorio-capillaris 
by  which  those  elements  are 
fed.  In  front  of  this  line  the  inner  surface  is  marked  with  delicate  stria- 
tions  resembling  fine  hairs  or  cilia,3  especially  well  marked  anteriorly,  due 
to  delicate  vessels  meridionally  arranged,  and  to  slight  folds  of  the  surface. 
These  were  noted  by  the  older  anatomists,  being  mentioned  by  Galen,  who 

1  Stellulce  vasculosce  Winslowii  (for  Jacques  Benigne  Winslow,  a  physician  of  Paris, 
1669-1760). 

2  Syn.  :  membrana  pigmenti  (Bruch,  Zur  Kenntniss  des  kornigen  Pigments,  Zurich, 
1844.     He  believed  it  to  be  composed  of  delicate  young  epithelial  cells,  from  which  the 
pigment  cells  of  the  retina  are  developed) ;  membrana  Bruchii;  Bruch's  layer;  m.  intra- 
chorwidea  (Luschka,  who,  however,  includes  with  it  under  this  designation  the  pigmentec 
layer  of  the  retina) ;  lamina  basilaris ;  1.  elastica  chorioidea  (Kolliker) ;  I.  vitrea  chorwidea 
(Arnold)  ;  vitreous  or  hyaline  layer  or  membrane. 

s  Filaments  of  Ammon.     (Wallace  ) 


The  ciliary  body  seen  from  behind.— a,  sclera;  6, 
chorioid;  c,  retina;  d,  d,  ora  serrata;  e,  e,  the  ciliary 
body,  divisible  into  /,  the  orbiculus  ciliaris,  a  ring  of 
radiating  filaments,  and  g,  the  corona  radiata,  com- 
posed of  a  ring  of  ciliary  processes,  h;  i,  iris;  *,  pupil. 


166  THE   ANATOMY   OF   THE   EYEBALL. 

considered  that  they  were  intended  partly  for  the  conveyance  of  nutriment 
to  the  lens,  and  partly  for  uniting  the  chorioid  with  the  iris  and  cornea. 
Vesalius l  described  them  in  similar  terms,  and  from  his  description  this 
part  of  the  eye  became  known  as  the  ciliary  region,  and  the  portion  of  the 
middle  coat  here  situated  as  the  ciliary  body.2 

Extending:  from  the  ora  serrata  to  the  attached  border  of  the  iris,  it  is 

O  ' 

from  six  to  seven  millimetres  broad.  It  is  divided  into  two  zones, — a  com- 
paratively smooth,  posterior  one,  the  orbiculus  ciliaris  (Fig.  39,  /),  showing 
merely  the  slight  striations  already  mentioned,  and  a  plicated  anterior  one, 
the  corona  radiata  (Fig.  39,  g\  much  thickened  by  a  number  of  folds  or 
elevations,  the  ciliary  processes,  arranged  meridionally,  encircling  the  de- 
tached border  of  the  iris  just  back  of  the  corneo-scleral  junction.  It  is 
thickened  by  the  interposition  between  it  and  the  sclera  of  a  zonular  band 
of  non-striated  muscle-fibres  forming  the  ciliary  muscle. 

The  orbiculus  ciliaris3  (Fig.  40,  h;  Fig.  39, /)  is  about  four  millimetres 
broad,  and  is  distinguished  from  the  chorioid  proper  not  only  by  the  absence 
of  the  chorio-capillaris,  but  also  by  some  changes  in  the  connective-tissue 
elements.  The  vessels  are  all  about  equal  in  calibre,  and  run  parallel  to 
each  other  without  branching.  The  connective  tissue  that  unites  the  vessels 
of  the  lamina  vasculosa  again  appears  here,  but  in  reduced  quantity.  The 
elastic  layer  of  Sattler  is,  on  the  contrary,  somewhat  increased.  Losing 
its  endothelial  elements,  it  extends  upon  the  inner  surface  of  the  vessels  as 
a  reticulum.  The  lamina  basilaris  is  continued  forward  without  change. 
Pigment  from  the  pigmented  layer  of  the  retina  usually  adheres  to  the 
inner  surface. 

The  corona  radiata 4  (Fig.  39,  g)  is  composed  of  about  seventy  folds 
of  the  inner  surface  of  the  ciliary  body,  arranged  like  a  fluted  collar  or 
run0  around  the  edge  of  the  iris.  Each  of  these  folds  is  termed  a  ciliary 
process5  (Fig.  39,  h;  Fig.  40,  d),  and  it  is  these  that  have  given  the  name 
to  the  entire  region. 

1  "  Verum  haec  tunica  tenuitate  aranese  telas  fere  superat;  et  processulis  ab  uvea  pro- 
natis  talibus  constat,  ut  nigra  cilia,  seu  palpebrarum  pilos  forma  insigniter  exprimat. " 
Vesalius,  De  corpore  human!  fabrica,  1543,  lib.  vii.  cap.  xiv.  p.  648, 

J  From  L.  ciliaris,  adjective  from  cilium,  an  eyelash  or  fine  hair.  "  Dicunt  autem 
ciliarem,  quia  ciliis  non  absimilis  est."  Casserius,  Pentaesthesion,  cap.  xxvi.  p.  286. 

Syn. :  tunica  ciliaris  (Vesalius) ;  corpus  ciliare  (Fallopius) ;  ligamentum  ciliare ;  I. 
sclerotico-chorioidale  (von  Ammon)  ;  circulus  or  annulus  ciliaris ;  orbiculus  ligamentosus 
(Krause) ;  Strahlenband,  G. ;  zone  choro'idienne  (Sappey)  ;  zone  ciliaire  (Testut).  Those 
terms  which  imply  a  ligament  refer  more  particularly  to  the  external,  whitish  portion  of 
the  ciliary  body.  Krause  calls  the  deeper,  soft  layer  the  orbiculus  gangliosus. 

Henle,  Gerlach,  and  some  other  modern  authors  apply  the  term  corpus  ciliare  to  the 
corona  radiata  -f-  the  ciliary  muscle. 

3  Syn. :  pars  non  plicata ;  p.  striata  (Zinn). 

*  From  L.  corona,  a  crown  or  garland,  and  radiata,  surrounding  like  a  halo. 

Syn. :  pars  plicata ;  Strahlen-  or  Faltenkranz,  G.  ;  corpus  ciliare  in  the  restricted 
sense  ;  the  ciliary  processes. 

6  Syn. :  processus  ciliaris  (Th.  Bartholin,  1655)  ;  plica  ciliaris;  ligamentum  ciliare. 


THE  ANATOMY  OF  THE  EYEBALL. 


167 


The  slight  striations  of  the  orbiculus  ciliaris  are  so  arranged  that  three 
or  four  of  them  unite  to  form  a  ciliary  process.  Like  those  striations  the 
processes  are  of  a  vascular  character,  each  being  composed  of  a  skein-like 
glomerulus  of  vessels  (Fig.  41)  covered  with  a  thickened  continuation  of 
the  basement  layer.  Although  of  different  shape,  their  structure  recalls 
that  of  the  glomeruli  of  the  kidney,  and  it  is  probable  that  the  analogy 
also  holds  good  in  respect  to  their  offices,  both  allowing  fluids  -from  the 
blood  which  they  contain  to  escape  through  their  walls.  It  is  thought  that 
they  are  the  principal  agents  in  the  secretion  of  the  aqueous  humor.  Cer- 
tain recesses  in  the  surface  of  the  processes  resemble  very  much,  on  section, 


FIG.  41. 


FIG.  40. 


Segment  of  the  ciliary  body  and 
of  the  iris.  (Sappey.)— a,  iris;  6,  its 
pupillary  zone;  c,  its  ciliary  zone; 
d,  a  ciliary  process ;  e,  its  apex ;  /,  its 
base;  g,  ciliary  folds;  h,  striations 
forming  the  orbiculus  ciliaris ;  i,  ora 
serrata ;  k,  chorioid  with  its  vessels. 


Vascular  plexuses  of  the  ciliary  processes.  (Sap- 
pey.)—!. 1,  two  ciliary  processes  formed  by  anasto- 
mosing veins;  2,  2,  portions  of  two  others;  3,  3, 
venules  passing  from  the  processes  to  the  vasa  vorti- 
cosa ;  4, 4,  veins  from  the  iris  discharging  into  the 
ciliary  processes. 


the  lumen  of  glands,  and  have  accordingly  been  named  by  Collins l  the 
ciliary  glands. 

The  arteries  of  the  processes  are  derived  from  the  long  posterior  ciliary 
through  a  plexus  formed  by  those  vessels  about  the  iris  (greater  arterial 
circle).  After  several  subdivisions  they  pass  into  a  set  of  very  large  con- 
voluted capillaries,  and  thence  into  veins  that  run  parallel  through  the 
orbiculus  ciliaris  to  empty  into  the  vorticose  veins. 

The  size  of  the  processes  is  somewhat  variable,  they  being  from  two  to 

1  Collins  (E.  T.).  The  Glands  of  the  Ciliary  Body  in  the  Human  Eye.  Tr.  Oph. 
Soc.  U.  Kingdom,  Lond.,  1890-91. 


168  THE   ANATOMY   OF   THE   EYEBALL. 

three  millimetres  in  length,  0.12  millimetre  in  breadth,  and  from  0.8  to 
one  millimetre  in  height.  Between  the  longer  ones  smaller  plications  may 
be  seen,  the  ciliary  folds l  (Fig.  40,  g),  which  appear  to  be  simple  continua- 
tions of  the  striae  of  the  orbicularis. 

The  most  elevated  portion  of  the  processes  is  a  little  in  front  of  the 
margin  of  the  lens  and  at  a  distance  of  0.5  millimetre  from  it,  so  that  it 
is  possible  to  see  the  iris  from  behind  through  the  interval.  The  anterior 
end  forms,  therefore,  a  portion  of  the  wall  of  the  posterior  chamber.  The 
zonula  ciliaris,  which  is  a  specialized  portion  of  the  membrane  enclosing 
the  vitreous  humor,  extends  over  the  posterior  portion  of  the  corona  radiata, 
being  applied  closely  to  the  processes  and  descending  into  all  the  interstices 
between  them.  A  well-marked  furrow  separates  the  corona  from  the  iris. 

The  numerous  vessels  of  the  corona  are  united  by  a  delicate  stroma 
of  fibrous  connective  tissue,  within  which  are  scattered  numerous  pigment- 
cells.  This  is  thickened  on  its  inner  surface  to  a  basement  membrane,  to 
which  there  usually  adheres  the  dark  pigmented  layer  of  the  retina.  In 
preparing  the  inner  coat  for  examination,  this  is  frequently  brushed  away 
from  the  ciliary  processes,  which  then  appear  grayish  white  upon  an  in- 
tensely black  ground. 

The  great  vascularity  of  this  region  makes  it  very  subject  to  inflamma- 
tion when  interfered  with.  It  is,  therefore,  avoided  as  much  as  possible  in 
all  operations  upon  the  eye. 

The  ciliary  muscle 2  has  a  curious  history.  It  was  early  thought  that 
muscular  fibres  must  exist  in  this  region  of  the  eye,  and  Eustachius 3  actu- 
ally figures  the  ciliary  body  as  a  muscle,  giving  it,  in  fact,  the  name  mus- 
culus  ciliaris.  This  seems,  however,  not  to  have  been  generally  known  at 
the  time,  owing  to  the  loss  of  his  plates,  Vesalius  and  Fallopius  not  men- 
tioning it.  Kepler,4  the  astronomer,  in  his  consideration  of  the  optical 
properties  of  the  eye,  was  led  to  suppose  that  muscular  fibres  existed  in  the 
region  about  the  lens,  by  which  it  was  moved  backward  and  forward  to 
adapt  it  to  vision  at  different  distances.  Schemer5  also  attributed  to  the 
ciliary  processes  a  faculty  of  movement  by  which  they  could  affect  the 
lens.  Plempi us6  supposed  muscular  fibres  to  exist  there  which  drew  the 

1  Syn. :  plicae  ciliares  (C.  Krause). 

J  Syn.  :  musculus  ciliaris  (Eustachius)  ;  ligamentum  ciliare  (Fallopius)  ;  annulus 
ciliaris  (Luschka)  ;  tensor  chorioidea  (Brucke)  ;  musculus  Brueckianus  (Bonders)  ;  ganglion 
ciliare  (Bochdalek) ;  Brucke's  muscle  ;  Bowman's  muscle. 

3  In  Fig.  VI.,  Plate  XL.,  of  the  beautiful  plates  of  Eustachius  there  is  a  good  repre- 
sentation of  an  eye  dissected  so  as  to  show  the  ciliary  processes,  with  the  following  descrip- 
tion :     [Figura  VI.,  patefacit]     "...  pupillam  cum  crystalline  humore,  et  ligamento 
seu  musculo  ciliare."     These  plates  were  prepared  by  Eustachius  from  1552  to  1574,  lost, 
found  by  Pope  Clement  XI.,  and  first  published  by  his  physician  Lancisi  in  1714      The 
text  accompanying  the  plates  has  never  been  recovered. 

4  Kepler  (Johannes).     Dioptrice.     Augustse  Vindelicorum  [Augsburg],  1611 

5  Scheiner  (Christopher).     Oculus.     (Eniponti  [Innspruck],  1619,  pp.  162,  163 

8  Plempius  (Vopiscus  Fortunatue).  Ophthalmographia.  Amstelodami  [Amsterdam], 
1632,  p.  169. 


THE  ANATOMY  OF  THE  EYEBALL.  169 

retina  forward.  Descartes1  appears  to  have  first  suggested  that  such  fibres 
altered  the  form  of  the  lens,  in  which  he  was  followed  by  Briggs.2  Many 
other  anatomists3  surmised  the  existence  of  this  muscle,  but  the  first  to 
demonstrate  it  microscopically  appears  to  have  been  William  Clay  Wallace,4 
of  New  York,  who  revived  Eustachius's  name,  ciliary  muscle.  Briicke* 


1  Descartes  (Rene).     La  dioptrique.     Leyden,  1637,  cap.  iii.-v. 

The  same.     L'Homme.     Paris,  1664. 

1  Briggs  (William).     Opbthalmographia.      Lugd.  Batav.  [Leyden],  1686. 

3  The  following  may  be  mentioned  among  others : 
Vesling  (J.).     Syntagma  anatomica.     Amstelodami,  1647. 
Bartholin  (Thomas).     Anatomia.     Hagae-Comitiis  [The  Hague],  1663. 
Willis  (Thomas).     De  anima  brutorum.     Genevae,  1676. 

Sturm  (J.  C.).  Oculus:  de  visionis  organo  et  ratione  genuina  dissertatio.  Altdorffi, 
1678,  p.  13. 

Zahn  (R.  P.  F.  J.).  Oculus  artificialis  teledioptricus.  Herbipoli  [Wiirzburg],  1685, 
p.  15. 

Bidloo  (Godberi).     Anatomia  humani  corporis.     Amstelodami,  1686. 

The  same.    Observationes  de  oculis  et  visu  variorum  animalium.    Lugd.  Batav.,  1715. 

Wedel  (Christ).     Epistola  ad  Fr.  Ruysch,  de  oculorum  tunicis.    Amstelodami,  1700. 

Keill  (James).     The  Anatomy  of  the  Humane  Body.     London,  1703,  p.  160. 

Walther  (Aug.  F.).     De  lente  crystallina  oculi  humani.     Lipsiae  [Leipsic],  1712. 

Morgagni  (J.  B.).     Epistolae  anatomic®.     1728. 

Porterfield  (William).  An  Essay  concerning  the  Motions  of  our  Eyes.  Med.  Essays 
and  Obs.  Soc.  Edinb.,  1735,  iii.  160;  1737,  iv.  124. 

The  same.     A  Treatise  on  the  Eye.     Edinburgh,  1759. 

Ruysch  (Fred.).     Responsio  ad  Wedelium  de  oculorum  tunicis.     1737. 

Plainer  (J.  Z.).     De  motu  ligamenti  ciliaris.     Lipsiae,  1738. 

Boerhaave  (H.).  Praelectiones  academicae,  edit,  et  not.  add.  Alb.  Haller.  Taurini 
[Turin],  1742-45. 

Camper  (P.).     De  quibusdam  oculi  partibus.     Lugd.  Batav.,  1746. 

Olbers  (H.  W.  M.).     De  oculi  mutationibus  internis.     Gottingae,  1780. 

Hildebrandt  (G.  F.).     Anatomic  des  Menschen.     1803. 

Graefe  (K.  A.).  Ueber  die  Bestimmung  der  Morgagnischer  Feuchtigkeit,  der  Linsen- 
kapsel  und  des  Faltenkranzes.  Archiv  f.  d.  Physiologic,  Halle,  1809,  ix.  225-336. 

Brewster  (D.).  Experiments  on  the  Structure  and  Refractive  Power  of  the  Coats  and 
Humors  of  the  Human  Eye.  Edin.  Phil.  Jour.,  1819,  i.  42-45. 

Jacobson.     Suppl.  ad  Ophthalm.     Copenhagen,  1821. 

Purkinje  (J.  E.).  Beobachtungen  und  Versuchezur  Physiologic  der  Si  nne.  Berlin,  1825. 

Miiller  (J.).    Zur  vergleichenden  Physiologic  des  Gesichtsinns.    Leipzig,  1826,  p.  172. 

Smith  (Thomas).  On  the  Muscular  Structure  and  Functions  of  the  Capsule  of  the 
Crystalline  Lens  and  Ciliary  Zone.  Lond.  and  Edinb.  Phil.  Mag.,  1833,  3d  ser.,  iii.  5. 

4  Wallace  (William  Clay).     On  the  Accommodation  of  the  Eye  to  Distances.    Amer- 
ican Journal  of  Sciences  and  Arts,  1835,  xxvii.  219-222. 

In  this  article  he  says,  "  At  the  base  of  the  ciliary  processes  upon  the  inner  surface 
of  the  chorioid  coat  there  is  a  range  of  muscular  fibres.  In  the  sheep  the  fibres  of  the 
upper  portion  run  transversely  to  the  ciliary  processes;  those  of  the  lower  portion  run 
parallel  to  them." 

See  also  the  following  by  the  same  author  :  Treatise  on  the  Eye,  New  York,  1839,  p. 
37.  Lond.  Med.  Gaz.,  1842-43,  i.  412.  Lectures  on  Myopia,  Boston  Med.  and  Surg. 
Jour.,  1844,  xxx.  289,  290.  Accommodation  of  the  Eye  to  Distances,  New  York,  1850. 

5  Brucke  (Ernst).    Ein  neuer  Muskel  im  Auge.     Med.  Zeitung,  Berlin,  1846,  xv.  130. 
The  same.     Ueber  den  Musculus  Cramptonianus  und  den  Spannmuskel  der  Choroidea. 

M  filler  :s  Archiv,  1846,  p.  370.     (Vorgetragen  in  der  Berliner  physikalischen  Gesellschaft 
am  29  Mai,  1846.) 


170  THE  ANATOMY  OF  THE  EYEBALL. 

published  a  description  of  the  muscle  in  1846,  and  its  discovery  is  often 
wrongly  ascribed  to  him.  Bowman '  gave  an  excellent  account  of  it  one 
year  later,  also  calling  it  by  its  present  name,  which  is  frequently  credited 
to  him.  Even  this  is  not  all.  The  deeper  circular  fibres  were  first  men- 
tioned by  Wallace,  afterwards  described  and  figured  by  Van  Reeken,2  and 
described  by  Rouget ; 3  yet,  since  somewhat  later  they  were  described  by 
H.  Miiller,4  the  honor  of  their  discovery  is  usually  allowed  him. 

The  muscle  in  question  is  a  zone-like  band  of  smooth  fibres  interposed 
between  the  ciliary  body  and  the  sclera,  and  may  perhaps  be  regarded  as  a 
condensation  of  the  scattered  bundles  of  smooth  muscle-fibres  found  else- 
where in  the  middle  coat.  Its  grayish  appearance  caused  it  to  be  long 
regarded  as  a  ligament.  It  occupies  a  zone  from  six  to  seven  millimetres 
wide,  taking  its  origin  from  the  thickened  band  already  described  as  the 
annular  ligament,  quickly  expanding  to  a  maximum  thickness  of  0.8  milli- 
metre, and  then  gradually  thinning  away  to  an  insertion  upon  the  chorioid, 
the  muscular  fibres  blending  with  the  stroma  and  passing  as  they  dis- 
appear into  some  peculiar  stellate  figures.  (Iwanoff.)  It  extends  a  little 
farther  on  the  temporal  side  than  on  the  nasal,  and  H.  Miiller  describes 
bundles  that  accompany  the  long  ciliary  arteries  as  far  as  their  scleral 
canals.  A  meridional  section  of  the  muscle  is,  therefore,  triangular,  the 
longest  sides  of  the  triangle  being  applied  to  the  sclera  and  to  the  ciliary 
body,  a  short  base  being  directed  towards  the  iris. 

Two  principal  divisions  of  the  muscle  may  be  made  out,  correspond- 
ing to  the  direction  of  its  fibres.  First,  an  external  condensed  portion,  in 
which  the  fibres  run  meridionally ;  this  was  the  portion  which  was  first  noted 
by  Briicke,  and  which  he  called  the  tensor  chorioidea.  Second,  an  internal 

1  Bowman  (William).  Lectures  on  the  Parts  concerned  in  the  Operations  of  the  Eye, 
and  on  the  Structure  of  the  Retina,  delivered  at  the  Royal  London  Ophthalmic  Hospital, 
Moorfields,  June,  1847. 

1  Van  Reeken  (C.  G.).  Ontleedkundig  onderzoek  van  den  toestel  voor  accommodatie 
van  het  oog.  Utrecht,  1855. 

8  Rouget  (Charles).  Recherches  anatomiques  et  physiologiques  sur  les  appareils  erec- 
tiles :  Appareil  de  1 'adaptation  de  1 'ceil  chez  les  oiseaux,  les  principaux  mammiferes  et 
Phomme.  Compte-rendu  de  1'Acad.  des  Sciences,  Paris,  19  Mai,  1856,  xlii.  937. 

The  same.  Sur  la  structure  de  1'ceil  et  en  particulier  sur  1'appareil  irio-choroidien. 
Compte-rendu  de  la  Soc.  de  Biologie,  Gaz.  m6d.  de  Paris,  1856,  3e  ser.,  xi.  563. 

The  same.  R6ponse  a  une  reclamation  de  priorite  adressee  par  M.  Miiller.  Comptes- 
rendus  de  1'Acad.  des  Sciences,  Paris,  xlii.  1255-1256. 

*  Miiller  (H.).  Sitzungsberichte  der  physikalisch-medicinischen  Gesellschaft  in 
"Wiirzhurg,  Sitzung  vom  24.  November,  1855.  Verhandl.  d.  phys.-med.  Gesellsch.  in 
Wiirzb.,  1856,  vi.  26. 

As  this  is  the  passage  upon  which  Miiller  rests  his  claims  to  priority,  it  is  given  entire 
for  comparison  with  Wallace's  notice: 

"  Hr.  H.  Miiller  theilt  eine  Notiz  iiber  eine  ringformige  Schicht  im  Ciliarmuskel  des 
Menschen  mit.  Dieselbe  liegt,  bedeckt  von  dem  Langsbiindeln  des  Muskels,  auf  dem  vor- 
dersten  Theil  des  Ciliarkorpers  und  Hr.  Miiller  glaubt  dass  sie  fur  die  Accommodation  des 
Auges  von  besonderer  Wichtigkeit  sei." 

The  same.  Ueber  ein  ringformigen  Muskel  am  Ciliarkorper  des  Menschen  und  iiber 
den  Mechanismus  der  Accommodation.  Arch.  f.  Ophth.,  Berl.,  1857,  iii.,  1  Abth.,  1-24. 


THE  ANATOMY  OP  THE  EYEBALL. 

portion  that  surrounds  the  margin  of  the  iris     nth  fl 
equatorial  direction;  this  is  ^^l^t 
st,tuteabout  one-tenth  of  the  .SSL     Its  L 
connections  with  other  portions  of  the  muscle 

Besides  these  clearly  denned  portions,  oiher  subdivisions  have  be, 
suggested.     Iwanoff*  describes  the  deeper  part  of  the  tensor  chorioidel" 


FIG.  42. 


Varieties  of  the  ciliary  muscle.    (Iwanoff.)— A.  form  found  in  emmetropic  eyes;  B.  In  myopic  eyes: 

C,  in  hypermetropic  eyes. 

which  is  arranged  in  a  loose,  reticulate  manner,  as  the  radial  portion. 
Waldeyer3  found  in  some  eyes  a  bundle  passing  from  the  annular  ligament 
to  the  sclera,  where  in  birds  is  situated  a  well-marked  band  of  striated 
muscular  fibres,  the  Cramptonian  muscle.  This  has  not  been  observed  by 

1  Syn.  :  Miiller's  muscle. 

2  Strieker's  Manual  of  Histology,  Arn.  ed.,  p  851. 
8  Grafe  und  Samisch,  Handbuch,  i.  231. 


172  THE  ANATOMY  OF  THE  EYEBALL. 

others,  so  far  as  I  am  aware,  and  it  seems  doubtful  if  structures  of  such 
distant  phylogenetic  relation  are  truly  homologous. 

Well-marked  variations  of  the  muscle  occur.  Iwanoff  has  pointed  out 
that  in  myopic  eyes  the  circular  fibres  are  deficient  or  entirely  absent,  while 
in  hypermetropic  eyes  they  are  greatly  increased,  so  as  to  cause  a  decided 
enlargement  of  the  ciliary  body,  and  they  then  amount  to  at  least  one-third 
of  the  entire  mass  of  the  muscle.  (See  Fig.  42.) 

The  action  of  the  muscle -has  been  a  subject  of  some  discussion.  It 
was  early  observed  that  the  eye  possesses  a  power  of  adjustment  for  the 
vision  of  near  or  remote  objects,  and  it  was  naturally  thought  that  this 
result,  which  is  called  accommodation,  was  produced  by  an  alteration  of  the 
relative  positions  of  the  lens  and  the  retina.  Most  early  observers  of  the 
ciliary  muscle  supposed  that  its  function  was  to  produce  an  effect  of  this 
kind  either  by  drawing  directly  upon  the  lens  or  the  retina  by  compressing 
and  thus  elongating  the  eyeball,  or  by  compressing  the  anterior  ciliary  veins, 
which,  thus  rendered  turgid,  displaced  the  lens. 

The  matter  was  finally  set  at  rest  by  observations  made  upon  images 
reflected  from  the  surfaces  of  the  lens  and  cornea.  Although  nearly  all 
light  passes  through  these  structures,  yet  if  a  small,  bright  flame  be  so 
placed  in  a  darkened  room  that  its  rays  fall  obliquely  upon  the  eye,  an 
observer  on  the*  opposite  side  can  see  three  images  of  the  flame  reflected, — 
the  first,  erect  and  bright,  from  the  outer  surface  of  the  cornea ;  the  second, 
erect  and  somewhat  larger  and  dimmer,  from  the  anterior  surface  of  the 
lens ;  the  third,  inverted  and  smaller,  from  the  posterior  surface  of  the  lens. 
(See  Fig.  43.)  ' 

These  are  called  the  images  of  Purkinje1  or  Sanson.2  The  second 
image  may  be  observed  to  change  in  size  and  position  when  the  vision  is 
directed  from  a  near  object  to  one  close  at  hand,  while  the  other  two  remain 
stationary.  This  shows  that  the  anterior  surface  of  the  lens  changes 
during  accommodation,  while  the  other  surfaces  remain  unaltered.  The 
matter  is  rendered  still  more  plain  by  using  two  sources  of  light,  making 
two  sets  of  images,  one  above  and  one  below  the  horizontal  meridian. 
(See  Fig.  44.) 

The  first  to  notice  the  change  of  the  images  during  accommodation  ap- 
pears to  have  been  Langenbeck,3  but  his  observations  were  imperfect.  In 
1851  both  Cramer4  and  Helmholtz5  improved  upon  his  experiments  and 

1  Purkinje  (J.    E.).      De  examine    physiologico  organi  visus  et  systematis  cutanei. 
Vratislaviffl  [Breslau],  1823,  p  28. 

2  Sanson  (L.  J.).     Leqons  sur  les  maladies  des  yeux.     Paris,  1838. 

*  Langenbeck  (Max).     Klinische  Beitrage  aus  dem  Gebiete  der  Chirurgie  und  Oph- 
thalmologie.     Gottingen,  1849. 

*  Cramer  (A.).  Mededeelingen  uit  net  gebied  der  opbthalmologie.    Tijdschr.  d.  Nederl. 
Maatsch.  t.  Bevord.  d.  Geneesk.,  1851,  ii.  99:  Nederl.  Lancet,  1851,  ser.  3,  i.  529. 

4  Helmholtz  (H.).     Beschreibung  eines  Augenspiegels.     Berlin,  1851. 
The  same.      Ueber  die  Accommodation  des  Auges.    Arch   f.  Ophth  ,  Berlin,  i  ,  Abth. 
ii.  1. 


FIG.  43. 


FIG.  44. 


Images  of  Purkinje  formed 
by  reflections  from  the  surfaces 
of  the  eye.  (Helmholtz.) — a. 
reflection  from  the  cornea;  6, 
from  the  anterior  surface  of  the 
lens ;  c,  from  the  posterior  sur- 
face of  the  lens. 


a        b        c  a        b        e 

Double  images  of  Purkinje  used  for 
showing  the  curvature  of  the  lens  during 
accommodation.  (Helmholtz.)— The  small 
letters  as  in  last,  illustration.  A,  appear- 
ance during  accommodation  for  distant 
vision ;  B,  for  near  vision. 


FIG.  45. 


Ciliary  muscle. 


Action  of  the  ciliary  muscle.    (Fuchs.)— The  shaded  portion  shows  the  parts  when  at  rest,  the  heavy 
line  their  displacement  when  the  ciliary  muscle  is  in  action. 


THE  ANATOMY  OF  THE  EYEBALL.  173 

showed  clearly  the  amount  of  curvature.  Helmholtz,  in  particular,  demon- 
strated that  the  anterior  surface  curved  most,  and,  indeed,  that  the  curva- 
ture can  be  seen  by  carefully  observing  any  eye  from  the  side  while  directed 
alternately  to  distant  and  near  objects.  The  posterior  surface  curves  but 
little,  and  the  lens  does  not  shift  its  position.  Accommodation  is  effected 
by  altering  the  curvature  of  its  surfaces  and  thus  changing  its  refractive 
power.  The  suspensory  ligament  of  the  lens,  which  will  be  hereafter  de- 
scribed, is  closely  connected  with  the  ciliary  body  and  with  the  chorioid. 
The  lens  is  held  by  it  in  a  state  of  tension,  and  when  this  tension  is  relaxed 
tends  to  assume  a  spherical  form.  The  ciliary  muscle,  having  its  punctum 
fixum  at  the  unyielding  corneo-scleral  junction,  has  its  punctum  mobile 
upon  the  chorioid,  which,  as  Briicke  surmised,  it  draws  forward.  When 
this  is  done,  the  suspensory  ligament  is  relaxed  (see  Fig.  45),  the  iris  and 
the  equator  of  the  lens  are  pushed  towards  the  axis  of  the  eye,  the  surfaces 
of  the  latter  are  more  curved  and  the  anterior  one  is  pushed  towards  the 
cornea.  The  circular  fibres  act  more  directly  and  effectively  than  the 
meridional  ones ;  hence  it  is  not  surprising  to  find  that  in  far-sighted 
persons,  who  have  to  accommodate  a  great  deal  (hypermetropes),  the  cir- 
cular fibres  are  greatly  developed,  while  in  those  who  are  near-sighted 
and  accommodate  but  little  (myopes)  there  is  little  or  no  development  of 
those  fibres. 

The  long  and  the  short  ciliary  nerves  supply  the  ciliary  muscle.  The 
former  are  derived  from  the  nasal  branch  of  the  ophthalmic,  and  are, 
therefore,  sensitive ;  the  latter  are  from  the  ciliary  ganglion,  and  are 
doubtless  of  a  mixed  character.  They  penetrate  the  sclera  near  the 
entrance  of  the  optic  nerve  (see  Fig.  35),  run  forward  in  the  supra- 
chorioidal  space,  enter  the  ciliary  muscle,  and  there  unite  in  a  plexus 
(the  ciliary  plexus)1  which  contains  scattered  nerve-cells.  From  this 
plexus  fibres  are  given  off  that  pass  to  the  cornea,  the  iris,  and  the  ciliary 
muscle. 

Arnstein  and  Agababow2  have  recently  described  the  following  nerve- 
endings  in  the  ciliary  body  :  1,  vaso-motor  endings  in  the  walls  of  the  ciliary 
vessels ;  2,  motor  endings  in  the  ciliary  muscle  like  those  characteristic  of 
smooth  muscle-fibre  elsewhere ;  3,  "  reticular  plates/'  or  extremely  fine 
reticulations  of  granular  nerve-fibres  ;  4,  terminal  arborescences,  or  "  telo- 
dendra,"  lying  in  the  connective  tissue  between  the  bundles  of  muscle-fibres. 
The  reticular  plates  probably  minister  to  ordinary  sensation,  the  arbores- 
cences to  "  muscular  sense,"  being  appropriately  situated  for  irritation  by 
the  contraction  of  the  muscle-bundles.  This  sense  would  be  of  great 
importance  in  an  organ  like  the  ciliary  muscle,  and  admit  of  an  accurate 
adjustment  of  accommodation. 

1  Syn.  :  plexus  gangliosus  ciliaris ;  or bicuhis  ganglia  sus  ciliaris.     (W.  Krause.) 

2  Die  Innervation  des  Ciliarkorpcrs.     Anatomischer  Anzeiger,  1893,  viii.  656. 


174 


THE   ANATOMY   OF   THE   EYEBALL. 


THE   IRIS.1 

This  anterior  section  of  the  vascular  coat  is  visible  on  looking  into 
the  eye  through  the  transparent  cornea,  appearing  as  a  thin,  contractile, 
variously  colored  curtain,  pierced  with  a  central  aperture,  the  pupil.  Its 
peripheral  or  ciliary  border  is  attached  to  the  inner  surface  of  the  eyeball ;  its 
central  or  pupillary  border  is  free,  and  rests  upon  the  anterior  capsule  of  the 
crystalline  lens,  which  gives  it  firm  support  in  its  movements.  It  therefore 
divides  the  aqueous  chamber  into  two  lesser  cavities,  the  anterior  chamber 
between  it  and  the  cornea,  and  the  posterior  chamber  between  it  and  the  lens. 

It  is  sometimes  stated  that  the  iris  lies  in  a  vertical  frontal  plane,  but  a 
little  examination  shows  that  this  is  not  strictly  correct.  As  its  pupillary 
border  rests  upon  the  lens,  which  projects  beyond  the  plane  of  its  ciliary 
border,  its  form  is  rather  that  of  a'  very  flat  truncated  cone  placed  with  its 
outer  rim  vertical.  When  the  lens  is  absent  it  may  hang  vertically,  and  then, 
losing  its  support,  may  tremble  and  shake  with  the  movements  of  the  ball. 

The  thickness  of  the  iris  is  only  0.4  millimetre,  decreasing  somewhat 
towards  the  pupillary  border.  With  wide  dilatation  of  the  pupil,  or  during 
inflammation,  its  thickness  may  be  nearly  doubled.  Its  total  diameter  is 
from  ten  to  twelve  millimetres,  and  when  at  rest  its  breadth  from  the  cil- 
iary border  to  the  pupil  is  from  four  to  five  millimetres,  a  little  less  on  the 
nasal  side,  the  pupil  being  slightly  eccentric.  The  diameter  of  the  pupil  is 
from  three  to  six  millimetres  when  the  iris  is  at  rest,  but  constantly  varies, 
ranging  from  one  to  eight  millimetres.2 

1  From  Ipif,  -«Jof,  the  rainbow  or  any  bright-colored  circumscribing  circle. 

Syn. :  tunica  coerulea ;  diaphragma  bulbi ;  Regenbogenhaut,  G. ;  Blendung,  G.  (lit. 
the  screen  or  blind). 

Galen  applies  the  term  to  the  annular  ligament  (De  usu  partium.  chap.  ii.).  Varolius 
and  others  of  the  older  authors  use  it  for  the  varied  colors  themselves.  Scheiner  calls  the 
iris  sol.  Vesalius  appears  to  have  been  aware  of  its  duplex  character,  describing  it  as 
composed  of  two  layers ;  and  Jacobus  Sylvius  calls  it  duplata  meninx. 

2  Different  authorities  vary  slightly  in  their  statements  of  these  dimensions.      The 
following  are  the  principal  ones  given : 


Thicknes 
of  Iris. 

Diameter 
of  Iris. 

Breadth  of 
Iris  Ring. 

Diameter  of 
Pupil. 

Quain     

Millimetres. 
04 

Millimetres. 
11 

Millimetres. 
5 

Millimetres. 
1  8 

Testut    

03 

12  13 

Sappey  .    . 

13 

3  4 

Henle     

04-02 

3  5-4  5 

8  6 

Rauber  

0  4 

10-12 

4  5 

3  6 

Merkel  

0  4-0  2 

4 

4 

Vierordt    

0  4 

11 

Gerlach  

03 

9  10 

3  6 

Schwalbe  

0  4-0  2 

4-5 

3  6 

Krause  -.    . 

04 

3  3 

Chauvel  and  Nimier 
(Diet,  encycl.  d.  sc.  med.) 
Budge    

13 

.    . 

3-4 
F>  33  7 

Huschke    

307   f\  7£ 

THE  ANATOMY  OF  THE  EYEBALL.  175 

According  to  Krause,  the  centres  of  the  pupils  are  distant  from  each 
other  58.5  to  67.5  millimetres. 

The  weight  of  the  iris  is,  according  to  Huschke,  fiay-six  milligrammes, 
being  y^T  of  that  of  the  whole  eye,  and  |  of  that  of  the  chorioid. 

The  attached  or  ciliary  border l  of  the  iris  is  continuous  behind  with  the 
ciliary  body,  in  front  with  the  posterior  limiting  membrane  of  the  cornea 
through  the  pectinate  ligament.  Its  place  of  attachment  is  at  the  annular 
ligament,  forming  the  inner  boundary  of  the  scleral  sinus,  as  shown  in  Fig. 
29.  It  coincides  with  the  inner  edge  of  the  corneo-scleral  bevel,  and  is 
therefore,  some  three  millimetres  behind  the  apparent  rim  of  the  cornea  as 
it  appears  externally.  When  it  is  desired  to  reach  the  lens  without  inter- 
fering with  the  iris,  it  is  necessary  to  insert  the  instrument  a  little  beyond 
this  interval.  The  tissue  of  the  iris  is  so  loose  that  it  is  easily  detached 
from  its  insertion.  Hyrtl 2  mentions  a  case  in  which  a  complete  separation 
of  the  iris  was  caused  by  a  blow  on  the  eye. 

The  free  or  pupillary  border3  is  very  thin,  and,  being  of  a  clear  black, 
owing  to  the  pigment  of  the  posterior  surface  that  here  turns  over  the 
border,  cannot  well  be  seen  against  the  dark  background  of  the  pupil  unless 
the  posterior  portion  of  the  eye  is  removed.  Examined  through  a  glass,  it 
presents  a  denticulate  or  beaded  appearance,  due  to  slight  elevations  formed 
at  the  junction  of  the  striae  of  the  anterior  surface,  which  will  presently  be 
described.  That  the  plane  of  this  border  lies  posteriorly  to  the  plane  of  the 
anterior  edge  of  the  sclera  can  easily  be  demonstrated  in  the  living  subject 
by  Helmholtz's  experiment  of  viewing  the  eye  from  the  side  in  such  a 
position  that  the  near  edge  of  the  sclera  almost  obscures  the  pupil.  (See 
Fig.  46.)  There  is  then  apparent  in  front  of  the  pupil  a  clear  strip,  due  to 
a  distorted  image  of  the  iris  refracted  by  the  cornea,  and  farther  forward, 
directly  against  the  convex  edge  of  the  cornea,  a  darker  strip,  which  is  the 
image  of  the  opposite  sclera.  When  the  observer  moves  his  eye  farther 
back  the  first  image  disappears,  but  the  second  remains,  which  shows  that 
the  iris  must  lie  behind  the  scleral  rim.  In  viewing  the  iris  in  this  manner 
it  appears  convex.  It  was  accordingly  described  as  of  that  shape  by  Galen, 
Vesalius,  and  many  subsequent  anatomists.  This  appearance  is  due,  how- 
ever, to  a  distortion  of  its  figure  by  the  passage  of  rays  through  the  cornea 
and  aqueous  humor.  .  When  the  eye  is  viewed  under  water— this  having 
nearly  the  same  index  of  refraction — it  appears  plane  or  nearly  so. 

The  fact  that  the  pupillary  border  touches  the  anterior  surface  of  the 
lens  can  also  be  shown  by  direct  observation  with  the  figures  of  Purkiuje, 
already  referred  to.  (See  Figs.  43  and  44.)  It  will  be  remembered  that  the 
second  of  these  figures  is  that  reflected  from  the  anterior  surface  of  the  lens. 
The  observer  may  easily  take  such  a  position  that  this  reflection  just 
touches  the  edge  of  the  pupil,  and  if  this  position  is  slightly  shifted  back- 

1  Syn.  :  margo  ciliaris. 

2  Topographische  Anatomic,  i.  245. 

3  Syn.  :  margo  pupillaris. 


176  THE  ANATOMY  OF  THE  EYEBALL. 

ward  the  image  at  once  completely  disappears.  Now,  if  the  pupillary 
border  were  not  in  contact  with  the  lens,  a  dark  band,  the  shadow  of  the 
iris,  would  have  intervened  between  the  image  and  the  border. 

The  structure  of  the  iris  cannot  be  clearly  understood  without  reference 
to  its  development.  This  is  treated  elsewhere  in  this  work,  and  it  is  only 
necessary  to  remark  here  that  the  pupil  is  the  opening  of  the  optic  cup, 
which,  it  will  be  remembered,  has,  by  the  iuvagination  of  the  optic  vesicle, 
become  two-layered,  like  the  toy  called  the  cup  of  Tantalus.  (See  Fig.  47.) 

The  two  layers  of  the  cup  (a,  6)  are  continued  forward  and  unite  at  the 
pupillary  border  of  the  iris,  constituting  the  black  seam  seen  there.1  The 
outer  layer  is  pigmented  throughout,  the  inner  layer  only  where  it  forms 
the  posterior  surface  of  the  iris.  Together  they  constitute  what  is  known 
as  the  retinal  portion  of  the  iris,2  being  not  derived  from  the  middle  coat. 
That  coat  is,  however,  continued  forward  as  an  envelope  of  the  optic  cup, 
forming  the  anterior  portion  or  stroma  of  the  iris.  This  is,  therefore, 
termed  the  uveal  portion 3  of  the  iris.  (Fig.  47,  c.)  The  two  portions  retain 
certain  characteristics  of  the  coats  from  which  they  are  derived,  the  retinal 
portion  being  essentially  epithelial  in  character,  the  uveal  portion  composed 
of  loose  connective  tissue,  rich  in  blood-vessels,  its  anterior  surface  formed 
of  flattened  endothelium  like  its  congener  the  posterior  layer  of  the  cornea, 
with  which  it  is  continuous  at  the  angle  of  the  anterior  chamber. 

At  first  the  investment  is  continued  from  the  iris  about  the  lens,  form- 
ing what  is  known  as  its  vascular  tunic,4  evidently  necessary  for  the  nutri- 
tion of  that  organ  while  growing.  The  part  of  this  tunic  then  seen  in 
front  appears  as  a  continuation  of  the  iris  closing  in  the  pupil,  and  is  there- 
fore called  the  pupillary  membrane.6  (Fig.  47,  d:  Fig.  48.)  It  usually 
disappears  during  the  latter  portion  of  intra-uterine  life,  but  vestiges  of  it 
may  remain  even  in  adults  as  fine  threads  passing  across  the  pupil.  Ste- 
phenson 6  found  vestiges  of  this  structure  sixty-eight  times  in  three  thou- 
sand four  hundred  and  fourteen  eyes  examined,  or  nearly  two  per  cent. 
It  was  more  frequent  in  females.  The  vestiges  did  not  interfere  with  sight 
or  with  the  action  of  the  pupil.  They  appear  to  occur  more  frequently  in 
members  of  the  same  family.  They  do  not  arise  from  the  margin  of  the 


1  This  arrangement  of  the  doubling  over  of  the  layers  of  the  iris  appears  to  have  been 
suspected  before  the  microscope  revealed  it.     Jacobus  Sylvius  speaks  of  it  as  reflected  and 
doubled  at  the  pupillary  margin.     "  Choroides  .  .  .  ibi  relicto  foramine  quam  vocamus 
pupillam,  reflexa,  et  quadantenus  duplata.  ..."      In  Hippocratis  et  Galeni  physiologiae 
partem  anatomicam  isagoge,  Paris,  1666,  1.  i.  cap.  iv. 

2  Syn. :  pars  retinalis  iridis ;  pars  iridica  retinae. 

3  Syn. :  pars  uvealis  iridis ;  pars  iridica  uvecp.. 
*  Syn. :  tunica  vasculosa  lentis. 

5  Syn. :  membrana  pupillaris ;  m.  capsulo-pupillaris ;  capsulo-pupillary    membrane; 
membrane  of   Wachendorf.      Wachendorf   first  published  a  description  of   it  in   1740. 
Albinus  is  said  to  have  noticed  it  some  years  previously. 

6  Stephenson  (Sydney).      Concerning  Persistent  Pupillary  Membrane  and  its  Fre- 
quency.    Trans.  Ophth.  Soc.  United  Kingdom,  xiii.,  1892-93,  139. 


Fio.  47. 


FIG.  46. 


Iris  and  sclera  viewed  from  the 
side.    (Helmholtz.) 


Development  of  the  eye.  shown  diagrammatically.— a  6 
outer  and  inner  layers  of  the  optic  cup,  forming  the  retinal 
portion  of  the  iris;  c,  the  uveal  portion  of  the  iris  derived 
from  the  connective-tissue  investment;  d,  a  portion  of  that 
investment  passing  in  front  of  the  lens  and  forming  the 
pupillary  membrane;  e,  portion  of  the  investment  forming 
cornea;  /,  cleft  in  the  investment  forming  anterior  chamber; 
g,  conjunctival  epithelium;  h,  lens;  i,  hyaloid  artery;  k, 
retina;  I,  chorioid  and  sclera. 


Fid.  48 


The  vascular  tunic  of  the  lens.  (Kolliker.)— A,  its  posterior  surface;  B,  its  anterior  surface,  form- 
ing the  pupillary  membrane.  1,  hyaloid  artery  cut  across;  2,  3,  radiating  vessels  directed  towards  the 
equator  of  the  lens ;  4,  the  same  vessels  that  have  passed  the  equator  and  appear  on  the  anterior  sur- 
face ;  5,  venous  trunks  passing  to  the  iris. 


THE   ANATOMY   OF   THE   EYEBALL.  177 

pupil,  but  from  the  anterior  surface  of  the  iris,  and  can  thus  be  distin- 
guished from  the  inflammatory  vegetations  of  iritis. 

The  following  arrangement  of  the  layers  of  the  iris  naturally  follows 
from  what  has  been  said  of  its  development : 

A.  Iris  Proper. 

1.  Anterior  endothelium. 

2.  Anterior  boundary  layer. 

3.  Stroma. 

4.  Basilar  layer. 

B.  Retinal  Iris. 

5.  Anterior  layer  of  epithelium. 

6.  Posterior  layer  of  epithelium. 

Layers  2  and  4  are  merely  specially  modified  portions  of  the  stroina. 

The  proper  relations  of  these  layers  will  become  evident  upon  inspec- 
tions of  Figs.  49  and  50. 

The  anterior  surface  of  the  iris,  being  visible  through  the  transparent 
cornea,  imparts  to  eyes  the  characteristic  color  by  which  they  are  described. 
This  color  is  due  partly  to  the  dark  pigmented  layer  of  the  posterior  sur- 
face showing  through  the  thin  stroma,  partly  to  pigmented  cells  lying  in 
the  stroma  itself.  When  these  cells  are  absent  or  nearly  so,  and  the  iris  is 
thin,  the  dark  background  shows  through  the  semi-opaque  stroma  as  blue,  a 
phenomenon  caused  by  interference,  as  is  the  color  of  the  cloudless  sky  or 
the  appearance  of  veins  through  a  delicate  skin.  When  the  iris  is  thicker 
and  the  opacity  greater  this  becomes  modified  to  gray,  and  when  pigment- 
cells  are  scattered  in  considerable  numbers  through  the  stroma  the  color 
assumes  various  shades  of  green,  yellow,  and  brown,  the  deepest  tints  of 
brown  being  the  so-called  black  eyes. 

On  close  inspection  it  will  be  seen  that  the  color  is  by  no  means  uni- 
formly distributed,  but  appears  in  irregular  flecks  or  spots  alternating  with 
lighter  tints.  On  this  account  Broca  advises  those  who  wish  to  note  with 
accuracy  the  color  of  the  eyes  to  observe  them  at  the  distance  of  one  metre, 
so  that  the  tints  may  blend.  The  color  is  also  distributed  in  two  zones 
concentric  with  the  pupil, — an  inner  or  pupillary  l  one,  from  one  to  two  mil- 
limetres wide,  darker  in  light  eyes  and  lighter  in  dark  eyes,  and  an  outer  or 
ciliary 2  one,  from  three  to  four  millimetres  wide,  darker  in  dark  eyes  and 
lighter  in  light  ones.  The  limit  between  the  two  is  a  zigzag  or  festooned 
series  of  ridges,  the  lesser  circle  3  of  the  iris. 

The  distribution  of  the  pigment  varies  greatly  in  different  individuals, 
so  much  so  that  it  has  been  proposed  to  make  a  systematic  record  of  the 
pattern  of  the  iris  for  the  purpose  of  identifying  criminals.4  The  varia- 

1  Syn. :    annulus  iridis  minor  or  internus  or  pupillaris ;   sphincter  zone ;    internal 
colored  ring. 

2  Syn. :  annulus  iridis  major  or  externus  or  dliaris ;  external  colored  ring. 

3  Syn. :  circulus  minor  iridis. 

4  See  Bertillon  (A.).     La  couleur  de  1'iris.     Rev.  scient.,  Paris,  1885,  xxxvi.  66-73. 

VOL.  I.— 12 


178 


THE  ANATOMY  OF  THE  EYEBALL. 


tions  are  more  numerous  in  the  ciliary  zone,  which  may  be  markedly  stri- 
ated with  radiating  lines  or  concentric  zigzags.  The  pigment-cells  may 
collect  in  spots,  giving  an  appearance  like  a  leopard's  skin.  Walker  sup- 
posed these  to  be  of  a  vascular  character,  resembling  the  congenital  vascular 
tumors  called  nsevi,  and  consequently  named  them  vwevi  iridis.  The  human 
imagination  has  not  neglected  to  exercise  itself  upon  these  flecks  and  mark- 
ings, and  we  consequently  find  that  strange  characters  are  deciphered  in  the 
eye.  Lavater  mentions  an  iris  on  which  an  ace  of  spades  could  be  seen ; 
Borelli  one  in  which  the  words  Loue  soil  Dieu  could  be  read ;  Tenon  saw 
the  letters  T  and  V  in  different  cases ;  in  others  the  names  of  Charles  XII., 
King  of  Sweden,  and  "  Napoleon,  empereur,"  or  mystical  Hebrew  charac- 
ters, have  been  found.  More  practical  in  their  bearing  are  those  cases 
sometimes  reported  in  which  the  deposits  of  pigment  simulate  a  second 

pupil,  or  a  coloboma. 

FIG.  51. 


Diagram  showing  the  prevalence  of  different-colored  eyes  among  European  peoples. 

It  was  noticed  by  Aristotle  that  the  eyes  of  new-born  children  are 
almost  always  blue.  This  is  due  to  the  fact  that  the  pigment-cells  of  the 
stroma  do  not  develop  until  some  time  after  birth,  the  coloration  not  being 
complete  until  after  the  second  year.  In  albinos  not  only  is  the  stroma 
pigment  wanting,  but  also  that  which  lines  the  posterior  surface.  The  iris 
consequently  takes  a  pinkish  color  from  the  numerous  blood-vessels  it  con- 
tains, and  the  eyes  share  with  the  rest  of  the  face  a  deeper  suffusion  of 
color  in  blushing.  This  want  of  pigmentation  is  a  serious  disadvantage,  as 
it  causes  great  sensitiveness  to  a  glare.  Hence  albinos  shun  bright  light, 
and  for  this  reason  the  Germans  call  them  Kakerlakken,  or  cockroaches. 

Considering  the  entire  population  of  the  world,  we  find  that  black  eyes 
are  by  far  the  most  numerous,  these  prevailing  throughout  the  dark  races, 
such  as  African,  Indian,  and  Malay,  and  in  a  considerable  proportion  of  the 
lighter  ones,  especially  among  peoples  inhabiting  tropical  climates. 


FIG.  49. 


-15 


Diagram  of  a  radial  section  of  the  iris.  (Testut.V- 1,  pupillary  border ;  2,  angle  of  the  iris ;  3,  pecti- 
nate ligament;  4,  anterior  endothelium;  5,  anterior  boundary  layer;  6,  stroma;  7,  fascicles  of  the 
sphincter  pupillae  cut  across;  8,  basilar  layer,  in  front  of  which  lie  the  radiating  fibres  of  the  dilatator 
pupillae,  not  here  shown :  9,  pigmented  layer  of  the  retina  continued  forward  as  the  anterior  layer  of 
epithelium  of  the  iris;  10,  cubical  epithelium  of  the  ciliary  portion  of  the  retina  continued  forward 
as  the  posterior  layer  of  epithelium  of  the  iris;  11,  internal  limiting  membrane  continued  forward  upon 
the  iris ;  12,  lamina  basalis  of  the  chorioid ;  13,  zonula ;  14,  zonular  space  or  canal  of  Petit ;  15,  lens. 


FIG.  50. 


Horizontal  section  through  the  ciliary  zone  of  the  iris.    (Gerlach.)-l,  anterior  endothelium ;  2, 
witli  vessels  cut  across;  3,  muscular  layer  containing  the  dilatator  pupillae ;  4,  pigmented  layers. 


THE  ANATOMY  OF  THE  EYEBALL. 


179 


The  color  of  the  eyes  usually  corresponds  with  that  of  the  hair  and 
complexion,  though  not  always,  as  it  occasionally  happens  that  blue  eyes 
may  accompany  a  bronzed  skin,  as  in  some  Afghans  (Fraser),  and  blue  eyes 
and  dark-brown  hair  are  not  a  very  unusual  combination.  It  is  considered 
by  ethnologists  that  a  close  relation  between  eyes  and  complexion  is  more 
persistent  in  the  lower  races,  and  that  want  of  agreement  is  an  indica- 
tion of  mixed  blood.  In  Europeans  the  iris  may  vary  greatly  in  color, 
being  generally  blue  in  Scandinavians,  Dutch,  Belgians,  North  Germans' 
Irish,  and  Norman  French  ;  generally  brown  in  the  Mediterranean  peoples' 
(See  Fig.  51.) 

Fio.  62. 


Diagram  showing  prevalence  of  different-colored  eyes  among  soldiers  native  to  the  United  States.— 
A,  the  six  New  England  States;  B,  New  York,  New  Jersey,  and  Pennsylvania;  C.  Ohio  and  Indiana; 
D,  Michigan,  Wisconsin,  and  Illinois;  E,  slave  States,  not  including  Florida  and  Georgia;  F,  Ken- 
tucky and  Tennessee ;  <?',  free  States  west  of  the  Mississippi ;  G,  slave  States  west  of  the  Mississippi. 

Beddoe l  considers  that  the  inhabitants  of  cities  have  generally  darker 
eyes  than  those  of  the  surrounding  country.  In  a  mixed  population  like 
that  of  the  United  States  no  great  value  can  be  attached  to  statistics  of 
coloration.  It  may,  however,  be  interesting  to  consider  the  proportion 
of  light  and  dark  eyes  in  different  parts  of  the  Northern  States,  as  shown 
in  519,645  native  enlisted  men  during  the  civil  war.2  This  is  shown  in 
Fig.  52. 

1  Beddoe  (John).     On  the  Testimony  of  Local  Phenomena  in  the  West  of  England 
to  the  Permanence  of  Anthropological  Types.    Mem.  Anthro.  Soc.,  Lond.,  1865-66,  ii.  37. 

2  Gould  (B.  A.)     Investigations  in  the  Military  and  Anthropological  Statistics  of  the 
American  Soldier.     New  York,  1869. 


180  THE  ANATOMY  OF  THE  EYEBALL. 

Generally  speaking,  the  two  eyes  of  the  same  individual  are  the  same  in 
color,  but  it  sometimes  happens  that  one  is  blue  or  gray,  while  the  other 
is  dark  (yeux  vairons  of  the  French).  The  color  pales  somewhat  with 
advancing  age,  and  many  change  during  life  as  a  result  of  pathological 
processes.  In  an  inflamed  eye  the  iris  may  change  from  blue  to  yellowish 
green,  and  if  it  becomes  permanently  thickened  it  may  remain  gray.  Being 
very  vascular,  it  is  extremely  prone  to  changes  under  inflammatory  dis- 
turbances :  hence  a  comparison  of  the  two  eyes  often  becomes  of  high  im- 
portance as  a  diagnostic  sign. 

On  viewing  the  anterior  surface  with  a  glass  (see  Fig.  53),  its  apparent 
uniformity  disappears,  and  it  is  seen  to  be  marked  by  striate  elevations 
having  a  somewhat  irregular  character  but  a  general  radial  direction. 
Those  of  the  pupillary  zone  are  fine  and  close,  and  are  called  by  Zinn  the 
radii  minores.  They  are  so  arranged  that  contiguous  ones  unite  at  very 
acute  angles,  leaving  between  them  deep  spaces,  at  the  bottom  of  which  the 
fibres  of  the  sphincter  muscle,  s,  may  be  seen  running  concentrically  about 
the  pupil. 

The  lesser  circle  (6)  is  composed  of  plexiform  elevations  or  trabeculae 
that  anastomose  with  each  other  in  a  wreath-like  manner,  forming  a  com- 
plete circumference.  These  are  mainly  the  vestiges  of  the  pupillary  mem- 
brane, which  in  the  foetus  was  attached  here.  When  this  membrane  became 
obliterated  its  vessels  shrank  away  to  cords  of  connective  tissue.  From 
this  circle  the  fine  radii  of  the  pupillary  zone  pass  towards  the  pupil  and 
spurs  extend  outward  into  the  ciliary  zone.  The  prominence  and  shape  of 
the  trabeculse  vary  with  the  degree  of  tension  of  the  iris.  (See  Fig.  54.) 
Between  them,  but  more  frequent  on  the  ciliary  side,  there  are  found  oval 
or  rhombic  depressions  called  the  crypts  of  the  iris.  These  penetrate  into 
the  stroma,  and  are  found  to  be  the  openings  of  lymph-spaces  by  which 
the  aqueous  humor  can  be  removed  from  'the  anterior  chamber.  (Fuchs.) 
The  trabeculse  may  bridge  them  over  or  send  out  processes  which  pass  into 
them  and  are  lost  on  the  floor. 

The  ciliary  zone  is  divided  by  Fuchs  into  three  concentric  regions, — the 
first  (c)  smooth,  not  folded  during  dilatation  of  the  pupil ;  the  second  (d) 
furrowed  by  arciform  ridges  that  increase  in  depth  during  dilatation ;  the 
third  (e)  cribriform,  showing  depressions  similar  to  the  crypts.  The  first 
and  second  regions  are  about  equal  in  width,  and  easily  inspected  in  the 
living  eye ;  the  third  is  narrow  and  concealed  in  the  living  subject  under 
the  edge  of  the  scleral  bevel.  THe  visible  portion  is  marked  by  radial 
striae  corresponding  to  the  vessels  and  nerves  of  the  iris,  the  radii  majores 
of  Zinn.  Between  these  appear  dark  spaces  often  filled  with  pigment. 
The  furrows  are  one  to  seven  in  number,  and  are  easily  visible,  describing 
incomplete  arcs  of  circles  concentric  with  the  pupil. 

The  posterior  surface  of  the  iris  is,  except  in  albinos,  deeply  and  uni- 
formly colored,  the  pigmentary  layer  of  the  retina  being  not  only  continued 
upon  it,  but  increased  in  thickness.  As  already  stated,  it  rests  by  its  lower 


FIG.  53. 


Segment  of  the  anterior  surface  of  the  iris 
X  20 ;  pupil  contracted.  (Fuchs.)— a,  pupillary 
zone;  b,  circulus  minor;  c,  smooth,  and  d, 
folded  part  of  the  ciliary  zone;  e,  marginal 
part  of  the  same;  /,  sphincter  muscle. 


The  same  surface  with  pupil  somewhat  dilated. 
(Fuchs.)— Letters  as  in  last  figure. 


THE  ANATOMY  OF  THE  EYEBALL. 

edge  upon  the  surface  of  the  lens,  a  contact  which  explains  the  importance 
of  expanding  the  pupil  in  iritis,  as  otherwise  inflammatory  exudations  are 
certain  to  glue  the  membrane  to  the  lens,  causing  posterior  adhesions  or 
synechise.  The  extent  of  the  contact  was  formerly  a  matter  of  dispute 
some  observers l  holding  that  the  entire  posterior  surface  of  the  iris  is  ap- 
plied to  the  lens  and  ciliary  bodies,  and  that  the  posterior  chamber  exists 
only  as  a  slit  between  two  surfaces  of  contact ;  others,  that  there  is  always 
a  cavity  of  some  dimensions  between  the  iris  and  the  lens.  The  latter 
opinion  is  now  universally  accepted. 

Heister 2  seems  to  have  been  the  first  to  recognize  the  posterior  chamber. 
The  following  evidence  as  to  its  existence  seems  conclusive.  1.  In  a  fresh 
eye  solidly  frozen  shortly  after  death,  a  thin  ring  of  ice,  the  frozen  aqueous 
humor,  is  found  between  the  iris  and  the  lens.  This  was  carefully  investi- 
gated by  Petit  in  1723.3  Those  who  deny  the  existence  of  the  chamber  cite 
cases  in  which  the  ring  was  not  found  (possibly  from  evaporation  of  the 
aqueous  humor  or  from  want  of  care  in  the  manipulation),  and  also  hold 
that  the  chamber  is  formed  post  mortem  as  a  result  of  the  emptying  of  blood 
from  the  ciliary  processes.  2.  On  suddenly  jarring  the  head  the  upper  part 
of  the  iris  can  be  seen  to  tremble,  which  it  would  not  do  were  it  completely 
supported  behind.  3.  If  a  minute  opening  be  made  at  the  edge  of  the 
cornea  and  the  aqueous  humor  partly  evacuated,  the  iris  will  swell  out 
towards  the  opening  because  of  the  pressure  of  the  aqueous  humor  behind 
it.  4.  A  needle  can  be  thrust  with  ease  and  certainty  between  the  ciliary 
border  of  the  iris  and  the  lens.  5.  Inflammatory  adhesions  occur  almost 
invariably  at  the  pupillary  edge  of  the  iris.  • 

The  modelling  of  the  posterior  surface  differs  somewhat  from  that  of 
the  anterior.  (See  Fig.  55.)  A  series  of  narrow  radiating  furrows  separates 
flat  ridges  (the  structural  folds  of  Schwalbe)  formerly  thought  to  be  con- 
tinuous with  the  folds  of  the  ciliary  processes,  but  which  are  usually  greater 
in  number  than  those.  These  ridges  are  cut  by  a  system  of  narrow  con- 
centric lines  so  that  the  entire  surface  is  divided  into  small  rectangular 
spaces,  the  arrangement  resembling  the  serial  succession  of  the  kernels  of 
an  ear  of  corn  as  set  upon  the  cob.  (Fuchs.)  Within  the  pupillary  zone 
the  number  of  radial  furrows  is  increased  (contraction  folds  of  Schwalbe) 
and  the  concentric  ones  are  nearly  obliterated,  so  that  the  arrangement 
resembles  a  plaited  ruffle.  All  furrows  do  not  reach  the  pupillary  margin, 
as  several  of  the  radial  plaits  may  unite  there,  forming  the  bead-like  heads 
already  mentioned. 

1  Among  others  the  following  may  be  cited  : 

Winslow  (J.  B.).     Mem.  de  1'acad.  royale  des  sciences,  1721. 

Stellwag  von  Carion.     Zeitsch.  d.  Gesell.  d.  Aerzte  z.  Wien,  1850,  vi.  133. 

Cramer.     Het  Accommodatievermogen  der  Oogen.     Haarlem,  1853,  p.  67. 

Helmholtz  (H.).     Physiologische  Optik.     Leipzig,  1867,  p.  19. 

Sappey  (P.  C.).     Anutomie  descriptive.     Paris,  1877,  iii.  820. 

3  Heifter  (L.).     Compendium  anatomicum,  1819. 

s  Mem.  de  1'acad.  royale  des  sciences,  1723. 


182  THE  ANATOMY  OP  THE  EYEBALL. 

The  pupil,1  which  Berger 2  picturesquely  cafts  janitrix  oculi,  is  normally 
round3  in  man,  whatever  may  be  its  state  of  contraction  or  dilatation. 
This  is,  however,  by  no  means  the  case  with  all  animals.  In  many  rep- 
tiles, fishes,  and  amphibians,  and  in  some  birds,  it  contracts  to  a  vertical 
slit.  In  mammals  it  is  not  invariably  round,  being  in  ungulates  (horse, 
ox)  transversely  oval  contracting  to  a  horizontal  slit ;  in  many  of  the  felidse, 
or  cat  tribe,  contracting  to  a  vertical  slit  and  showing  during  expansion 
various  elliptic  or  lozenge-shaped  forms. 

As  excess  of  light  injures  the  retina,  and  too  divergent  rays  impair  the 
definition  of  images  thereon  projected,  the  pupil  contracts  and  expands 
during  life  according  to  the  necessities  of  vision.4  This  is  a  reflex  phe- 
nomenon dependent  on  the  action  of  light  upon  the  retina,  and  not  caused, 
as  was  formerly  thought,  by  rays  falling  upon  the  iris.8  If  a  bright  light 
passing  through  an  aperture  smaller  than  the  pupil  reaches  the  otherwise 
shaded  eye,  no  effect  is  produced  unless  it  penetrates  to  the  retina,  and  when 
that  occurs  the  pupil  at  once  contracts.  In  man,  stimulation  of  a  single 
eye  affects  both  pupils  simultaneously,  and  any  difference  in  their  size  may 
at  once  be  considered  as  of  pathological  origin. 

Long  before  Marshall  Hall  established  our  knowledge  of  reflex  move- 
ments, similar  theories  were  applied  to  these  phenomena.  Thus  Morgagni 6 
supposed  that  the  retina,  excited  by  light,  conveyed  its  vibrations  to  the 

1  From  L.  pupilla,  a  little  girl  or  doll,  probably  because  of  the  diminutive  image  of 
the  observer  that  is  reflected  from  the  cornea  on  looking  at  the  black  background  of  the 
pupil.     Kindlein  for  pupil  is  found  in  old  German  works,  and  the  Greek  ndpq  is  used  for 
both  girl  and  pupil.     Isidorus  (A.D.  636)  says,  "Vocatur  autem  pupilla  quod  sit  pura 
atque  impolluta  ut  sint  puellae." 

Syn. :  from  its  appearance,  nigrum  oculi ;  the  apple  of  the  eye  (used  as  a  comparison 
for  something  to  be  carefully  kept  from  injury,  Deut.  xxxii.  10,  Ps.  xvii.  8,  Prov.  vii.  2, 
Zech.  ii.  8).  From  its  necessity  for  vision,  the  sight  of  the  eye  ;  fi  bfyq  (Kufus  Ephesius) ; 
visio,  fenestra,  or  lumen  oculi ;  Sehe  or  Seheloch,  G. 

2  Bergerus  (J.  G.).     Physiologica  medica.     Vitembergse,  1701. 

3  Oval  pupils  are  occasionally  seen  in  man.     Cases  are  cited  as  follows : 
Plempius  (V.  P.).     Ophthalmographia,  1.  iii.  cap.  viii. 

Tode  (J.  C.).     Soc.  med.  Havn.  Collect.,  1775,  ii.  145. 
Hagstrom.    Abh.  d.  schw.  Acad.,  xxxvi. 
Ephem.  nat.  cur.,  viii.  34. 
Archiv  f.  die  Physiologic.     Halle,  v.  63. 
Richter's  Chir.  Bibl.,  ii.  58;  iv.  230;  vii.  104. 

Foucher  (Rev.  med.-chir.  de  Paris,  1852,  xii.  207)  found  thirty-four  cases  out  of  one 
hundred  and  fifty-four  in  which  the  pupil  was  more  or  less  elliptical. 

4  This  movement  was   apparently  known  to  Galen.     (De  usu  partium,  lib.  x.  cap. 
v.)     The  Arabian  physician  Rhazes  (A.D.  852-932)  was  first  to  note  its  connection  with 
the  intensity  of  light.     He  says,  "In  [uveae]  medio  in  loco  scilicet  ubi  grandineo  [i.e., 
crystalline]    opponitur   humori,  est   foramen   quod  quandoque   dilatur,  quandoque   con- 
stringitur  prout  grandineo  humori  causa  luminis  necessarium  fuit.  .  .  .  Hoc  foramen  est 
pupilla."     Ad  Almansor,  lib.  i.  cap.  viii. 

6  See,  for  experimental  proof  of  this,  Zinn  (J.  G.),  De  motu  uveae,  1757;  Mviller 
(J.  R.),  De  irritabilitate  iris  hincque  pendente  motu  pupillse,  Basil.,  1760,  p.  9  et  seq. ; 
Fontana  (Felice),  Dei  moti  dell'  iride,  1765. 

6  Epistola  anatomica,  xvii.  48. 


FIG.  55. 


Segment  of  the  posterior  surface  of  the  iris  X  25 ;  medium  dilation  of  the  pupil.    (Fuchs.)— a,  pupillary 
zone ;  6,  ciliary  zone ;  c,  ciliary  processes. 


FIG.  56. 


Reproduction  of  photomicrograph  showing  a  portion  of  a  radial  section  of  the  iris  after  bleaching 
with  euchlorine.  (Juler.)— a,  posterior  epithelium ;  b,  layer  of  muscle-fibres  constituting  the  dilatator 
pupillae ;  c,  stroma.  The  oval  cells  with  normal  nuclei  between  a  and  6  probably  represent  the  anterior 
epithelial  layer. 


THE  ANATOMY  OF  THE  EYEBALL.  183 

iris,  and  Blumenbach  1  actually  anticipated  the  modern  theory  of  nervous 
action  by  supposing  the  vibrations  of  light  to  be  conveyed  to  a  sensorium 
commune,  which  in  turn  affected  the  iris. 

From  an  early  period  anatomists  have  attempted  to  determine  the 
essential  structures  upon  which  these  movements  depend.  The  investiga- 
tion is  one  of  unusual  difficulty,  owing  to  the  fact  that  they  are  covered 
over  and  greatly  obscured  by  the  pigmeuted  cells  of  the  iris,  which  cannot 
be  removed  without  damaging  to  some  extent  the  subjacent  tissues.  It  is 
not  surprising,  therefore,  that  a  number  of  conflicting  views  should  have 
been  held  regarding  these  structures,  and  that  a  long  and  sometimes  acri- 
monious controversy  concerning  them  should  have  lasted  up  to  the  present 
time. 

The  earliest  view  was  that  the  iris  enlarged  and  contracted  by  the  filling 
of  its  fibres  (vessels,  nerves,  etc.)  with  some  fluid,  either  the  pneuma* — 
animal  or  vital  spirits — of  the  old  physiologists,  or  blood.  Fabricius 
ab  Aquapendente  considered  its  properties  those  of  erectile  tissue.3  Me"ry 4 
had  a  similar  view,  holding  that  the  active  state  was  during  contraction 
of  the  pupil,  dilatation  being  caused  by  the  elasticity  of  the  posterior  mem- 
brane, thus  explaining  the  dilatation  after  death.  Vieussens5  described  a 
"  vasculo-lymphatic-nervous  sphincter"  at  the  pupillary  margin,  and  Fer- 
rein,6  Haller,7  Zinn8  in  part,  Foutana,9  and  Sommering,10  also  maintained 
the  erection  theory.  Even  during  the  present  century  many  u  have  ascribed 
the  movements  either  wholly  or  in  part  to  an  influx  and  efflux  of  blood. 
The  question  appears  to  have  been  finally  settled  by  the  experiments  of 
Brown-Se"quard,12  who  showed  that  the  pupillary  orifice  is  but  slightly  nar- 
rowed by  injecting  the  blood-vessels  of  the  iris. 

1  De  oculis  leucsethiopium  et  iridis  motu.     Getting®,  1786. 

2  Galen,  loc.  cit. 

3  Tract,  anat.  de  oculo,  aure  et  larynge,  1613,  p.  58.     After  comparing  the  iris  to  the 
muscle  of  the  heart,  which  he  considers  has  a  special  faculty  of  its  own,  he  continues, 
"  Melius  autem  forte  fuerit  virilis  pudendi  motui  uveae  foraminis  motum  assimilare;  ita  ut 
sicuti  penis  per  insitam  quandam  facultatem  erigitur." 

4  Mem.  de  1'acad.  royale  des  sciences.     Amsterdam,  1704,  p.  353. 

6  Traite  nouveau  des  liqueurs  du  corps  humain.     Toulouse,  1715,  p.  211. 

6  Mem.  de  1'acad.  royale  des  sciences,  1741,  p.  495. 

7  Primae  lineae  physiologic.     Gottingse,  1761.     Elementa  physiologis,  1763,  xvi.,  ii.  p. 
371. 

8  Descriptio  anatomica  oculi  humani.     Gottingse,  1756. 

9  Dei  moti  dell'  iride.     Lucca,  1765. 

10  In  his  commentary  on  Haller's  Primae  lineae,  Berlin,  1788,  p.  391. 

11  Portal  (A.).     Cours  d'anatomie  medicale.     Paris,  1804. 

Gaddi  (P.).  Argomenti  dimostrativi  della  fondamentale  struttura  vascolare  dell'  iride. 
Eaccoglitore,  Fano,  1845,  xvi.  258-266. 

Guarini  (L.).  L 'iride  se  muove  per  simplice  erettismo  vascolare,  oppure  per  opera  de 
fibre  muscolari  ?  Ann.  univ.  de  med.,  Milano,  1844,  cxii.  21-48. 

Letheby.  On  the  Structure  and  Movements  of  the  Iris.  Royal  London  Ophth.  Hosp. 
Kep.,  London,  1859,  ii.  18-20. 

13  Compt.-rend.  Soc.  de  biol.,  1849.     Paris,  1850,  i.  116-118. 


184  THE  ANATOMY  OF  THE  EYEBALL. 

Other  and  more  chimerical  views  with  regard  to  the  cause  of  the  move- 
ments have  been  occasionally  advanced.  Weitbrecht l  believed  them  to  be 
due  to  the  expansion  and  contraction  of  the  vitreous  body.  Delia  Torre  2 
ascribed  them  to  the  contractile  power  of  the  very  numerous  nerves  with 
which  the  iris  is  supplied. 

Blumenbach s  held  that  there  was  some  special  vital  property  in  the 
tissue  of  the  iris  that  endowed  it  with  contractility,  but  that  it  was  not 
muscular.  Essentially  similar  views  were  maintained  by  other  anatomists.4 

Since,  however,  the  movements  of  the  iris  are  like  those  produced  by 
muscle-fibres,  it  was  early  thought  by  some  to  be  a  muscular  organ.  Thus 
we  find  that  Avicenna,5  the  chief  of  the  Arabian  school  (A.D.  980-1036), 
calls  it  "  lacerius  motus  pupttlse"  and  Descartes 6  held  a  similar  view.  This 
was  based,  however,  on  theoretical  grounds,  as  no  means  then  existed  for 
demonstrating  muscular  fibres. 

The  arrangement  and  direction  of  the  fibres  were  naturally  matters  of 
discussion.  The  radial  folds  formed  by  the  vessels  resembled  to  the  naked 
eye  muscle-bundles,  and  we  accordingly  find  that  almost  without  exception 
those  who  upheld  the  muscular  nature  of  the  organ  believed  it  to  be 
arranged  radially.  Vesling 7  held  that  fibres  were  prolonged  into  the  iris 
from  the  ciliary  processes.  Kiolan 8  and  DrSlincourt 9  seem  to  have  had  a 
clear  idea  of  such  radial  fibres  without  suspecting  any  orbicular  ones,  and 
Valsalva,10  O'Halloran,11  and  Zinn12  all  expressly  deny  orbicular  fibres, 
while  admitting  radial  ones. 

1  Comm.  Petrop.,  xiii.  349. 

2  Nuove  osservat.  microscopiche,  p.  68. 
8  Op.  cit. 

4  Domling.     Arch.  f.  d.  Physiol.     Halle,  1802,  v.  335. 

Bichat  (M.  F.  X.).     Anatomic  descriptive.     Paris,  1801-03,  ii.  444. 
Grapengieser.     Asklepeion,  1811,  p.  1314. 
Weber  (E.  H.).     De  motu  iridis.     Lipsise,  1821. 
Rudolphi  (K.  A.).     Physiologic.     Berlin,  1821-28,  ii.  218. 

Arnold  (Fr.).  Anatomische  und  physiologische  Untersuchungen  uber  das  Auge  des 
Menschen.  Heidelberg  and  Leipzig,  1832,  p.  74. 

5  Canon.     Tr.  3,  Fen.  1,  c.  i. 

6  "  Ce  trou  [la  prunelle]  n'est  pas  tousiours  de  mesme  grandeur,  car  la  partie  de  la  peau 
dans  laquelle  il  est,  nageant  librement  dans  I'humeur,  qui  est  fort  liquide,  semble  estre 
comme  un  petit  muscle,  qui  s'elargit  ou  s'etre"cit  par  la  direction  du  cerveau,  selon  que  Fusage 
le  requiert."     L'Homme,  Paris,  1664,  p.  39. 

I  Syntagma  anatomica.     Patavii  [Padua],  1659,  p.  202. 

8  Anthropographia  et  osteologia.     Parisii,  1626,  pp.  416-429. 

9  Opera  varia,  1693. 

10  Opera.     Venetiis,  1741. 

II  A  New  Treatise  on  the  Glaucoma,  or  Cataract.     Dublin,  1750,  p.  74. 

12  Descriptio  anatomica  oculi  humani,  1755,  p.  89.  He  also  states  that  he  bases  his 
conclusions  rather  on  the  known  properties  of  vascular  tissue  than  on  actual  demonstra- 
tions. "  Dum  autem  phenomena,  vascula,  molem  nervorum  iridem  adeuntium,  ejusque 
analogiam  cum  aliis  partibus  corporis  humani  musculosis  attentius  considero,  parum 
abest,  quin  ad  credendum  adducar,  fibras  rnusculosas  reliquis  vasculis  et  nervulis  in  ante- 
riore  facie  iridis  intermistas  esse." 


THE  ANATOMY  OF  THE  EYEBALL.  Ig5 

Berger l  appears  to  have  been  the  first  to  recognize  clearly  the  exist- 
ence of  fibres  encircling  the  pupil.  Ruysch  2  mentioned  and  figured  them 
shortly  after,  with  some  doubt,  and  they  were  soon  recognized  by  a  large 
number  of  anatomists,3  most  of  whom  seem  to  have  been  led  rather  by  the 
known  functions  of  the  organ  than  by  any  special  anatomical  appearances 
noted.  All  these  authors  admitted  the  existence  of  the  radial  or  dilatator 
fibres  without  question.  Demours,4  however,  declared  that  the  radial  fibres 
were  not  muscular. 

The  first  careful  microscopical  demonstration  of  the  sphincter  was  made 
by  Maunoir5  in  birds.  Treviranus6  afterwards  showed  its  fibres  to  be 
striated  in  those  animals.  Valentin7  found  them  smooth  in  mammals,  and 
Krohn,8  Lauth,9  and  Schwanh 10  confirmed  this  for  man.  Since  these  inves- 

1  "  Duplex  quoque  idem  est  circulus  major,  alter,  qui  desinit,  ubi  librae  processus  ciliaris 
terminantur,  et  reflectuntur  arteriolae,  alter  multo  minor,  qui  ad  pupillam  desinit,  et  ex 
fibrillis  circularibus,  in  orbemque  scite  flexis,  et  annuli  in  modo  circumductis,  constructus 
videtur. "  Physiologia  medica.  Vitembergae,  1701,  p.  405,  Amstelodami,  1702. 

1  Thesaurus  anatomicus,  ii.      Amstelodami,  1702,  pi.  i.  fol.  5. 

8  See,  among  others  : 

Maitrejan  (Antoine).     Traite  des  maladies  de  Poeil,  1707. 

Morgagni  (J.  P.).     Adversaria  anatomica  omnia,  1719,  i.  337,  vi.  88. 

The  same.     Epistola  xvi.,  No.  9. 

Heister  (Laurenz).  Compendium  anatomicum,  1727,  p.  215.  He  describes  these 
fibres  as  the  sphincter  jmpiltce. 

Palfyn  (J.).     Anatomic  chirurgicale..     Paris,  1734. 

Lobe  (J.  P.).     De  oculo  humano.     Lugd.  Bat.,  1742,  p.  22. 

Boerhaave  (H.).     Praelectiones  academics,  1742-45. 

Petsche  (J.  Z.).     In  Haller's  Disputationes  anatomicae.     Gottingae,  1751,  vi.  768. 

Mauchart  (B.  D.).  De  mydriasis.  In  Haller's  Disputationes  chirurgicales,  1755,  i.  p. 
558.  Has  an  excellent  description  of  the  preparation  of  the  iris  by  brushing  off  the  posterior 
pigmented  layer,  and  compares  the  muscle-fibres  to  those  of  blood-vessels. 

Winslow  (J.  B.).     Exposition  anatomique  de  la  structure  du  corps  humain,  1757. 

Porterfield  (William).     A  Treatise  on  the  Eye.     Edinburgh,  1759,  i.  153. 

Gataker  (Thomas).     An  Account  of  the  Structure  of  the  Eye.     London,  1761,  p.  52. 

Deverney  (J.  G.).     (Euvres  anatomiques.     Paris,  1761,  i.  146. 

AVhytt  (Robert).  Essay  on  the  Vital  and  other  Involuntary  Muscles  of  Animals. 
Edinburgh,  1763.  He  appears  to  have  been  the  first  to  name  the  radial  fibres  the  laxator 
or  dilatator  pupillse. 

De  St.  Yves  (Charles).     Nouveau  traite  des  maladies  des  yeux,  1767,  p.  12. 

Janin  de  Combe  Blanche  (Jean).  Hemoires  et  observations  anatomiques,  physi- 
ologiques  et  physiques  sur  1'ceil.  Lyon,  1772,  p.  8. 

Cheselden  (William).     Anatomy,  1792. 

Monro  (Alexander).     On  the  Brain,  Eye,  and  Ear,  1797,  p.  112. 

*  Dissertation  sur  le  mecanisme  des  mouvements  de  la  prunelle  ou  Ton  examine  quelle 
est  la  structure  et  la  maniere  d'agir  des  fibres  droites  de  1'uvee.  Recueil  des  pieces  par  des 
savants  etrangers,  publie  par  1'Academie  des  Sciences. 

5  Memoire  sur  1'organisation  de  1'iris.     Paris,  1812. 

6  Vermischte  Schriften,  1820,  iii.  167. 

7  Repertorium  f.  Anat.  u.  Physiol.,  1837,  p.  248. 

8  Ueber  die  Structur  der  Iris  der  Vogel  und  ihren  Bewegungsmechanismus.     Arch.  f. 
Anat.,  Physiol.  u.  wissensch.  Med.,  Berlin,  1837,  p.  379. 

9  Institut.,  Nos.  57,  70,  73. 

10  In  Job.  Miiller's  Handb.  d.  Physiologic,  1840,  ii.  36. 


186  THE   ANATOMY   OF   THE   EYEBALL. 

tigations  the  existence  of  the  sphincter  pupillae l  has  never  been  seriously 
questioned.  It  consists  of  a  well-marked,  flat  ring  of  plain,  muscular 
fibres  about  one  millimetre  in  width,2  lying  next  the  pupillary  margin. 
It  is  from  0.07  to  0.1  millimetre  thick,  and  lies  behind  the  vessels  against 
the  basilar  layer.  At  its  outer  edge  it  is  looser  in  texture,  certain  fibres 
arching  away  from  the  ring  and  assuming  a  radial  direction. 

With  regard  to  the  existence  of  true,  radiating  muscle-fibres  constituting 
a  veritable  dilatator  pupillae  3  much  controversy  has  arisen.  The  early  ob- 
servers seem  to  have  based  their  conclusions  upon  theoretical  grounds 
rather  than  upon  actual  observation  of  the  structure  in  man.  After  its 
description  by  Briicke,4  Kolliker,5  and  Budge,6  its  presence  was  generally 
conceded,  although  these  authors  did  not  fully  agree  as  to  the  details  of  its 
situation,  origin,  and  insertion.  In  1864,  however,  its  existence  was  again 
denied  by  Griinhagen,  who  has  up  to  the  present  time  so  persistently 
opposed  it  that  his  views  have  been  widely  accepted.7  On  the  one  side  are 
found  Henle,8  Kolliker,9  Luschka,10  Merkel,11  Iwanoff,12  Faber,13  Sappey,14 
Dogiel,15  Gerlach,16  Retzius,17  Schafer,18  Bohm,  and  v.  Davidoff,19  all  *of 

1  Syn. :  contractor  pupillce ;  sphincter  iridis  ;  musculus  circularis  iridis. 

2  This  is  the  dimension  given  by  Briicke,  Budge,  Henle,  Luschka,  Stohr,  and  others. 
Kolliker  gives  it  as  0.56  millimetre;  Faber  as  0.8  millimetre;  Merkel  as  from  0.8  to  1 
millimetre. 

3  Syn.  :  laxator  pupillce ;  dilatator  iridis  ;  musculus  radialis  iridis. 

4  Anatomische  Beschreibung  des  menschlichen  Augapfels.     Berlin,  1847,  p.  18. 
6  Mikroskopische  Anatomie.     Leipzig,  1854,  ii. 

6  Ueber  die  Bewegung  der  Iris.     Braunschweig,  1855. 

7  Ueber  Irisbewegung.     Arch.  f.  path.  Anat.,  etc.,  Berlin,  1864,  xxx.  481-524. 
Ueber  das  Vorkommen  eines  Dilatator  pupillse  in  der  Iris  des  Menschen  und  der 

Saugethiere.     Zeitschr.  f.  rat.  Med..  Leipzig  u.  Heidelb.,  1866,  3  K.  xxviii.  176-189. 

Zur  Iris-Bewegung.    Arch.  f.  d.  ges.  Physiol.,  Bonn,  1870,  iii.  440-448. 

Zur  Frage  uber  die  Iris-Musculatur.    Arch.  f.  mikr.  Anat.,  Bonn,  1873,  ix.  286-292. 

Ueber  die  hintere  Begrenzungschichte  der  menschlichen  Iris.     Ibid.,  726-729. 

Ueber  die  Muskulatur  und  Bruchsche  Membran  der  Iris.  Anatomischer  Anzeiger, 
1888,  iii.  27. 

8  Handbuch  der  systematischen  Anatomie  des  Menschen.    Braunschweig,  1873,  ii.  654. 

9  Handbuch  der  Gewebelehre  des  Menschen,  5te  Aufl..  1867,  p.  667. 

10  Anatomie  des  Menschen.     Tubingen,  1867,  iii.,  Abth.  2,  p.  416. 

11  Die  Muskulatur  der  menschlichen  Iris. 

12  Grafe  u.  Samisch's  Handbuch.     Leipzig,  1874,  i.  283-287. 

13  Der  Bau  der  Iris  des  Menschen  und  der  Wirbelthiere.     Leipzig,  1876. 

14  Anatomie  descriptive.     Paris,  1877,  iii.  785. 

16  Ueber  den  Musculus  dilatator  pupillae  bei  Saugethieren  und  Vogeln.    Centralb.  f.  d. 
med.  Wissensch.,  Berlin,  1869,  vii.  337-340 

Ueber  den  Musculus  dilatator  pupillae  bei  Saugethieren,  Menschen  und  "Vogeln. 
Arch.  f.  mikr.  Anat.,  Bonn,  1870,  vi.  89-99. 

Neue  Untersuchungen  uber  den  pupillerweiternden  Muskel  der  Saugethiere  und 
Vogel.  Arch.  f.  mikr.  Anat.,  Bonn,  1886,  xxvii.  403. 

18  Handbuch  der  speciellen  Anatomie  des  Menschen.  Miinchen  und  Leipzig,  1891,  p. 
199. 

17  Zur  Kenntniss  vom  Bau  der  Iris.     Biologische  Untersuchungen,  N.  F.,  1893,  vii. 

18  Quain's  Elements  of  Anatomy.     London,  1894,  vol.  iii.,  part  iii.,  pp.  32,  33. 

19  Histologie  des  Menschen.     Wiesbaden,  1895,  p.  336. 


THE  ANATOMY  OF  THE  EYEBALL.  187 

whom,  after  careful  examination,  affirm  the  existence  of  the  dilatator- 
on  the  other,  Griinhagen,  Fuchs,1  Boe,2  Koganei,3  Retterer,*  Schwalbe,* 
Debierre,6  and  Testut,7  who  deny  it. 

The  matter  under  contention  is  the  interpretation  of  the  appearances 
in  the  posterior  portion  of  the  stroma  of  the  iris.  Here  are  seen  a  large 
number  of  nuclei  resembling  those  of  unstriped  muscular  fibre  lying  in  a 
tissue  that  appears  to  be  radially  striated.  Those  who  deny  the  existence 
of  the  dilatator  hold  that  these  nuclei  belong  to  the  epithelial  layer,  and  that 
no  proper  muscular  fibre  exists ;  while,  on  the  other  hand,  many  observers 
hold  that  fibres  are  demonstrable,  and  even  that  they  can  be  isolated. 

One  of  the  most  recent  and  apparently  conclusive  proofs  of  the  exist- 
ence of  the  muscle  was  made  by  Juler  at  the  Ophthalmological  Congress  at 
Edinburgh  in  1894.8  He  exhibited  specimens  of  the  iris  in  which  the  pig- 
mented  epithelium  of  the  posterior  surface  had  been  bleached  by  euchloriue, 
so  that  the  subjacent  structures  were  plainly  visible.  In  front  of  the  epi- 
thelial layer  was  seen  a  continuous  layer  of  muscular  fibres  two  or  three 
deep.  The  fibres  were  fusiform,  with  rod-shaped  nuclei,  that  showed  no 
local  bulging  such  as  is  often  seen  in  connective-tissue  fibres.  (See  Fig.  56.) 

Juler  considers  them  absolutely  identical  with  unstriped  muscular 
fibres  found  elsewhere.  They  appear  to  run  from  the  pectinate  ligament 
to  the  sphincter,  with  which  they  blend. 

Besides  the  anatomical  proofs  of  the  existence  of  two  orders  of  muscu- 
lar fibres  in  the  iris,  there  are  others,  seemingly  conclusive,  that  depend  upon 
physiological  experimentation.  There  is  apparently  an  antagonism  existing 
between  the  nerve-supply  of  the  sphincter  and  that  which  presides  over  the 
dilatation  of  the  pupil.  Herbert  Mayo9  showed  that  the  sphincter  is  sup- 
plied by  the  oculo-motor  nerve.  Section  of  the  nerve  paralyzes  the  muscle 
and  increases  the  size  of  the  pupil,  while  stimulation  of  it  causes  contraction 
of  the  pupil.  On  the  other  hand,  Petit 10  discovered  that  section  of  the  sym- 
pathetic in  the  neck  apparently  paralyzes  the  apparatus  for  dilatation,  as  it 
somewhat  diminishes  the  size  of  the  pupil,  and  Biffi  u  showed  that  stimula- 

I  Beitrage  zur  normalen  Anatomie  der  menschlichen  Iris.    Arch.  f.  Ophthalmologie, 
Berlin,  1885,  xxxi.,  Abth.  iii.  pp.  39-86. 

3  Quelques  recherches  sur  la  couche  pigmentaire  de  Piris  et  sur  le  soi-disant  muscle 
dilatateur  de  la  pupille.     Arch,  d'ophtalmologie,  Paris,  1885,  v.  311. 
3  Arch.  f.  mikr.  Anat.,  Bonn,  1885. 
*  Bull.  Soc.  de  Biologic,  1885. 

5  Anatomie  der  Sinnesorgane.    Erlangen,  1887,  205-209. 

6  Sur  le  muscle  de  Piris  de  Phomme.     Comptes-rendus  de  la  Soc.  de  Biologic,  Paris, 
1888,  8  ser.,  v.  361. 

7  Anatomie  humaine.     Paris,  1894,  iii.  149,  150. 

8  A  Contribution  to  the  Anatomy  and  Physiology  of  the  Iris.    Trans.  Eighth  Internal. 
Ophthalm.  Congress,  Edinburgh,  1894,  p.  67. 

9  Jour,  de  physiol.  exper.,  Paris,  1823,  iii.  349. 

10  Mem.  de  1'acad.  royale  des  sciences,  1727,  p.  1. 

II  Biffi  (Serafino).     Intorno  all'  influenza  che  hanno  sull'  occhio  i  due  nervi  grande 
simpatico  e  vago.     Dissert,  inaug.,  Par.,  1846. 


188  THE  ANATOMY  OF  THE  EYEBALL. 

tfon  of  the  same  nerve  dilates  the  pupil.  It  is,  therefore,  natural  to  sup- 
pose that  this  nerve  supplies  fibres  having  a  dilating  function. 

%In  1852,  Claude  Bernard l  discovered  that  the  cervical  sympathetic  con- 
tained vaso-constrictor  fibres,  and  the  opponents  of  the  dilatator  then  held 
that  it  was  the  action  of  these  fibres  upon  the  vessels  of  the  iris,  causing 
either  a  decrease  of  turgescence  or  a  contraction  of  radial  arteries,  that 
effected  the  dilatation  of  the  pupil.  To  this  it  may  be  replied  that  dila- 
tation of  the  pupil  may  be  caused  by  excitation  of  the  cervical  sympathetic 
after  an  animal  has  bled  to  death,  that  contraction  of  the  blood-vessels  in 
other  parts  of  the  head  is  not  synchronous  with  the  dilatation  of  the  pupil, 
and  that  atropine  has  been  seen  to  dilate  the  pupil  in  white  rats  without 
causing  any  change  in  the  blood-vessels.2  Further,  a  direct  examination 
of  the  iris  during  the  stimulation  of  the  sympathetic  in  the  neck  shows 
that  contraction  of  the  vessels  does  not  occur  until  after  the  pupil  dilates. 
(Langley  and  Anderson).3 

Griinhagen  and  others 4  have  also  maintained  that  the  dilatation  occur- 
ring after  excitation  of  the  cervical  sympathetic  may  be  due  to  inhibition 
of  the  sphincter,  the  basilar  layer  of  the  iris  proper  (the  posterior  portion 
of  the  stroma  already  alluded  to)  being  a  highly  elastic  tissue  that  at  once 
dilates  the  pupil  as  soon  as  the  tension  of  the  sphincter  is  abolished. 

Langley  and  Anderson  found  no  evidence  of  inhibition  of  the  sphincter, 
as  excitation  of  the  sympathetic  caused  contraction  of  sectors  of  the  iris 
severed  from  the  remainder  by  radial  incisions,  and  this  without  the  least 
relaxation  in  the  tone  of  the  pupillary  border.  When  such  a  sector  is  kept 
in  an  exteuded  state  for  a  short  time  it  does  not  retract  on  being  released, 
as  it  would  did  it  contain  sufficient  elastic  tissue  to  act  as  an  efficient  dila- 
tator. Neither  does  it  retract  after  complete  death  of  the  iris  muscles  when 
the  sphincter  portion  is  removed.  Besides,  a  local  stimulation  upon  the 
solera  or  by  means  of  the  sympathetic  (all  the  ciliary  nerves  but  one  being 
cut)  causes  a  local  dilatation  with  contraction  of  the  sphincter,  and  also 
drags  over  to  the  stimulated  side  that  part  of  the  iris  opposite  the  stimulus. 
(See  Fig.  57.)  This  cannot  be  explained  by  Griinhagen's  hypothesis. 

The  combined  anatomical  and  physiological  evidence  of  the  existence 
of  a  radially  arranged,  dilatator  muscle  now  appears  conclusive. 

The  causes  of  the  movements  of  the  iris  may  be  briefly  summarized  as 
follows :  contraction  of  the  pupil  may  occur  not  only  from  the  stimulation 
of  the  retina  by  light,  as  in  excess  of  illumination,  but  also  from  the  gen- 


1  Comptes-rendus  de  la  Soc.  de  Biol.,  1852,  Paris,  1853,  iv.  155. 

J  Zeglinski  (N.).     Experimentelle  Untersuchungen  iiber  die  Irisbewegung.    Arch.  f. 
Anat.  u.  Physiol.,  Abth.  i.,  1855,  p.  1. 

3  Langley  and  Anderson.     On  the  Mechanism  of  the  Movements  of  the  Iris.    Jour, 
of  Physiol.,  Cambridge,  1892,  xiii.  544-597. 

4  Griinhagen  and  Samkowy.     Arch.  f.  d.  ges.  Physiol.,  1875,  x.  165. 
Fran9ois-Franck.     Travaux  de  la  laboratoire  de  M.  Marey,  1880,  iv.  55. 
Gaskell.     Jour,  of  Physiol.,  Cambridge,  1886,  vii.  38. 


THE  ANATOMY  OF  THE  EYEBALL. 


189 


eral  stimulation  of  electricity  or  strychnine ;  from  the  deadening  of  the 
reflexes,  as  in  sleep,  coma,  narcosis,  and  the  first  stages  of  chloroform-  or 
alcohol-poisoning ;  from  the  topical  application  of  myotics,  such  as  eserine 
(physostigmine)  and  pilocarpine ;  from  central  disorders,  such  as  encepha- 
litis and  meningitis ;  from  local  disorders,  such  as  iritis  and  many  affec- 
tions of  the  globe  j  from  increase  of  blood -pressure,  as  in  forced  expiration, 
deficiency  of  the  aqueous  humor,  or  anything  else  causing  congestion.  Con- 
traction also  appears  to  be  associated  with  certain  movements  of  the  eye. 
Thus  it  occurs  in  accommodating  for  near  objects l  and  in  turning  the  eye 
inward. 

Dilatation  occurs  from  an  opposite  set  of  causes,  such  as  deficiency  of 

FIG.  57. 


Effect  of  local  stimulation  of  the  sclera  in  the  cat.  (Langley  and  Anderson.) — A,  first  stage,  con- 
traction of  the  sphincter ;  B,  second  stage,  radial  contraction  synchronous  with  contraction  of  the 
sphincter.  The  two  dots  indicate  the  position  of  the  stimulating  electrodes. 

light ;  anything  tending  to  make  the  retina  insensitive,  such  as  amblyopia 
or  amaurosis  ;  the  application  of  mydriatics,  such  as  atropine,  daturine,  or 
hyoscyamine ;  depression  of  the  nervous  system,  as  in  fright,  shock,  fatigue, 
the  latter  stages  of  chloroform-  or  alcohol-poisoning ;  lowering  of  the  blood- 
pressure,  as  in  the  application  of  cocaine ;  in  dyspnoea ;  in  strong  muscular 
exertion ;  in  excessive  distention  of  the  anterior  chamber.  Accommodation 
for  distant  objects  also  dilates  the  pupil. 

When  fully  contracted  the  pupil  is  so  small  that  it  is  easy  for  it  to  be 
totally  occluded  by  inflammatory  exudations;  hence  the  importance  of 
dilating  it  during  iritis.  Its  diameter  varies  somewhat  with  age.  Closely 
contracted  in  the  newly  born,  it  is  rather  large  in  children  whose  reflexes 
are  active,  somewhat  smaller  in  adult  life,  and  still  more  reduced  in  old 

1  Hence  Haller  (Elementa  physiologise,  1743,  v.  616),  Morton  (Am.  Jour.  Med.  Sci., 
November,  1831),  and  others  held  that  accommodation  is  produced  by  adjustment  of  the 
pupil.  This  is  incorrect,  for  accommodation  is  not  affected  by  viewing  objects  through  a 
hole  smaller  than  the  pupil. 


190 


THE  ANATOMY  OF  THE  EYEBALL. 


age.    In  the  aged  the  iris  is  somewhat  stiffened  by  the  increase  of  connective 
tissue,  and  it  therefore  reacts  less  readily. 

The  nerves  of  the  iris  are  derived  from  the  ciliary  plexus,  which  has 
already  been  described  in  connection  with  the  ciliary  muscle.  They  are  at 
first  medullated,  and  quickly  reunite  within  the  ciliary  zone  to  form  another 
or  iridian  plexus,  which  is  denser  as  it  approaches  the  sphincter.  Three 
orders  of  fibres  are  derived  from  this  plexus, — first,  pale,  nou-medullated 
fibres,  apparently  belonging  to  the  sympathetic,  that  pass  backward  towards 
the  dilatator  and  are  believed  to  supply  it ;  second,  medullated  fibres,  appar- 
ently sensitive,  that  pass  to  the  anterior  surface ;  third,  medullated  fibres 
that  pass  to  the  sphincter  and  probably  give  it  its  motor  influence.  Certain 
vaso-motor  fibres  supply  the  coats  of  the  vessels.  There  are  no  ganglion- 
cells  in  the  iris,  as  was  formerly  supposed. 


Arteries  of  the  iris.  (Sappey.)— 1, 1,  long  posterior  ciliary  arteries;  2,  3,  their  branches  of  bifurca- 
tion ;  4,  recurrent  arteries  destined  for  the  chorioid ;  5,  5,  6,  6,  anterior  ciliary  arteries  anastomosing 
with  the  long  ciliary  to  form  the  greater  arterial  circle  of  the  iris ;  7,  the  lesser  arterial  circle  of  the  iris. 

The  tactile  sensibility  of  the  iris  is  not  great.  Operations  upon  the 
membrane  are  not  very  painful  if  traction  is  avoided. 

The  arteries  of  the  iris  are  derived  from  the  long  posterior  ciliary  and 
the  anterior  ciliary  arteries.  The  long  posterior  ciliary,  two  in  number, 
arise  from  the  ophthalmic  artery  where  it  lies  below  the  optic  nerve,  and 
penetrate  the  sclera  near  its  junction  with  that  nerve.  Their  course  within 
the  chorioid  has  already  been  described.  (See  Fig.  35 ;  Fig.  36,  b ;  Fig. 
58,  1,1.)  Just  before  reaching  the  posterior  border  of  the  ciliary  muscle, 
at  four  to  seven  millimetres  behind  the  cornea,  each  bifurcates  into  an 
ascending  and  a  descending  branch ;  .these  soon  assume  a  direction  parallel 
to  the  equator,  the  arteries  of  opposite  sides  finally  anastomosing  with 
each  other  and  with  the  anterior  ciliary  arteries.  The  latter  vessels  (see 
Fig.  35 ;  Fig.  36,  c;  Fig.  58,  5,  5,  6,  6),  six  to  eight  in  number,  are  derived 


THE    ANATOMY    OF   THE   EYEBALL.  191 

from  the  muscular  or  lacryrnal  branches  of  the  ophthalmic  artery,  and  pierce 
the  sclera  at  or  near  the  annular  ligament.  Uniting  with  the  branches  of 
the  long  posterior  ciliary,  they  form  a  vascular  anastomosis  about  the 
ciliary  border  of  the  iris  between  the  two  portions  of  the  ciliary  muscle. 
This  is  known  as  the  greater  arterial  circle  of  the  iris.1  (See  Figs.  35,  36, 
and  58.) 

From  the  concavity  of  this  arcade  arterioles  pass  radially  towards  the 
pupil,  lying  in  the  stroma  and  dividing  dichotomously  with  frequent  cross- 
unions  in  a  way  which  has  been  compared  to  the  behavior  of  the  vasa 
intestini  tenues  of  the  mesentery.  (Morgagni,  Zinn.) 

At  the  periphery  of  the  sphincter  the  branches  form  a  fine  circular 
mesh-work,  the  lesser  arterial  circle  of  the  iris.2  (Fig.  58,  7.)  It  is  from 
this  that  branches  are  given  off  during  foetal  life  to  supply  the  pupillary 
membrane. 

The  veins  of  the  iris  arise  from  the  capillary  net-work  of  the  sphincter 
and  from  the  delicate  branchlets  of  the  anterior  surface,  and  gather  into 
trunks  that  run  meridionally  backward  through  the  orbiculus  ciliaris,  to 
empty  into  the  vorticose  veins. 

The  vessels  of  the  iris  are  deficient  in  muscular  fibres,  but  have  un- 
usually thick  coats  of  circular  connective-tissue  fibres.  (Bohm  and  von 
Davidoff.) 

There  are  no  proper  lymphatics  in  the  iris,  but  very  numerous  lacunar 
spaces,  by  which  the  lymphatic  circulation  is  kept  up,  occur  in  the  stroma. 
In  the  crypts  of  the  anterior  surface  these  present  open  mouths,  and  the 
aqueous  humor  doubtless  passes  freely  by  means  of  these  spaces  into  the 
general  circulation. 

THE  INNER  OR  NERVOUS  COAT.5 

Formed  by  an  outgrowth  from  the  brain, — that  is  to  say,  by  the  primi- 
tive optic  vesicle  or  ophthalmencephalon, — the  inner  coat  is  in  many  respects 
analogous  to  the  cerebral  cortex.4  It  contains  the  essential  portion  of  the 

1  Syn.  :  circulus  arteriosus  iridis  major. 

2  Syn.  :  circulus  arteriosus  iridis  minor. 

3  Syn.  :  tunica  interna  ;  t.  nervosa ;  retina ;  Netzhaut,  G. ;  retine,  F.     The  term  retina 
is  derived  from  L.  rete,  &  net. 

Hyrtl  (Onomatologia  anatomica,  p.  452)  comments  on  the  absurdity  of  this  name, 
as  the  retina  is  not  a  net.  The  name  first  appears  in  a  translation  of  the  Canon  of  Aver- 
roes  made  by  Gerardus  Cremonensis.  The  passage  is  as  follows  :  "  Extremitas  nervi  con- 
cavi  [the  optic  nerve,  so  called  because  he  considered  it  hollow]  comprehendit  vitrcum 
[the  vitreous  body],  sicut  rete  comprehendit  venationem  [the  catch],  quapropter  nominatur 
retina."  (Lib.  iii.,  fen. '3,  tract,  i.,  cap.  i.) 

Galen  used  the  term  a^i^r/arpoeid^  (*«•(&»),  which  indicates  primarily  an  investment 
(from  afj^iftdXXu,  to  throw  around),  secondarily  a  net  which  invests  the  captured  fish. 
His  Arabic  translators  took  it  in  the  latter  sense  only,  and  called  it  rescheth,  meaning  reti- 
formis.  To  express  this  Gerardus  invented  the  well-sounding  but  barbaric  term  retina. 

*  Casserius  says,  "  Retina  convoluta  cerebri  substantial  similis."  Pentsesthesion,  1.  5, 
tab.  v.,  Fig.  9. 


192 


THE  ANATOMY  OF  THE  EYEBALL. 


FIG.  69. 


: 


organs  of  vision,1  the  other  coats  being  subsidiary  to  it  and  serving  for  its 
protection  and  nourishment.  Originally  vesicular  in  form,  with  the  optic 
nerve  for  a  pedicle,  the  development  of  the  lens  and  of  the  vitreous  body 
dimples  and  finally  invaginates  it  in  front  much  as  the  pressure  of  the 
finger  would  a  ball  of  thin  rubber,  so  that  in  its  completed  form  it  is  a 
two-layered  cup  with  its  lip  at  the  pupillary  border.  (See  page  176  and 
Fig.  59.) 

The  outer  layer  retains,  throughout,  its  primitive  epithelial  character, 
becoming  strongly  pigmented.  Its  close  apposition  to  the  middle  coat  and 

its  ready  separation  from  the  inner  layer 
caused  it  to  be  formerly  incorrectly  reckoned 
as  belonging  to  that  coat,  although  it  is  now 
known  to  be  genetically  distinct.  The  inner 
layer  is  variously  modified  in  the  different 
regions  which  it  traverses.  Behind,  where 
it  is  fully  nourished  by  the  chorioid,  it  has 
become  strongly  developed;  in  the  ciliary 
region,  where  the  nourishment  is  less,  it  re- 
tains its  embryonic  condition  ;  while  in  front, 
where  it  lines  the  iris,  it  has  become  reduced 
to  a  mere  epithelial  investment.  There  are, 
therefore,  three  portions  of  the  coat  showing 
successive  degrees  of  reduction :  first,  the 
retina  proper  or  chorioidal  retina,  extending 
from  the  optic  nerve  entrance  to  the  ora 
serrata ;  second,  the  ciliary  retina,  extending 
from  the  ora  serrata  to  the  iris ;  third,  the 
iridian  retina,  forming  the  posterior  layers  of  the  iris.  The  latter  has 
already  been  described  with  the  iris. 

THE   RETINA   PROPER.2 

The  inner  layer  of  this  portion  of  the  nervous  coat  is  conterminous  with 
the  chorioid  portion  of  the  middle  coat,  and  is  characterized,  except  where 
the  fibres  of  the  optic  nerve  enter,  by  a  complicated  arrangement  of  cellular 
layers  which  constitute  the  receiving  apparatus  for  vibrations  of  light.  It 
extends  from  the  optic  nerve  entrance  on  the  posterior  two-thirds  of  the 
globe,  a  little  farther  forward  on  the  medial  than  on  the  lateral  aspect, 
ending  at  the  wavy  line  called  the  ora  serrata,  by  losing  its  essential  nervous, 
elements. 


Model  showing  formation  of  optic 
cup.  (Bonnet.)— Sn,  optic  nerve  or 
hollow  pedicle  of  the  cup,  enlarged 
forward  into  a  vesicle  with  an  outer 
wall  (ab),  an  inner  wall  (ib),  and  a 
cavity  (A),  which  later  disappears. 
The  furrow  (aus)  on  the  lower  side  of 
the  stalk  is  the  chorioid  fissure,  which 
farther  forward  is  filled  in  with  the 
vitreous  body  (gl)  and  the  lens  (I). 


1  That  the  seat  of  vision  is  in  the  retina  has  by  no  means  been  universally  held  by 
investigators.     Mariotte,  Mery,  Le  Cat,  and  Sir  David  Brewster  were  of  the  opinion  that 
the  chorioid  is  the  active  agent  in  perception.     In  1793,  John  Taylor  published  a  treatise 
entitled  "An  Important  Inquiry  into  the  Seat  of  the  Immediate  Organ  of  Sight, — viz., 
whether  Retina  or  Choroides." 

2  Syn.  :  pars  optica  retinae;  physiological  retina  ;  chorioidal  retina. 


THE    ANATOMY   OF   THE    EYEBALL.  193 

It  gradually  decreases  from  a  thickness  of  0.4  millimetre  behind  to 
about  0.2  millimetre  near  the  ora  serrata.  In  health  it  is  perfectly  smooth 
being  well  stretched  over  the  chorioid,  but  after  death  it  rapidly  swells  up 
by  imbibition,  so  that  folds  appear,  usually  directed  meridionally.  Its  trans- 
parency is  so  great  that  during  health  it  can  be  distinguished  only  by  its 
blood-vessels,  which  seem  to  float  within  it,  or,  after  it  has  been  for  a  time 
deprived  of  light,  by  a  purplish-red  tinge  due  to  a  disseminated  coloring 
matter  termed  the  visual  purple.  Hence  in  the  interior  of  the  eye  the 
outer  or  pigmented  layer  shows  through  the  inner  transparent  layer, 
forming  a  dark  background  which  prevents  reflection  of  the  rays  of  light 
and  consequent  interference.  Immediately  after  death  the  retina  becomes 
clouded,  then  appearing  as  a  thin  grayish  pellicle.  In  a  short  time  it 
grows  soft  and  diffluent.  Pathological  causes  may  also  change  its  trans- 
parency during  life,  and  its  examination  by  means  of  the  ophthalmoscope 
gives  us  important  information  as  to  its  blood-vessels  and  the  general 
nutrition  of  the  tissues. 

The  general  transparency  of  the  retina  is,  however,  obscured  in  two 
places, — one,  where  the  optic  nerve  enters,  called  the  optic  disk  ;  the  other, 
an  elliptical  area  lying  in  the  ocular  axis,  called  the  yellow  spot. 

The  optic  disk l  is  a  whitish  spot  opposite  the  attachment  of  the  optic 
nerve,  composed  of  the  fibres  of  that  nerve  that  have  penetrated  the  lamina 
cribrosa  and  are  bending  at  nearly  right  angles  to  diverge  meridionally  to 
all  parts  of  the  retina.  (See  Fig.  60.)  It  appears  to  be  circular,  or  nearly 
so,  but  accurate  measurements  show  it  to  be  slightly  elliptical,  with  its  long 
axis  directed  vertically.  Its  diameters  vary  from  1.4  to  1.7  millimetres. 
The  divergence  of  the  fibres  causes  its  surface  to  be  depressed  in  a  varying 
degree ;  sometimes  it  is  a  mere  dimple,  or  it  may  be  deepened  to  a  con- 
siderable funnel-like  hollow,  the  excavation.2  This  is  not  situated  at  the 
centre  of  the  disk,  but  somewhat  towards  its  nasal  side,  where  the  wall  of 
the  excavation  is  somewhat  steeper  and  the  retinal  vessels  are  found,  they 
having  penetrated  at  the  bottom  of  the  funnel.  The  passage  by  which  they 
penetrate  is  known  as  the  poi*us  opticus,  a  name  erroneously  applied  by 
some  to  the  entire  disk.  It  may  be  noted  that  the  excavation  is  the  natural 
result  of  the  invagination  of  the  optic  vesicle  in  the  embryo,  and  should 
be  considered  as  a  vestige  of  the  chorioidal  fissure.  A  vestige  of  the  hyaloid 

1  Syn. :  porus  opticus ;  colliculus  opticus ;  papilla  optica ;  optic  papilla ;  optic  entrance ; 
head  of  the  optic  nerve;  punctum  caecum;  blind  spot;  Mariotte's  spot  (discovered  as  to 
this  property  by  Mariotte  in  1668) ;  macula  albida,  Luschka.     The  designations  papilla 
and  colliculus  were  used  by  the  older  anatomists  for  the  reason  that  when  viewed  in  a 
fresh  eye  the  disk  appears  elevated.     This  is,  however,  an  optical  illusion  produced  by  the 
whitish  nerve-fibres  showing  through  the  transparent  surroundings.     There  is  really  no 
elevation  of  the  disk  as  a  whole,  but,  on  the  contrary,  a  slight  excavation.    For  this  reason 
the  term  optic  disk  is  to  be  preferred. 

2  Syn. :  excavatio  papillce  nervi  optici ;  excavatio  physialogica ;  physiological  excava- 
tion.   The  latter  names  contrast  it  with  the  much  larger  pathological  excavation  involving 
the  whole  disk  that  at  times  results  from  a  considerable  increase  of  intra-ocular  pressure. 

VOL.  I.— 13 


194 


THE  ANATOMY  OF  THE  EYEBALL. 


artery  of  foetal  life  may  often  be  found  here,  appearing  as  a  thread  of  con- 
nective tissue  running  from  the  disk  into  the  vitreous  body. 

The  whitish  appearance  of  the  disk  is  due  to  the  fact  that  the  lamina 
cribrosa  and  the  myelinated  fibres  beyond  it  show  through  the  transparent 
axis-cylinders  which  alone  form  the  disk.  The  proper  perceptive  elements 
of  the  retina  are  here  entirely  absent. 

Viewed  through  the  ophthalmoscope  (see  Fig.  61),  the  disk  presents 
some  appearances  that  depend  upon  its  structure.  Immediately  surround- 
ing it  there  is  usually  seen  a  whitish  circle,  the  scleral  ring,  it  being  an 


Radiation  of  the  optic  nerve  fibres  upon  the  retina.    (Michel.)— -4,  optic  disk ;  B,  macula  lutea. 

The  image  is  reversed. 

edge  of  sclera  that  shows  through  the  somewhat  larger  chorioidal  aperture. 
Exterior  to  this  there  is  often  seen  a  dark  circle,  the  chorioidal  ring,  fre- 
quently broken  by  the  passage  of  vessels  so  as  to  form  two  or  more  crescents. 
This  is  due  to  the  showing  of  the  pigment  of  the  chorioid,  often  especially 
well  developed  in  this  locality.  Within  this  a  fine  reddish-gray  line  indi- 
cates the  proper  edge  of  the  nerve.  These  rings  are  somewhat- obscured 
on  the  nasal  side  because  there  a  greater  number  of  fibres  pass  over  them. 
The  yellow  spot1  is  an  oval  area  of  a  reddish-brown  color  that  appears 

1  Syn.  :  macula  lutea;  limbus  luteus,  Sommering.     It  was  first  discovered  by  Bozzi, 
not  by  Sommering,  as  is  usually  stated. 


The  optic,  disk  viewed  with  the  ophthalmoscope.  (Jager.)-X,  the  optic  disk;  a,  scleral  ring; 
b.  chorioidal  ring;  c,  central  artery  of  retina;  d,  d,  its  branches;  e,  central  vein  of  retina;  /,  /,  its 
branches;  N,  nasal  side ;  T,  temporal  side. 


FIG.  G'2. 


n      b 


Vessels  of  the  retina.  (Testut.)— a,  solera;  b,  chorioid;  c,  retina;  1,  macula  lutea;  2,  optic  disk; 
3,  superior  nasal  artery;  4,  inferior  nasal  artery;  5,  inferior  temporal  artery;  6,  superior  temporal 
artery;  T,  temporal  side;  A,  uusal  side. 


THE  ANATOMY  OF  THE  EYEBALL.  195 

somewhat  darker  than  the  rest  of  the  retina  because  of  pigment  granules 
diffused  throughout  its  tissue.  Its  horizontal  diameter  is  about  two  milli- 
metres, and  its  vertical  diameter  half  that  distance.  This,  however,  can 
be  determined  only  approximately,  because  the  coloring- matter  fades  away 
gradually.  In  the  retina  which  has  been  removed  from  the  chorioid  it  is 
of  a  golden-yellow  hue.  The  color  soon  disappears  after  death,  probably 
dissolved  by  extravasated  fluids. 

Near  the  centre  of  the  yellow  spot  there  occurs  a  funnel-shaped  depres- 
sion known  as  the  central  fovea.1  This  is  situated  nearly  in  the  axis  of 
the  globe  at  an  average  distance  of  3.915  millimetres2  from  the  centre  of  the 
optic  disk  and  0.785  millimetre  below  the  horizontal  meridian  (Landolt), 
a  distance  which  varies  according  to  the  shape  of  the  ball,  being  greater  in 
hypermetropes  and  less  in  myopes.  It  is  the  region  of  most  acute  vision, 
and  it  is  because  of  the  localized  character  of  this  acuity  that  the  eye  must 
be  moved  when  scanning  carefully  a  surface  of  any  extent.  Its  diameter  is 
from  0.2  to  0.4  millimetre,3  and  it  is  so  deep  that  the  retina  at  its  bottom  or 
fundus  is  thinner  than  at  any  other  place,  being  only  0.1  to  0.08  millimetre 
thick.  With  the  ophthalmoscope  it  can  usually  be  discerned  as  a  clear 
speck  situated  in  the  darker  area  of  the  yellow  spot. 

In  retinas  examined  any  considerable  time  after  death  a  small  fold, 
the  plica  centralis,  is  seen  running  from  the  disk  to  the  macula  lutea.  This 
was  formerly  thought  to  be  an  anatomical  feature  of  the  living  eye,  but  is 
now  known  to  be  a  post-mortem  phenomenon. 

The  vessels  of  the  retina  (see  Fig.  62)  form  a  system  distinct  and  inde- 
pendent from  that  of  the  chorioid,  communicating  with  the  latter  only  at 
the  optic  nerve  head  by  means  of  small  twigs  that  join  the  circlet  of  Zinu. 

The  pathological  consequences  of  this  may  be  readily  deduced ;  the  in- 
tegrity of  the  central  artery  is  necessary  to  the  preservation  of  the  sight ; 
should  it  at  any  time  become  occluded  or  compressed,  vision  at  once  fails 
for  lack  of  retinal  nutrition.  Besides,  the  retinal  arteries  do  not  anastomose 
with  one  another,  every  arteriole  supplying  its  appropriate  area  and  ter- 
minating in  its  own  capillaries.  In  this  respect  the  vessels  behave  very 
much  like  those  supplying  the  cortex  of  the  brain.  (See  Fig.  63.)  From 
this  method  of  termination,  quite  distinct  from  the  ordinary  distribution, 
such  vessels  are  called  terminal  arterioles.  In  case  any  branch  is  plugged, 
the  area  it  supplies  is  at  once  deprived  of  a  proper  circulation,  and  loss  of 
vision  at  once  ensues  there. 

The  trunk  artery  of  the  retinal  system  is  the  central  artery  of  the 

1  Syn.  :  fovea  centralls  ;  foveola  centralis ;  Netzhautgrube,  G. ;  foramen  central*,  S6m- 
mering.  The  retina  being  very  thin  here,  the  subjacent  pigment  shows  through  very  dis- 
tinctly, even  after  the  tissue  becomes  clouded  after  death.  The  fovea  then  appears  as  a 
clear  punctiform  spot,  and  was  at  first  supposed  to  be  a  foramen. 

a  3.38  millimetres,  Schafer ;  3.8  millimetres,  Weber;  3.28  to  3.6  millimetres,  Krause; 
4  millimetres,  Gerlach  and  Macalister. 

3  0.18  to  0.225  millimetre,  Kolliker. 


196  THE  ANATOMY  OF  THE  EYEBALL. 

retina,  a  branch  of  the  ophthalmic,  which  in  the  foetus  supplies  the  anterior 
part  of  the  primitive  optic  vesicle,  and  becomes  enfolded  in  the  optic  nerve 
when  the  invagination  of  that  vesicle  forms  the  optic  cup.  It  enters  the 
nerve  some  1.5  centimetres  behind  the  eyeball,  and  runs  within  it  as  far  as 
the  fundtis  of  the  excavation  of  the  disk.  Within  this  excavation  it  usually 
divides  into  two  principal  branches,  the  superior  and  inferior  papillary 
arteries.  Each  of  these  soon  divides  into  a  temporal  and  a  nasal  branch  : 
so  that  there  are  four  principal  arteries  supplying  the  four  principal  aspects 

of  the  retina, — viz.,  the  superior  and  inferior 
FIG.  63.  temporal  and  the  superior  and  inferior  frontal 

arteries.  It  should  be  noted  that,  owing  to 
the  situation  of  the  optic  disk  in  the  inferior 
nasal  quadrant,  the  artery  for  that  quadrant 
has  much  less  area  to  supply  than  the  others, 
while  the  superior  temporal  artery  has  much 
more  than  the  others ;  the  former  is  conse- 
quently the  smallest,  the  latter  the  largest,  of 
these  branches.  Some  small  twigs  given  off 
from  the  papillary  arteries  pass  towards  the 
region  of  the  yellow  spot,  and  are  designated 
as  the  superior  and  inferior  macular  arteries. 
Others  pass  towards  the  median  line,  and 
Blood-vessels  of  the  retina  injected,  are  called  the  median  arteries. 

(Bohm  and  v.  Davidoff.) — A,  vein;  B,  „,  ,  ,,     ,  .      , 

artery;  C,  free  area  near  artery.  J-he  mam  branches  OI   the  retinal  system 

run  in  the  deeper  layers  of  the  retina,  send- 
ing vertical  offshoots  to  supply  the  immediately  contiguous  layers.  The 
most  superficial  layers  are  not  supplied  from  that  system,  their  nutrition 
depending  upon  transudation  from  the  chorioidal  vessels,  as  has  already 
been  explained.  This  lack  of  blood-vessels  in  the  external  layers  explains 
why  none  are  found  in  the  immediate  region  of  the  central  fovea.  The 
retina  is  reduced  at  that  place  to  its  outer  layers,  and  there  is  consequently 
an  area  of  about  one-sixth  of  a  square  millimetre  that  is  entirely  destitute 
of  blood-supply.  The  region  of  the  yellow  spot  is,  however,  one  of  the 
best  supplied  of  the  whole  retina,  it  receiving  a  multitude  of  fine  twigs 
from  both  the  temporal  and  macular  arteries  (see  Fig.  64) :  so  there  is 
every  reason  to  think  that  its  nutrition  is  especially  active. 

The  capillary  vessels  are  arranged  in  an  internal,  large-meshed  net- 
work and  an  external,  much  finer  one.  From  these  arise  the  veins,  that 
follow  courses  corresponding  inversely  to  those  of  the  arteries  and  discharge 
into  the  cavernous  sinus  or  the  superior  ophthalmic  vein. 

Numerous  variations  of  the  distribution  of  the  retinal  arteries  have 
been  found,  depending  upon  the  place  at  which  the  central  artery  divides. 
This  division  may  occur  at  the  margin  of  the  disk,  immediately  at  the  exit 
of  the  artery  from  the  nerve  (normal  form),  or  within  the  trunk  of  the 
nerve  (quite  frequent),  and  the  arteries  may  indeed  again  subdivide  into 


PIG.  64. 


Large  loops. 


Non-vascular 
region  of  fovea. 


Blood-vessels  of  the  yellow  spot  injected.    (Bohm  and  v.  Davidoff.) 


The  hyaloid  artery.  (Testut.)— 1,  retina  with  its  vessels;  2,  vitreous  body;  3,  crystalline  lens; 
4,  iris;  5,  central  artery  of  the  retina,  with  5',  5',  its  branches;  6,  hyaloid  artery;  7.  7,  its  branches 
supplying  the  posterior  surface  of  the  lens;  8,  8,  iridian  arteries;  9,  anastomosis  of  the  two  systems 
upon  the  capsulo-pupillary  membrane. 


THE    ANATOMY    OF   THE    EYEBALL.  197 

nasal  and  temporal  branches  within  the  nerve,  and  appear  upon  the  disk 
as  four  trunks.1 

During  foatal  life  the  retinal  system  supplies  the  contents  of  the  opt  it- 
cup  as  well  as  the  cup  itself.  A  vessel  passes  from  the  central  artery  at  the 
disk  forward  through  the  vitreous  body  and  supplies  the  vascular  tunic 
of  the  lens.  (See  Fig.  65.)  This — the  hyaloid  artery — may  persist  to  adult 
life  and  offer  no  serious  obstacle  to  vision.  The  connection  of  this  vessel 
with  the  arteries  of  the  iris  has  already  been  mentioned. 

The  layers  of  the  retina  are  fully  considered  in  the  article  that  treats 
of  its  microscopical  anatomy.  It  is  only  necessary  to  state  here  that  the 
outer  layer  of  the  optic  cup  forms  a  sheet  of  densely  pigmented  epithe- 
lium, while  the  inner  one  is  differentiated  into  neuro-epithelial  elements, 
constituting  the  visual  cells,  or  rods  and  cones,  and  the  neural  elements 
proper.  There  is  also  a  system  of  supporting  fibres,  probably  of  the  nature 
of  neuroglia,  known  as  Miiller's  fibres.  The  arrangement  of  the  nerve- 
cells  is  not  unlike  that  of  those  found  in  the  cortex  of  the  brain,  there 
being  evidently  receiving,  associative,  and  transmitting  elements,  as  well 
as  filaments  apparently  derived  from  the  central  organs  that  seem  to  repre- 
sent afferent  fibres. 

At  the  region  of  the  yellow  spot  the  inner  layers  thin  away  and  become 
reduced  until  at  the  central  fovea  the  neuro-epithelium  alone  remains. 

THE  CILIARY   RETINA.2 

At  the  ora  serrata  the  nervous  elements  of  the  retina  suddenly  cease, 
and  there  remain  only  the  external,  pigmented  epithelium  and  an  undiffer- 
entiated,  internal  layer  representing  the  primitive  condition  of  the  optic 
vesicle.  This  portion  of  the  retina  is  reduced  in  thickness  to  0.2  milli- 
metre. It  is  continuous  forward  with  the  posterior  pigmeuted  epithelium 
of  the  iris.  The  width  of  this  region — that  is  to  say,  the  distance  from  the 
ora  serrata  to  the  attachment  of  the  iris — is  five  millimetres. 

CONTENTS  OF  THE  EYEBALL. 

Within  the  coats  just  described  are  contained  certain  transparent  media. 
These  were  formerly  known  as  the  humors  of  the  eye,  and  were  distin- 
guished, according  to  their  characters,  as  the  aqueous,  crystalline,  and 
vitreous  humors, — the  first  being  liquid  and  watery,  the  second  a  soft  solid 
having  a  definite  structure,  and  the  third  having  a  jelly-like  consistency. 
With  the  decline  of  the  doctrine  of  the  humors  these  names  have  been  gradu- 
ally abandoned,  the  first  only  being  retained,  the  crystalline  humor  being 
known  as  the  crystalline  lens,  and  the  vitreous  humor  as  the  vitreous  bod  it. 

The  three  have  a  diversity  of  origin :  the  aqueous  humor  is  a  secretion 
derived  from  the  blood,  especially  through  the  ciliary  processes ;  the  lens 

1  See  Magnus,  Die  makroskopische  Gefasse  der  menschlichen  Netzhuut,  Leipzig,  187 

2  Syn. :   pars  ciliaris  retinae ;  corpus  ciliaris  retinae ;  margo  facculo&us  retinae ;  pre- 
retina  (Leidy). 


198  THE  ANATOMY  OF  THE  EYEBALL. 

is  an  ectodermic  invagination ;  while  the  vitreous  body  is  of  mesodermic 
origin,  being  an  excellent  example  of  a  mesenchymal  structure. 

THE   AQUEOUS   HUMOR.1 

This  is  a  sparkling,  transparent  lymph  filling  the  aqueous  chamber  or 
space  in  front  of  the  lens,  which  is  morphologically  a  lymph-space.  As 
already  stated,  it  is  separated  from  the  blood  by  the  action  of  the  vessels 
of  the  middle  coat,  particularly  those  of  the  ciliary  processes.  It  was 
formerly  believed  to  be  essential  to  vision  and  fixed  in  its  quantity,  so  that 
if  evacuated  it  was  not  renewed  and  blindness  necessarily  ensued.  Zinn 2 
'showed  this  not  to  be  the  case. 

Many  theories  have  at  various  times  been  brought  forward  to  account 
for  its  production.  Palfin  supposed  that  he  had  discovered  special  glands 
for  its  secretion  at  the  edge  of  the  iris ;  Nuck  indicated  special  conduits 
for  its  transmission,  which  were,  however,  shown  to  be  the  long  anterior 
ciliary  arteries.  Some  have  supposed  that  it  was  an  oozing  from  the  vit- 
reous body, — that  it  was  the  secretion  of  a  special  membrane,  the  mythical 
aqueo-capsulary  membrane,  believed  to  line  the  entire  surface  of  the  aqueous 
chamber.  The  view  of  Collins,  that  it  is  mainly  secreted  by  certain  gland- 
like  bodies  found  upon  the  surface  of  the  ciliary  processes,  has  been  already 
mentioned.  The  older  view  of  Me>y,3  that  the  ciliary  processes  throughout 
their  whole  extent  are  the  agents  of  the  secretion,  seems  to  be  the  correct 
one.  With  these  should  probably  be  included  the  vascular  ridges  that 
extend  from  the  ciliary  processes  to  the  posterior  surface  of  the  iris.  It 
is  pointed  out  by  Leber  that  the  tubular  imaginations  of  Collins  have 
not  the  anatomical  character  of  glands,  as  they  have  no  secretory  duct,  no 
proper  lumen,  and  no  glandular  epithelium.  Nicati 4  endeavors  to  show 
that  in  addition  to  the  ciliary  processes  the  chorioid  itself  must  be  consid- 
ered as  a  secreting  organ  for  this  liquid,  of  which  he  distinguishes  two 
kinds, — one,  non-fibrinous  or  ordinary,  which  occupies  the  chambers  when 
no  pathological  conditions  are  present ;  the  second,  fibrinous  or  neuro-para- 
lytic,  secreted  during  pathological  conditions,  such  as  a  sudden  evacuation 
of  the  chambers,  or  special  affections  of  the  sympathetic  nerve.  In  oppo- 
sition to  this  view,  Leber 5  remarks  that  there  is  no  anatomical  structure 

1  From  L.  aqueus,  -a,  -Mm,  watery,  and  humor  (orwmor),  fluid,  moisture. 

Syn.  :  humor  aqueus ;    Wasserfeuchtigkeit,  G. 

*  "  Secernitur  et  renovatur  ut  ex  notissimis  et  plurimis  illis  observationibus  apparet, 
quibus  constat,  humorem  aqueum,  qui  per  vulnusculum  corneas  inflictum  effluxit,  ut  oculus 
collaboretur,  intra  quadraginta  et  octo  horas,  perfecte  renasci  et  oculutn  illo,  ut  antea 
repletum  inveniri."  Zinn,  Descriptio  oculi  humani,  Gottingae,  1755,  chap.  vi.  \\  2,  3. 

3  Mery.     Sqavoir  si  le  glaucoma  et  la  cataracte  sont  deux  differentes  ou  une  seule  et 
meme  maladie.     Mem.  de  1'academie  des  sciences,  1707,  p.  498. 

4  Nicati.     La  glande  de  1'humeur  aqueuse.     Archives  d'ophtalmologie,  Paris,  1890. 
x.  481,  and  1891,  xi.  24,  152. 

5  Leber  (Th.).     Der  gegenwartiger  Stand  unserer  Kenntnisse  vom  Flussigskeitswech- 
sel  des  Auges.     Ergebnisse  der  Anatomic  und  Entwickelungsgeschichte,  1894,  Bd.  iv., 
Wiesbaden,  1895. 


THE   ANATOMY   OF   THE    EYEBALL.  199 

behind  the  ciliary  processes  that  could  serve  as  a  secretory  organ.  Nicati 
apparently  overlooked  the  fact  that  the  chorio-capillary  layer  of  the  middle 
coat  does  not  extend  forward  of  the  ora  serrata. 

The  importance  of  these  investigations  is  very  considerable,  as  it  is 
almost  certain  that  glaucoma  is  dependent  in  some  way  upon  the  production 
of  this  fluid. 

When  the  chamber  is  evacuated  the  secretion  is  quite  rapid,  refilling  the 
cavity  in  a  few  moments.  It  does  not  seem,  however,  that  under  ordinary 
normal  conditions  there  can  be  any  very  rapid  production  of  the  fluid,  for 
if  the  normal  interocular  pressure  is  maintained  after  puncture  there  is  no 
sensible  increase  in  the  fluid.  That  some  production  is  constantly  going 
on  appears  clear  from  the  fact  that  after  death  the  interocular  pressure  de- 
creases gradually,  which  would  not  be  the  case  were  it  dependent  merely 
upon  vascular  tension. 

Those  who  hold  that  other  portions  than  the  ciliary  processes  assist  in 
this  production  note  that  the  fluid  is  freely  secreted  before  the  pupillary 
membrane  is  obliterated, — that  is  to  say,  while  the  anterior  and  posterior 
chambers  are  still  separated  from  each  other.  On  the  other  hand,  it  is 
stated  that  the  anterior  chamber  is  not  formed  until  the  pupillary  mem- 
brane begins  to  disappear,  and  that  the  slightest  break  in  the  continuity  of 
the  latter  would  necessarily  fill  the  newly  developed  lymph-space.  Again, 
the  humor  is  said  to  be  found  in  normal  quantity  after  inflammations  which 
occasion  a  complete  adhesion  of  the  iris  to  the  lens,  shutting  off  the  ciliary 
processes.  This,  however,  is  by  no  means  conclusive,  as  there  is  nothing 
to  show  that  the  fluid  which  certainly  existed  in  the  anterior  chamber  before 
the  adhesion  occurred  has  been  absorbed.  That  it  is  more  freely  formed  in 
the  posterior  chamber  is  shown  by  the  fact  that  when  posterior  adhesions  of 
the  iris  exist,  a  forward  bulging  of  that  organ  occurs,  indicating  a  collection 
of  fluid  behind  it.  The  experiments  of  Deutschmann1  show  that  extirpa- 
tion of  the  ciliary  body  is  followed  by  a  complete  cessation  in  the  produc- 
tion of  the  aqueous  humor;  indeed,  that  in  a  short  time  the  vitreous 
humor  also  is  absorbed,  so  that  there  remains  little  else  than  the  crystal- 
line lens  within  the  contracted  eyeball.  Whether  such  extensive  inter- 
ference with  the  anatomical  structure  of  the  organ  as  is  necessary  for  the 
complete  removal  of  the  ciliary  body  can  be  effected  without  involving  the 
relations  and  functions  of  other  parts  seems  open  to  question.  The  humor 
leaves  the  chamber  by  the  spongy  tissue  of  the  spaces  of  Fontana,  as 
already  described  in  speaking  of  the  sinus  venosus.  It  also  passes  out  by 
the  lymph-crypts  of  the  iris.  The  lowering  of  the  intra-ocular  tension 
which  necessarily  occurs  when  the  humor  is  evacuated  causes  the  blood  to 
pour  in  increased  quantity  into  the  iris  and  the  ciliary  body,  which  greatly 
aids  the  transudation  of  fluid. 

1  Deutschmann  (K.).  Ueber  die  Quellen  des  Humor  Aqueus  im  Auge.  Archiv  f. 
Ophthalm.,  1880,  xxvi.,  Abth.  iii.,  117-134. 


200  THE   ANATOMY   OF   THE   EYEBALL. 

The  tension  of  the  eyeball  is  kept  up  mainly  by  the  active  secretion 
of  the  aqueous  humor,  and,  as  this  tension  amounts  to  the  force  exerted 
by  a  column  of  mercury  twenty-six  millimetres  in  height,  it  is  not  sur- 
prising that  when  the  cornea  is  punctured  the  fluid  should  rush  with  some 
force  towards  the  avenue  of  escape.  This  rush  is,  in  fact,  so  great  that  it 
sweeps  the  iris  with  it :  hence  great  care  must  be  taken  in  operating  upon 
the  cornea  to  prevent  the  iris  from  being  carried  into  the  lips  of  the  wound 
and  becoming  fixed  there. 

According  to  Nicati,  the  liquid  of  the  anterior  chamber  is  never  quies- 
cent, but  has  an  incessant  rotary  movement  which  prevents  any  deposits 
from  being  formed  on  the  surfaces  of  the  chamber. 

The  aqueous  humor  is  an  active  agent  in  removing  material  from  the 
eye.  Blood  diffused  into  the  anterior  chamber  may  be  seen  to  disappear 
in  the  course  of  a  few  days,  being  dissolved  in  the  aqueous  humor  and 
then  absorbed.  It  has  also  a  solvent  power  for  other  organic  substances, 
such  as  the  substance  of  the  crystalline  lens.  If  the  capsule  of  the  lens 
be  wounded  so  as  to  permit  the  access  of  the  aqueous  humor  to  it,  the  lens 
will  be  gradually  absorbed  and  in  favorable  cases  disappear  entirely.  This 
is  taken  advantage  of  in  operating  for  cataract  by  the  process  of  discission. 

The  index  of  refraction  of  the  aqueous  humor  is  of  value  in  considering 
the  optical  properties  of  the  eye.  It  is  variously  stated  by  different  author- 
ities as  follows : 

Observer.  Index  of  Index  of 

Aqueous  Humor.  Distilled  Water. 

Chossat1 1.338  13358 

Brewster2 1.3366  13358 

W.  Krause8 1.3420  13342 

Helmholtz* 1.3365  13354 

Fleischer5 1.8373  13340 

Hirschberg8 1.3375 

Krause  found  that  in  the  eyes  of  calves  the  refractive  indices  were  not 
materially  changed  within  twenty-four  hours  after  death. 

The  quantity  of  the  aqueous  humor  in  the  eye  is  not  great,  being  but 
from  two  hundred  and  thirty-one  to  three  hundred  and  twenty-three  cubic 
millimetres.  Its  weight  is  from  0.233  to  0.325  gramme,  and  its  specific 
gravity  is  nearly  that  of  water,  being  1.0053.7 

The  latest  determinations  of  its  chemical  composition  are  those  of  Michel 
and  Henry  Wagner,8  who  experimented  on  swine,  finding  the  following : 

1  Bulletins  de  la  Societe  philomathique  de  Paris,  1818,  p.  95. 

2  Edinburgh  Philosophical  Journal,  1819,  i.  43. 

3  Die  Brechungsindices  des  menschlichen  Auges.     Hannover,  1855. 

4  Handbuch  der  physiologischen  Optik,  1867,  S.  78. 

6  Neue  Bestiramungen  der  Brechungsexponenten  der  durchsichtigen  flussigen  Medien 
des  Auges,  1872. 

6  Centralblatt  fur  die  medicinischen  Wissenschaften,  1874,  xii.  193. 

7  These  figures  are  from  Vierordt,  Daten  und  Tabellen,  1893. 

8  Archiv  fur  Ophthalmologie,  1886,  xxxii.,  Abth.  ii.,  156. 


THE    ANATOMY   OP   THE   EYEBALL.  201 

Per  Cent. 
Water 98.71 

Ash    . 


Albumin 


0.89 
.107 


Other  organic  substances 293 

100. 

The  extraordinary  solvent  power  of  the  aqueous  humor  makes  it  easily 
affected  by  drugs  circulating  in  the  blood.  One  of  the  tests  for  poisoning 
by  belladonna  or  atropine  is  to  drop  a  little  of  the  aqueous  humor  from  the 
eye  of  the  suspected  subject  into  the  eye  of  another  animal.  If  atropine  is 
present,  dilatation  of  the  pupil  of  that  animal  soon  ensues.  Some  drugs 
color  the  humor  sufficiently  to  affect  vision.  This  is  the  case  with  santo- 
nin, even  in  medicinal  doses,  the  patient  often  "  seeing  yellow"  for  some 
hours  after  taking  it. 

THE   CRYSTALLINE   LENS.1 

This  was  early  recognized  as  one  of  the  most  important  of  the  acces- 
sory structures  of  the  eye,  its  optical  properties  being  described  by  Kepler, 
Descartes,  and  other  early  writers  upon  vision.  Maurolycus2  appears  to 
have  been  the  first  to  compare  it  to  a  lens  of  glass,  and  to  affirm  that  it  had 
the  power  of  refracting  rays  of  light. 

Scheiner3  was  the  first  to  demonstrate  its  function,  and  to  prove  that 
the  rays  are  focussed  upon  the  retina.  This  he  did  by  stripping  away  the 
solera  behind  and  allowing  a  beam  of  light  to  pass  through  the  eye  and 
fall  upon  a  screen. 

Possessing,  as  it  does,  the  property,  unique  among  optical  apparatus, 
of  changing  its  form  for  the  purpose  of  producing  greater  or  less  refraction 
of  the  rays  of  light,  it  was  thought  by  Leeuwenhoek  to  be  of  a  muscular 
character,  and  he  accordingly  named  it  the  musculus  crystallinus. 

It  is,  as  its  name  implies,  a  transparent  lenticular  body,  biconvex  in 
shape,  with  a  circular  margin,  situated  between  the  aqueous  and  vitreous 
chambers,  touching  the  iris  on  the  one  side  and  the  vitreous  body  on  the 
other,  its  anterior  surface  being  applied  to  the  pupillary  orifice,  and  its  pos- 
terior surface  fitting  into  a  saucer-shaped  depression  of  the  vitreous.  It  is 
surrounded  and  held  in  place  by  a  suspensory  apparatus  known  as  the  zonula, 
derived  from  the  hyaloid  membrane  of  the  vitreous  body.  The  distance  of 
its  rounded  edge  from  the  ciliary  processes  is  from  0.5  to  0.6  millimetre. 

Its  anterior  pole  is  distant  from  the  cornea  about  2.3  millimetres,  and 
its  posterior  pole  from  the  yellow  spot  15.6  millimetres.  For  rough  estima- 

1  From  L.  lens,  lentis,  a  lentil,  in  allusion  to  its  biconvex  shape. 

Syn. :  lens  crystallina ;  humor  crystallinus ;  corpus  crystallinum ;  cristallin,  F. ; 
Linse,  Krystalllinse,  G.  Some  of  the  older  anatomists  called  it  ffutta,  from  the  expression 
of  Celsus,  "  gutta  humoris,  ovi  albo  similis."  Hence  those  cases  of  amaurosis  in  which  the 
lens  is  not  affected  were  called  gutta  serena.  (Hyrtl.) 

2  Fr.  Maurolyci.     Photismi  de  lumine  et  umbra  ad  perspectivam  etradiorum  inciden- 
tiam  facientes.     Venetiis,  1575. 

8  Scheiner  (C.).     Oculus,  sive  fundamentum  opticum.    (Eniponti  [Innspruck],  If 


202 


THE  ANATOMY  OF  THE  EYEBALL. 


tions  it  may  be  stated  that  its  posterior  pole  is  nearly  at  the  junction  of  the 
anterior  and  middle  thirds  of  the  optical  axis. 

Tscherning l  has  shown  that  the  antero -posterior  axis  of  the  lens  does 
not  correspond  exactly  to  the  axis  of  the  eye,  the  deviation  being  such  as 
would  occur  if  the  structure  had  been  rotated  around  a  vertical  axis  out- 
wardly from  three  to  seven  degrees.  A  slight  rotation  may  also  be  observed 
in  some  cases  around  a  transverse  or  frontal  axis,  the  deviation  here  ranging 
between  zero  and  three  degrees. 

In  order  to  remove  the  lens  for  examination  it  is  necessary  to  cut  away 
the  cornea,  raise  the  iris,  and  carefully  snip  with  small  blunt  scissors  the 
suspensory  ligament  around  its  whole  circumference.  It  can  then  be  lifted 
out,  care  being  taken  to  detach  any  adherences  that  may  exist  between  it 
and  the  vitreous  body. 

The  lens  is  about  nine  millimetres  in  its  transverse  diameter,  varying  in 
size  somewhat  with  age  and  stature,  being  larger  in  the  old  and  in  very  tall 
persons.  Its  antero-posterior  diameter,  or  thickness,  varies  according  as  it 
is  accommodated  for  near  or  for  distant  vision,  as  do  also  the  curvatures 
of  the  anterior  and  posterior  surfaces.  Practically  these  may  be  stated  in 
round  numbers  as  follows : 

Distant  Vision.  Near  Vision. 

Millimetres.  Millimetres. 

Thickness 3.7  4 

Anterior  radius 10.  6 

Posterior  radius 6.  5 

In  distant  vision  the  anterior  radius  is  to  the  posterior  about  in  the  pro- 
portion of  three  to  two.  Since  the  invention  of  the  ophthalmometer  more 
minute  measurements  have  been  attempted.  The  following  are  those  of 
Woinow,2  and  are  probably  the  most  accurate  : 


Accommodated  for 

Distant  Objects. 

Near  Objects. 

Thickness  

3.0247-3.96269 
8.9452-10.2091 
9.1139-12.5804 
6.248-  7.1905 
6.06353-  8.0013 

3.5129-  4.4784 
5.1466-  6.8507 
7.3104-10.1731 
4.9714-  6.2817 
4.6941-  6.3792 

Radius  of  anterior  surface  in  young  persons  .... 
Radius  of  anterior  surface  in  old  persons  

Radius  of  posterior  surface  in  young  persons  .  .  . 
Radius  of  posterior  surface  in  old  persons  ... 

It  will  be  seen,  therefore,  that  the  curvature  of  the  anterior  surface  is 
much  more  affected  than  that  of  the  posterior  by  the  action  of  the  ciliary 
muscle.  It  should  be  remembered  that  the  action  of  that  muscle,  by  with- 
drawing the  tension  of  the  suspensory  ligament,  allows  the  lens  to  assume 
its  own  unmodified  shape.  The  length  of  a  meridian  is  about  twelve 
millimetres. 


1  Tscherning.     Sur  la  position  du  cristallin  de  1'ceil  humain. 
domadaires  dcs  seances  de  I'Academie  des  Sciences,  Paris,  1888. 

2  Woinow  (M.)     Ophthalmometrie.     Wien,  1871. 


Comptes  rendus  heb- 


THE    ANATOMY    OF   THE   EYEBALL.  203 

Attempts  have  been  made  to  classify  the  curved  surfaces  of  the  lens 
under  some  regular  form.  Kepler  thought  the  anterior  surface  to  be  that 
of  a  spheroid,  the  posterior  that  of  a  hyperboloid.  Chossat,  Vallee,  and 
Kratise  considered  both  surfaces  as  ellipsoidal,  formed  by  the  revolution 
of  an  ellipse  about  its  minor  axis.  Briicke  believed  the  anterior  surface  to 
be  ellipsoidal,  and  the  posterior  paraboloidal.  It  is  probable  that  neither 
curvature  is  perfectly  regular. 

The  weight  of  the  lens,  as  determined  by  Sappey l  from  eleven  speci- 
mens, is  from  .20  to  .25  gramme,  the  lowest  weighing  .201  gramme,  the 
highest  .252  gramme,  the  average  being  .218  gramme.  The  weight  increases 
slightly  with  age,  which  may  account  for  the  estimations  given  by  Vierordt,2 
who  states  the  weight  of  the  lens  as  ranging  from  .28  to  .29  gramme. 

Its  specific  gravity  is  greater  than  that  of  either  the  aqueous  or  the 
vitreous  humor,  being  stated  by  Nunneley 3  as  1121.  Consequently,  when- 
ever the  lens  is  displaced  by  the  rupture  of  the  suspensory  ligament  it  falls 
to  the  bottom  of  the  eyeball. 

Monoyer4  gives  its  volume  as  one-fourth  of  a  cubic  centimetre. 

The  lens  is  so  nearly  transparent  that  in  early  life  it  appears  to  have 
no  color,  or,  if  any,  a  very  slight  bluish  tinge,  when  it  is  seen  by  oblique 
illumination.  With  advance  of  years  it  assumes  a  yellowish  hue,  or  may 
present  a  slight  trace  of  opalescence,  this  latter  being  one  of  the  causes 
which  occasion  the  slight  grayish  or  grayish-green  dimness  that  appears  in 
the  pupils  of  aged  people  when  compared  with  the  intense  black  of  youth. 
The  coloration  affects  first  the  central  portion  of  the  lens,  and  gradually 
extends  to  the  cortical  part.  In  some  cases  it  may  be  sufficient  to  affect 
the  appreciation  of  colors.  It  is  said  of  the  painter  Mul ready  that  in  all 
the  compositions  executed  by  him  during  the  later  years  of  his  life  he  had 
a  false  scale  of  color,  which  greatly  reduced  the  yellow  tints  and  increased 
the  blue  ones,  so  that  in  order  to  judge  properly  of  his  works  it  is  necessary 
to  view  them  through  a  glass  slightly  tinted  with  yellow. 

When  exposed  to  the  air  the  lens  becomes  dulled  in  lustre  and  slightly 
wrinkled,  and  also  loses  a  portion  of  its  transparency.  Immersed  in  water 
it  swells,  becomes  milky,  and  bursts  its  capsule,  which,  after  letting  out 
a  small  amount  of  opaline  fluid,  again  contracts  closely  upon  it.  All 
agents  that  coagulate  albumin,  such  as  silver  nitrate,  gold  chloride,  osmic 
acid,  etc.,  harden  it. 

When  pressed  between  the  fingers  or  by  a  blunt  instrument  the  lens 
readily  changes  its  figure,  springing  back  to  its  original  shape  as  soon  as 
the  pressure  is  removed.  In  fact,  it  behaves  like  a  soft  material  enclosed 

1  Traite  d'anatomie  descriptive. 

2  Vierordt.     Daten  und  Tabellen  fur  Medizirier,  S.  106. 

3  Nunneley,  in  the  Quarterly  Journal  of  Microscopical  Science,  1868,  p.  188.     Davy 
(Transactions  of  the  Medico-Chirurgical  Society  of  Edinburgh,  1829,  iii.  436)  states  the 
specific  gravity  as  1.100,  and  Chenevix  (Annales  de  chimie,  xlviii.  74)  fixes  it  at  1079. 

4  Nouveau  dictionnaire  de  medecine  et  chirurgie  pratique.     Paris,  1869,  x  259. 


204  THE  ANATOMY  OF  THE  EYEBALL. 

in  an  elastic  envelope.  It  is  easy  to  demonstrate  that  this  is  really  the  case 
by  slightly  staining  the  envelope  with  dilute  osmic  acid,  immersing  it  in 
water,  and  scratching  it  open  with  a  small  instrument,  when  the  substance 
of  the  lens  will  peel  out,  leaving  the  external  envelope  or  capsule l — a  deli- 
cate, shell-like  membrane — floating  on  the  water. 

It  can  be  easily  seen  by  the  unassisted  vision  that  this  membrane  is 
thicker  in  front  than  behind,  and  that  the  two  parts  meet  at  a  rounded 
edge,  the  equator  of  the  lens.  The  anterior  portion  is  somewhat  more  than 
twice  as  thick  as  the  posterior  part,  and  consequently  is  more  prone  to  take 
on  pathological  changes.  This  difference  in  the  two  parts  has  led  anato- 
mists to  apply  to  them  the  names  of  anterior  capsule 2  and  posterior  capsule? 
These  names  are  not  to  be  commended,  as  they  seem  to  imply  that  there 
are  two  distinct  investments,  when  in  fact  there  is  no  clear  demarcation 
between  the  two. 

The  elasticity  and  strength  of  the  capsule  are  considerable.  It  is  also 
very  brittle,  and  if  scratched  or  cut  by  a  sharp  instrument  readily  tears, 
breaking  like  thin  glass  along  irregular  angular  lines,  that  are  usually  per- 
pendicular to  the  surface.  This  has  led  some  writers  to  describe  the  capsule 
as  the  vitreous  membrane.  Its  cut  edges  always  roll  outward,  so  that  the 
outer  surface  lies  innermost  in  the  roll,  behaving  in  this  respect  like  the 
internal  limiting  layer  of  the  cornea,  with  which,  indeed,  it  presents  many 
analogies.  It  is  entirely  structureless,  but  is  readily  permeable  by  coloring 
matters.  Near  the  equator  of  the  lens  the  zonula  or  suspensory  ligament 
is  attached,  there  being  no  visible  line  of  union.  The  capsule  is  remark- 
ably resistant  to  reagents,  as  it  still  retains  its  transparency  after  treatment 
with  boiling  water,  acids,  and  alcohol.  It  also  resists  putrefactive  changes 
for  a  considerable  time.  Bowman  has  pointed  out  that  during  life  its 
immunity  from  change  is  by  no  means  very  great,  any  injury  to  the  capsule 
being  almost  certainly  followed  by  an  opacity.  Special  operative  precau- 
tions are  necessary  to  prevent  its  forming  an  obstruction  to  light  after  the 
removal  of  the  substance  of  the  lens  in  the  needle  operation  for  cataract. 

It  is  well  known  that  the  substance  of  the  lens  is  developed  by  an  in- 
vagi nation  of  the  ectoderm,  and  that  it  may,  therefore,  be  considered  as 
epithelial  in  character.  The  capsule,  however,  is  probably  of  mesodermic 
origin.  This  matter  is  more  fully  discussed  in  the  chapters  on  the  develop- 
ment and  the  microscopical  anatomy  of  the  eye. 

The  main  body  of  the  lens  is  of  a  much  less  firm  consistency  than  the 
capsule.  At  an  early  age  this  consistency  is  nearly  the  same  throughout 
the  whole  substance,  but  in  advanced  years  there  is  a  difference  between  the 
deeper  or  more  central  portions  of  the  lens  and  those  at  its  exterior.  There 
is  no  abrupt  transition  between  these  parts,  and  the  condensation  appears 
to  be  due  entirely  to  the  absence  of  metabolic  changes  in  the  interior.  As 

1  Syn. :  capsula  lentis. 

2  Syn. :  cristalloide  anterieure,  F. 

3  Syn.  :  cristalloide  posterieure,  F. 


FIG.  66. 


Concentric  layers  of  lens-stars. 


FIG.  67. 
A 


Appearance  of  fibres  in  the  adult  lens  viewed  laterally  (A),  anteriorly  (#),  and  posteriorly  (C). 


THE  ANATOMY  OF  THE  EYEBALL. 

the  adult  lens  has  no  blood-vessels,  it  is  nourished  entirely  by  intercellular 
transmission  of  fluids,  and  the  condensing  process  consists  mainly  in  a  loss 
of  water.  This  hardening  commences  in  childhood,  but  it  is  not  until 
adult  years  that  a  well-marked  nucleus  can  be  distinguished.  In  old  age 
almost  the  entire  lens  becomes  condensed,  so  that  any  alterations  in  its 
form  are  much  more  difficult.  Hence  arises  the  defect  in  the  power  of 
accommodation  so  well  marked  in  the  eyes  of  persons  past  middle  life. 
The  central  condensed  portion  is  usually  called  the  nucleus l  of  the  lens, 
while  the  external  softer  part  is  termed  the  cortical  substance.2 

The  change  in  the  density  of  the  lens  produces  other  results.  For 
example,  as  the  nucleus  reflects  more  light  than  the  cortical  portion,  the 
lens  can  more  readily  be  seen  in  the  eyes  of  older  people,  and  the  jet  black 
of  the  pupil  is  slightly  dimmed,  as  above  mentioned.  Again,  the  increased 
density  of  the  lens  affects  its  power  of  refracting  the  rays  of  light. 

The  following  table  shows  the  refractive  power  of  the  lens  as  determined 
by  different  observers  : 


Lens. 

Observer. 

Cortex. 

Middle 
Layer. 

Nucleus. 

Total. 

Water. 

Young  3           .        ....        .... 

1.4026 

1  4385 

1.383 

1.395 

1.420 

1.3368 

Brewster5       

1.3767 

1.3786 

1.3896 

1.3368 

~W.  Krause6  

1.4053 

1.4294 

1.4541 

1.3342 

Helmholtz7    

1.4189 

1.4467 

1.3354 

S.  Fleischer8     

1.4371 

1.3340 

TVoinow  9   ... 

1.3968 

1.4216 

1.4351 

1.4387 

1.3354 

Aubert10         

1.3967 

1.4067 

1.4093 

The  following  optical  constants  of  the  lens  were  determined  by  Helm- 
holtz : 

Millimetres.  Millimetres. 

Focal  length 45.144  47.435 

Distance  of  the  first  principal  point  from  the  anterior 

surface  .  ' 2.268  2.810 

Distance  of  the  second  principal  point  from  the  ante- 
rior surface  .  1.646  1.499 


1  Syn. :  nucleus  lentis. 

1  Syn.  :  substantia  corticalis. 

3  Philosophical  Transactions  of  the  Koyal  Society  of  London,  1801,  pt.  i.  p.  23. 

4  Bulletin  de  la  Societe  philomathique  de  Paris,  1818,  p.  95. 

5  Edinburgh  Philosophical  Journal,  1819,  i.  43. 

6  Die  Brechungsindices  des  menschlichen  Auges.     Hannover,  1855. 

7  Handbuch  der  physiologischen  Optik,  1867,  Ss.  78,  84. 

8  Neue  Bestimmungen  der  Brechungsexponenten  der  durchsichtigen,  fliissigen  Medien 
des  Auges.     Jena,  1872,  S.  26. 

9  Klinische  Monatsblatter  fur  Augenheilkunde,  1874,  xii.  407. 

10  Grafe  und  Siimisch,  Handbuch  der  gesammten  Augenheilkunde,  1876,  ii.  409. 


206  THE  ANATOMY  OF  THE  EYEBALL. 

After  careful  consideration  of  the  optical  properties  of  the  lens,  he 
concludes : 

1.  That  the  focal  distances  are  less  than  they  would  be  if  its  entire  mass 
had  the  index  of  refraction  of  its  nucleus. 

2.  The  distance  which  separates  the  principal  points  is  less  in  the  crys- 
talline lens  than  in  a  lens  of  glass  having  the  same  form  and  whose  refract- 
ing power  is  equal  to  that  of  the  nucleus. 

3.  A  lens  having  the  form  of  the  crystalline  lens  and  the  index  of  re- 
fraction of  its  nucleus  would  have  its  principal  points  distant  from  each 
other  about  one-fourth  millimetre. 

Some  interesting  considerations  may  be  raised  with  .regard  to  the  effect 
on  the  lens  of  certain  rays  at  the  extremities  of  the  spectrum.  It  is  well 
known  that  beyond  the  limits  of  vision  there  are  rays  of  energy  extending 
on  the  one  hand  above  the  violet  rays  (ultra-violet)  and  on  the  other  below 
the  red  rays  (infra-red).  The  first  of  these  especially  effect  chemical  action, 
the  latter  verge  towards  heat-rays. 

Are  these  rays  stopped  in  any  way  by  the  media  of  the  eye,  or  do  they 
exert  an  appreciable  influence  upon  the  retina  ? 

Certain  substances,  such  as  solutions  of  quinine,  guaiacum,  etc.,  have  the 
power  of  absorbing  some  of  the  ultra-violet  rays  and  causing  them  to  ap- 
pear as  a  bluish  opalescence.  This  property  of  these  substances  is  known 
as  fluorescence.  It  is  possessed  to  some  degree  by  the  lens.  If  resin  of 
guaiacum,  properly  prepared  in  the  dark,  is  exposed  to  diffused  light,  it  ap- 
pears blue.  If,  however,  the  light  be  passed  through  the  lens  of  a  bullock's 
eye,  the  resin  appears  greenish  yellow.  This  shows  that  the  blue  rays  are 
absorbed  by  the  lens. 

At  the  other  end  of  the  spectrum  it  is  also  found  that  a  considerable 
number  of  rays  are  absorbed.  It  is  well  known  that  glass-blowers,  stokers, 
and  others  are  able  to  face  very  intense  heat  without  apparently  injuring  the 
eyes.  Fritz  and  Jansen  have  shown  that  at  least  thirteen  per  cent,  of  the 
heat-rays  are  absorbed  by  the  lens. 

It  may  well  be  asked  whether  such  a  powerful  force  as  this  can  be 
arrested  by  the  tissues  of  the  body  without  producing  any  marked  result. 
Probably  a  prolonged  exposure  to  intense  heat  does  lead  to  degenerative 
changes  in  the  lens.  It  is  said  that  cataracts  are  much  more  frequently 
found  in  persons  who  have  been  subjected  to  such  exposure. 

Within  a  short  time  after  death  a  small  amount  of  fluid  appears  among 
the  fibres  of  the  lens  or  between  those  fibres  and  the  capsule.  This  fluid, 
formerly  believed  to  be  present  during  life,  and  called  the  liquor  Morgagni, 
is  now  known  to  be  due  to  post-mortem  changes,  and  is  either  an  infiltra- 
tion of  the  aqueous  humor  or  a  product  of  decomposition. 

The  substance  of  the  lens  offers  a  great  contrast  to  its  capsule  as  regards 
the  effect  of  physical  and  chemical  reagents.  When  exposed  to  the  air  it 
rapidly  loses  its  moisture,  becomes  dry  and  brittle,  and  assumes  a  light  straw 
color.  Very  cold  water  causes  it  to  lose  its  transparency,  which  is  restored 


THE  ANATOMY  OF  THE  EYEBALL. 


207 


by  placing  it  in  water  of  the  temperature  of  the  air.  Boiling  water  renders 
it  permanently  opaque,  as  do  also  alcohol  and  acids. 

The  structure  of  the  body  of  the  lens  will  be  only  briefly  referred  to 
here.  It  is  composed  of  two  layers  of  epithelium  lengthened  out  into 
long,  ribbon-like  fibres  which  are  so  arranged  as  to  pass  from  the  anterior 
to  the  posterior  surfaces,  leaving  on  each  side  certain  lines  of  implantation 
or  dehiscence,  the  so-called  lens- stars.  When  treated  by  appropriate  re- 
agents the  lens  may  be  made  to  cleave  along  these  lines  in  concentric  layers. 
(See  Fig.  66.)  The  lens-stars,  which  are  different  on  the  anterior  and 
posterior  surfaces  of  the  lens,  are  occasionally  sufficiently  marked  to  be 
observed  by  the  ophthalmoscope  or  by  the  subject  himself  as  an  entoptic 
phenomenon.  They  are  simpler  in  the  young  than  in  the  adult,  giving  in 
front  the  appearance  of  a  Y  turned  upside  down,  behind  of  the  same  letter 
in  its  erect  position.  The  adult  appearance  is  shown  in  Fig.  67. 

The  lens  is  early  to  develop.  It  is  at  first  nearly  spherical  in  form,  and 
does  not  attain  its  adult  shape  until  near  puberty.  Collins1  found  the  fol- 
lowing measurements  in  foetal  eyes  : 

Diameters  of  Foetal  Eyes  and  of  their  Lenses. 


Age. 

Diameters  of  Eyeball. 

Diameters  of  Lens. 

Antero- 
posterior. 

Lateral. 

Vertical. 

Antero- 
posterior. 

Transverse. 

Four  months  

Millimetres. 
8.1 
11.75 
12.5 
14.3 
16.75 
24.3 

Millimetres. 
7.8 
11.5 
12. 
13.2 
16. 
23.6 

Millimetres. 
7.5 
10.5 
11.1 
12.6 
15.3 
23.4 

Millimetres. 
2.8 
3.5 
3.8 
4. 
4.3 
3.7 

Millimetres. 
3.3 
4. 
4.5 
5. 
6.75 
9. 

Five  months  .    ,    .    .    . 

Six  months         .        .... 

Seven  months    

Nine  months      

Adult  (Merkel)  

From  the  above  it  will  be  seen  that  during  foetal  life  the  lens  is  much 
larger  in  proportion  to  the  eyeball  than  it  is  afterwards.  At  the  fourth 
month  the  ciliary  processes  touch  the  lens,  and  it  is  by  the  subsequent  growth 
of  the  various  parts  that  they  assume  their  final  position. 

It  should  be  remembered  that  this  rapid  development  of  the  lens  during 
foetal  life  is  rendered  possible  by  the  nutrition  afforded  by  the  net-work  of 
blood-vessels  forming  the  tunica  vasculosa  lentis  already  referred  to.  This 
disappears  at  birth  or  before,  and  the  growth  of  the  lens  is  thenceforward 
stationary,  it  merely  changing  its  shape  presumably  in  accordance  with  the 
growth  of  the  other  parts  of  the  eye  and  the  various  strains  to  which  it  is 
subjected. 

What  nutritive  fluid  the  lens  requires  is  probably  supplied  from  the 
ciliary  body.  It  enters  the  lens  near  the  region  of  the  equator,  and  circu- 

1  Collins  (E.  Treacher).  Lectures  on  the  Anatomy  and  Physiology  of  the  Eye, 
Lancet,  London,  December  8  and  December  22,  1894. 


208  THE  ANATOMY  OF  THE  EYEBALL. 

lates  in  the  interfibrillary  clefts.  Fuchs  cites  a  case  in  which  a  small  frag- 
ment of  iron  penetrated  the  lens,  followed  by  an  opacity  that  assumed  a 
yellowish-green  color.  Then  a  circle  of  rust-colored  points  appeared  corre- 
sponding nearly  to  the  margin  of  the  dilated  pupil.  He  believed  that  the 
liquid  circulating  in  the  lens  was  removed,  and  discolored  the  lens-capsule 
in  this  situation. 

THE   VITREOUS   BODY.1 

The  space  between  the  crystalline  lens  and  the  retina,  constituting  about 
two-thirds  of  the  area  of  the  cavity  of  the  eyeball,  is  filled  with  a  trans- 
parent, watery  jelly,  enclosed,  like  the  lens,  in  a  delicate  capsule.  The 
entire  structure  is  known  as  the  vitreous  body.  It  is  by  far  the  largest  of 
the  transparent  media  of  the  eye.  Its  shape,  almost  wholly  determined  by 
that  of  the  cavity  which  contains  it,  is  that  of  an  oblate  spheroid,  hollowed 
out  in  front  by  a  saucer-like  depression,  the  hyaloid  fossa,2  into  which  the 
lens  fits.  It  is  perforated  by  a  central  lymph-space,  the  hyaloid  canal,3 
that  runs  somewhat  eccentrically  from  the  optic  disk  to  the  lens.  (See 
Figs.  3  and  65.) 

The  mechanical  function  of  this  structure  is  of  considerable  importance. 
If  for  any  cause  it  is  permanently  decreased  in  volume,  the  lens  is  no 
longer  held  firmly  in  position,  but  becomes  tremulous,  and  the  retina, 
ceasing  to  be  supported  in  front,  loses  its  close  attachment  to  the  subjacent 
layers. 

The  capsule  of 'the  vitreous  body,  called  the  hyaloid  membrane*  is  a 
very  delicate  investment  in  contact  behind  with  the  internal  limiting  mem- 
brane of  the  retina.  It  is  thickened  in  front,  where  it  forms  the  suspen- 
sory ligament  of  the  lens,  and  is  again  thinned  to  extreme  tenuity  in  the 
hyaloid  fossa.  It  has  no  demonstrable  structure,  and  its  very  existence  is 
denied  by  many  eminent  authorities,  among  whom  may  be  mentioned 
Henle,5  Iwanoff,6  and  Merkel.7  These  observers  hold  that  the  appearance 
of  a  membrane,  which  is  unquestionably  present  in  specimens  suitably 
prepared  for  microscopical  examination,  is  due  to  the  reagents  that  are 

1  From  L.  vitreus,  -a,  -um,  glassy,  alluding  to  its  glassy  and  hyaline  character. 

Syn. :  corpus  vitreum  ;  vitreum ;  humor  vitreus ;  vitreous  humor ;  vitreous  ;  corpus 
hyaloideum ;  Glaskdrper,  G.  The  term  vitreous  humor  is  often  used  for  the  fluid  portion 
only,  as  distinguished  from  the  capsule. 

2  Syn.  :  fossa  hyaloidea ;  fossa  patellaris ;  fossa  lenticularis. 

3  Syn.  :  canalis  hyaloideus;    canalis  Cloqueti ;  Cloquet's  canal  (mentioned  by  Jules 
Cloquet,  an  anatomist  of  Paris,  1787-1840,  in  his  "  Memoire  sur  la  membrane  pupillaire 
et  sur  la  formation  du  petit  cercle  arteriel  de  1'iris,"  Paris,  1818)  ;  canalis  Stillingi ;  canal 
of  Stilling  (being  first  fully  described  by  J.  Stilling  in  his  article  "  Zur  Theorie  des  Glau- 
coms,"  Archiv  f.  Ophth.,  Berlin,  1868,  xiv.,  Abth.  iii.,  259. 

4  From  tiaXof,  glass,  and  eMof,  resemblance. 

Syn. :  membrana  hyaloidea ;  tunica  hyaloidea ;  hyaloidea ;  hyaloidea  interna  ;  tunica 
arachnoidea ;  membrana  vitrea. 

5  Eingeweidelehre,  p.  669. 

6  Strieker's  Handbook,  American  edition,  p.  876. 

7  Topographische  Anatomie,  1887,  i.  266. 


FIG.  68. 


Attachment  of  the  suspensory  ligament  of  the  lens.  (Testut.)— 1,  posterior  surface  of  the  lens; 
2,  its  equator;  3,  the  suspensory  ligament  improperly  shown  as  a  continuous  membrane;  4,  fibres 
passing  forward  to  the  anterior  portion  of  the  capsule;  5,  fibres  passing  to  the  posterior  portion  of  the 
capsule ;  6,  6,  the  zonular  spaces  or  segments  of  the  canal  of  Petit. 


FIG.  69. 


The  suspensory  ligament  of  the  lens.    (Testut.)-;!,  non-injected :  B,  injected :  1,  lens;  2,  posterior 
portion  of  the  ligament;  3,  anterior  portion;  4,  its  insertion  into  the  lens  in  B,  showin 
spaces  inflated  with  air. 


THE  ANATOMY  OF  THE  EYEBALL. 

necessarily  employed  in  the  preparation.  As  the  entire  vitreous  body  may 
be  removed  from  its  chamber  and  lie  as  a  self-limited  mass  from  which 
fluid  gradually  drains  away,  it  seems  unreasonable  to  doubt  the  existence 
of  this  envelope. 

Behind,  at  the  optic  disk,  where  the  retinal  vessels  enter,  and  where  in 
the  foetus  the  vessels  supplying  the  vitreous  body  are  given  off,  the  hyaloid 
•membrane  is  somewhat  firmly  attached.  At  the  ciliary  part  of  the  retina 
also  its  adhesion  is  intimate.  Along  the  corona  ciliaris,  however,  while  the 
membrane  becomes  thicker,  it  is  not  so  adherent  to  the  adjacent  layers. 
Though  intimately  attached  to  the  apices  of  the  ciliary  processes,  it  passes 
over  the  valleys  between  those  processes  with  their  secondary  folds,  thus 
leaving  a  series  of  radial  spaces  that  communicate  in  front  with  the  pos- 
terior aqueous  chamber  and  contain  aqueous  humor.  These  are  known  as 
the  recesses  of  the  posterior  chamber.1 

Leaving  the  ciliary  processes,  the  hyaloid  membrane  is  strengthened  by 
the  fibres  belonging  to  the  suspensory  ligament  of  the  lens.2  These  fibres 
do  not  form  a  continuous  sheet,  as  was  formerly  supposed,  but  leave  the 
neighborhood  of  the  ora  serrata  in  several  sets,  some  of  which  are  inserted 
on  the  lens  near  the  equator,  others  in  front,  and  still  others  behind  it. 
A  more  complete  description  of  this  arrangement  is  given  in  the  article  on 
the  microscopic  anatomy  of  the  eye.  The  insertion  of  the  fibres  is  shown 
in  an  imperfect  manner  in  Fig.  68.  In  this  figure  the  suspensory  ligament 
appears  as  a  continuous  sheet,  which  it  probably  is  not,  there  being  many 
interspaces  between  the  fibres  by  which  fluid  can  pass  between  the  aqueous 
and  vitreous  chambers. 

The  regularity  of  the  insertion  of  the  fibres  about  the  equator  of  the 
lens  is  such  that  it  is  possible  with  a  very  fine  canula  to  inject  air  into 
a  series  of  spaces  between  the  zonula  and  the  hyaloid  membrane,  where  it 
remains,  showing  a  series  of  bullae.  This  experiment  was  first  performed 
by  Petit,  and  he  naturally  supposed  that  it  demonstrated  the  existence  of 
a  canal  extending  about  the  lens  in  the  folds  of  the  suspensory  ligament. 
He  named  it  the  canal  godronne,  or  ruffed  canal,  from  its  similarity  in 
appearance  when  injected  to  a  certain  kind  of  silverware  (vaisselle  godronnee) 
ornamented  in  this  manner.  The  appearance  of  the  non-injected  and  that 
of  the  injected  ligament  are  shown  in  Fig.  69.  This  supposed  passage  is 
usually  called  the  canal  of  Petit*  The  imperfect  chambers  in  which  the 

1  Syn. :  recessus  camerce  posterioris ;  spaces  of  Kuhnt,  being  described  by  him  in  his 
article  "  Ueber  einige  Altersveranderungen  im  menschlichen  Auge,"  Ber.  ii.  d.  Vereamral. 
d.  ophth.  Gesellsch.,  Stuttgart,  1881,  xiii.  38. 

2  Syn.  :  ligamentum  suspensoria  lentis ;  zonula  Zinnii;  zone  of  Zinn,  for  J.  G.  Zinn, 
who  first  described  it  in  his   "  Descriptio  anatomica  oculi  humani,"  Getting®,    1756; 
zonula;  ciliary  zone;  orbiculus  capsulo-ciliaris ;  lamina  ciliaris. 

3  From  Francois  Pourfour  de  Petit,  oculist  at  Paris,  1664-1741.     See  Mem.  de  1'acad. 
de  Paris,  1723,  p.  54. 

Syn.  :  canalis  Petiii ;  circulus  Petiti ;  camera  tertia  aquosa ;  post-zonular  lymphatic 
space. 

VOL.  I.— 14 


210 


THE  ANATOMY  OF  THE  EYEBALL. 


air  is  engaged  during  the  experiment  are  sometimes  called  the  zonular 
spaces.1  It  is  also  possible  to  inject  a  viscous  material  like  albumin  be- 
tween the  anterior  and  the  posterior  fibres  of  the  zonula,  making  an  arti- 
ficial passage  known  as  the  canal  of  Hannover. 

Most  anatomists  consider  that  the  hyaloid  membrane  blends  completely 
in  front  with  the  capsule  of  the  lens,  which,  in  fact,  represents  it;  but 
Stuart 2  describes  it  as  continuous  over  the  hyaloid  fossa. 

FIG.  70. 


Lymphatic  spaces  of  the  eyeball.  (Fuchs.)— 1,  sinus  venosus ;  2, 3,  Tenon's  capsule ;  4,  perichorioidal 
space;  5,  hyaloid  canal;  6,  perivascular  space  about  vorticose  veins;  7,  Tenon's  space;  8,  supra- 
vaginal  space ;  9,  intervaginal  space. 

The  hyaloid  canal  is  a  passage,  some  two  millimetres  in  width,  that 
directly  traverses  the  substance  of  the  vitreous,  being  the  vestige  of  a  peri- 
vascular  space  that  in  foetal  life  surrounded  the  hyaloid  artery.  In  front 
of  the  optic  disk  it  enlarges  to  a  space  of  about  the  same  diameter  as  the 
disk,  known  as  the  area  Martegiani.3  A  similar  enlargement  behind  the 
lens  is  called  the  post-lenticular  space*  (See  Fig.  65.)  These  lymph- 

1  Syn.  :  spatia  zonularia. 

J  Stuart  (T.  P.  A.).  On  a  Membrane  limiting  the  Fossa  Patellaris  of  the  Corpus 
Vitreum.  Proceedings  of  the  Koyal  Soc.,  Lond.,  1891,  xlix. 

8  From  Francesco  Martegiani,  who  described  it  in  his  "  Novae  observationes  de  oculo 
humano,"  Napoli,  1814,  p.  19. 

*  Berger.    Anatomie  normale  et  pathologique  de  1'oeil. 


THE  ANATOMY  OF  THE  EYEBALL. 


211 


passages  communicate  in  front  by  means  of  the  zonular  spaces  with  the 
anterior  chamber;  behind  they  are  continuous  with  the  intervaginal  sna« 
of  the  optic  nerve.  (See  Fig.  70.)  Coloring  matter  injected  into  theT 
tenor  chamber  passes  into  the  hyaloid  canal  (Michel),  and  from  thence 
along  the  nerve  (Schwalbe).  Remains  of  the  hyaloid  artery  may  usually 
be  found  in  the  canal,  even  in  the  adult. 

The  substance  of  the  vitreous,  as  distinguished  from  its  capsule  is  com- 
posed of  a  large  amount  of  watery  fluid,  the  vitreous  humor,1  and  from  one 
and  a  half  to  two  per  cent,  of  structural  material,  known  as  the  stroma  of 
the  vitreous.2 

It  is  a  perfectly  transparent  material,  having  a  refractive  index  about 
that  of  distilled  water.3  It  is  very  liable  to  be  affected  by  materials  absorbed 
from  the  blood ;  for  example,  it  may  be  colored  orange  by  the  administra- 
tion of  madder,  and  it  may  be 

tinged    by    the    bile-pigments  FIG.  71. 

during  an  attack  of  jaundice. 
Its  weight  is  from  6.7  to  8.3 
grammes,  and  its  specific 
gravity,  according  to  Cheuevix, 
is  1.005. 

When  freshly  removed  from 
the  eye  the  vitreous  substance 
has  a  somewhat  sirupy  consist- 
ency; but  placed  upon  the 
meshea  of  a  sieve  its  watery 
constituents  drain  away,  leaving 
a  slight  residue  that  represents 
its  structural  elements. 

A  long  controversy  has  been 
waged  with  regard  to  the  inti- 
mate structure  of  the  vitreous. 
The  entire  substance  appears  to 
be  separated  into  compartments  by  means  of  septa  which  at  the  periphery 
are  concentric,  or  nearly  so,  while  towards  the  centre  they  become  radial. 
(See  Fig.  71.)  The  number  of  radii  is  differently  estimated  by  different 
observers,  Schwalbe  counting  about  thirty-seven,  while  Hannover  reckons 
them  at  one  hundred  and  eighty.  It  should  be  stated  that  the  existence  of 
a  real  structure  in  the  vitreous  is  doubted  by  many  competent  observers. 

Certain  connective-tissue  cells  and  leucocytes  are  found  in  small  num- 
bers within  the  vitreous.  These  may  occasionally  give  rise  to  those  entoptic 
phenomena  known  as  muscse  volitantes,  or  floating  specks.  Crystals  of 

1  Syn.  :  humor  vitreus  ;  vitrina  ocuii. 

2  Syn.  :  stroma  vitrei. 

*  Krause  gives  its  refractive  index  as  1.33485,  that  of  water  being  1.3342.  Helmholtz 
fixes  it  at  1.33382,  water  being  1.33365. 


Diagram  of  supposed  structure  of  the  vitreous  body 
as  shown  by  an  equatorial  section.  (Testut.)— 1, 1,  radial 
divisions ;  2,  2,  concentric  divisions.  The  hyaloid  canal 
is  seen  at  the  centre. 


212  THE  ANATOMY  OF  THE  EYEBALL. 

cholesterin  are  occasionally  found  floating  in  it,  and  present  a  peculiar 
glittering  appearance  under  the  ophthalmoscope. 

A  certain  amount  of  the  vitreous  may  be  removed  without  seriously 
injuring  vision,  and  it  seems  to  be  rapidly  renewed.  Operative  interference 
with  it  is,  however,  usually  to  be  avoided.  It  may  be  wounded  during  the 
extraction  of  cataract  by  cutting  through  the  suspensory  ligament,  and  may 
then  drain  away  and  cause  an  embarrassing  complication. 

In  reaction  the  vitreous  humor  is  alkaline.  Half  of  its  solids,  which 
are  but  from  1.69  to  1.98  per  cent,  of  the  whole,  are  chloride  of  sodium 
and  carbonate  of  sodium.  Its  composition  seems,  in  fact,  to  differ  from 
that  of  the  aqueous  humor  mainly  in  a  slight  excess  of  albumin. 

Numerous  experiments  have  been  made  to  determine  the  source  of  the 
secretion  of  the  vitreous  humor.  Those  of  Deutschmaun,  which  show  its 
absorption  after  removal  of  the  ciliary  processes,  have  already  been  men- 
tioned. Its  greater  richness  in  albumin  may  probably  be  accounted  for  by 
its  less  rapid  renewal.  Experiments  made  by  injecting  into  the  eyes  of 
animals  ferrocyanide  of  potassium  or  fluoresceiue,  although  somewhat  con- 
tradictory in  their  results,  seem  to  show,  on  the  whole,  that  fluid  passes 
from  the  ciliary  processes  through  the  zonula  into  the  vitreous  chamber.1 

Stilling 2  was  of  the  opinion  that  the  vitreous  humor  was  removed  by 
way  of  the  optic  disk  and  along  the  optic  nerve.  His  views  are  not  sup- 
ported, however,  by  recent  observers,  and  it  seems  probable  that  Leber  is 
correct  when  he  states  that  the  only  well-established  channel  for  the  exit 
of  fluids  from  the  eye  is  that  at  the  angle  of  the  anterior  chamber,  and 
that  if  any  fluid  leaves  along  the  nerve  it  must  be  very  slight  in  amount. 

During  foetal  life  the  vitreous  body  is  well  supplied  with  blood-vessels 
by  the  hyaloid  artery,  a  branch  of  the  central  artery  of  the  retina  that  runs 
in  the  hyaloid  canal  and  gives  off  numerous  branches  to  the  substance. 
These  all  degenerate,  and  appear  only  as  vestiges  during  adult  life.  As  is 

1  Upon  this  subject  see  the  following : 

Ulrich  (R.).  Neue  TJntersuchungen  iiber  die  Lymphstromung  im  Auge.  Arch.  f. 
Augenheilk.,  1889,  xx.  270. 

Leplat  (L.).  Etudes  sur  la  nutrition  du  corps  vitre.  Ann.  d'oculistique,  1887, 
xcviii.  89. 

Gifford.  "Weitere  Versuche  iiber  die  Lymphstrome  und  Lymphwege  des  Auges. 
Arch.  f.  Augenheilk.,  1893,  xxvi.  308. 

Nicati.  La  glande  de  1'humeur  aqueuse.  Archives  d'ophtalmologie,  1890,  1891,  x. 
481,  xi.  24,  152. 

Pfliiger.     Zur  Lymphcirkulation  im  Auge.     Arch.  f.  Augenheilk.,  1894,  xxviii.  351. 

Schick  (H.).  Experimentelle  Beitrage  zur  Lehre  von  Fliissigkeitswechsel  im  Auge. 
Inaug.  Diss.  von  Marburg,  1885. 

Ehrenthal  (W.).  Kritisches  und  Experimentelles  zur  Lehre  vom  Fliissigkeitswechsel 
im  Auge.  Inaug.  Diss.  von  Konigsberg,  1887. 

Panas.     Etudes  sur  la  nutrition  de  1'oeil.     Arch,  d'ophtal.,  1887,  vii.  97. 

Rampoldi.  Sul  passagio  sperimentale  della  fluoresceina  nella  camera  anteriore.  Ann. 
di  ottalm.,  1887,  xvi.  250. 

'Stilling  (J.).     Ueber  die  Pathogenese  des  Glaucoms.     Arch.  f.  Augenheilk.,  1886, 


THE    ANATOMY    OF   THE   EYEBALL.  213 

well  remarked  by  Rauber,  the  arteries  supplying  this  region  tend  towards 
degeneration,  in  contrast  with  those  of  the  retina,  which  tend  towards 
greater  development. 

Since  the  adult  vitreous  possesses  no  proper  vessels  of  its  own,  it  depends 
upon  those  of  the  retina  or  of  the  chorioidal  tract  for  its  nutrition.  Hence 
it  is  almost  always  involved  in  retiuitis  or  chorioiditis. 

THE   INTRA-ORBITAL   PORTION   OF   THE   OPTIC   NERVE. 

When  we  compare  the  optic  nerve l  with  the  spinal  nerves  and  most  of 
the  cranial  ones,  such  striking  diiferences  are  disclosed  that  it  becomes  evi- 
dent that  if  the  nomenclature  were  to  be  revised  on  a  strictly  morphological 
basis,  we  should  not  class  the  structure  as  a  nerve.  It  is  formed  in  the 
course  of  development  as  an  outgrowth  from  the  sides  of  the  third  ven- 
tricle, being  the  pedicle  of  the  optic  vesicle,  and  is,  therefore,  a  commissural 
tract,  like  the  peduncles  of  the  cerebellum  or  the  olfactory  tract.  This  view 
is  also  sustained  by  its  intimate  structure,  as  it  does  not,  like  nerves,  con- 
tain well-developed  nerve-fibres  ensheathed  with  a  neurilemrna,  but  those 
which  have,  like  the  white  fibres  of  the  brain  and  spinal  cord,  a  sheath 
of  myelin  sustained  by  interfibrillary  neuroglia.  Neuroglia  or  "spider" 
cells  are  also  found  within  its  substance.  This  similarity  to  white  fibre- 
tracts  is  also  shown  in  pathological  conditions.  Atrophy  of  its  fibres  often 
occurs  simultaneously  with  atrophy  of  other  tracts  within  the  brain  and 
cord. 

On  entering  the  orbit,  the  optic  nerve,  which  is  closely  united  only  to 
the  upper  side  of  the  optic  foramen,  loses  the  flattened  character  which  it 
had  within  the  cranium  and  becomes  approximately  round.  While  passing 
through  the  foramen  the  ophthalmic  artery  lies  to  its  outer  side  and  below 
it.  Immediately  afterwards  it  traverses  the  fibrous  funnel  formed  by  the 
conjoined  tendon 2  of  the  four  recti  muscles  of  the  eye.  In  this  situation  it 
is  accompanied  by  the  artery  and  by  those  structures  which  have  entered 
the  funnel  by  passing  through  the  lower  part  of  the  sphenoidal  fissure. 
These  are  the  ophthalmic  vein,  and  the  oculo-motor,  abducens,  and  naso- 
ciliary  nerves,  the  latter  being  a  branch  of  the  ophthalmic  division  of  the 
trigeminus.  The  trochlear  nerve  and  the  other  branches  of  the  ophthalmic 
division  pass  through  the  upper  part  of  the  sphenoidal  fissure,  lie  exterior 
to  the  muscles,  and  do  not  enter  the  funnel. 

It  receives  several  investments  derived  from  the  neural  structures  within 
the  cranial  cavity.  The  dura,  continuous  with  the  periosteum  of  the  orbit, 
sends  upon  the  nerve  an  external  sheath  that  passes  as  far  as  the  eyeball, 
blending  there  with  the  sclera.  In  a  similar  way  the  pia  also  envelops  the 
nerve  with  a  vascular  sheet  which  serves  for  its  nutrition,  forming  an  inner 
sheath  of  the  nerve.  Between  these  there  is  left  a  lymphatic  space,  the 

1  Syn.  :  nervus  opticus ;  second  pair. 

2  Syn.  :  annulus  tendineus  communis ;  annulus  Zinnii ;  tendon  of  Zinn.     First  men- 
tioned by  Zinn  in  his  Descriptio  oculi  humani,  Gottingae,  1755. 


214  THE   ANATOMY   OF   THE   EYEBALL. 

intervaginal  space.  An  incomplete  sheath  of  delicate  trabecular  tissue, 
derived  from  the  arachnoid  of  the  brain,  lies  between  the  dural  and  pial 
sheaths,  dividing  the  intervaginal  space  into  two, — an  external,  narrow, 
subdural  space,  and  an  internal,  wider,  subarachnoid  space.  These  com- 
municate through  the  optic  foramen  with  the  corresponding  spaces  of  the 
cerebral  meninges  and  contain  cerebro-spinal  fluid. 

Within  the  orbit  the  nerve  passes  a  little  sideways  and  downward  to 
reach  the  eyeball,  describing  a  long  sigmoid  curve  in  the  horizontal  plane. 
In  the  vertical  plane  there  is  a  somewhat  abrupt  downward  sweep,  fol- 
lowed by  a  straight  course.  These  curves  cannot,  however,  be  said  to  be 
constant,  as  numerous  individual  varieties  occur.  They  are  apparently 
connected  with  the  mobility  of  the  eyeball,  being  more  marked  in  animals 
that  have  very  movable  eyes, — such,  for  example,  as  the  chameleon,  that 
can  direct  each  eye  separately  in  any  direction, — and  only  slightly  developed 
in  birds,  who  move  their  eyes  but  little,  directing  the  vision  rather  by 
movements  of  the  entire  head.  In  exophthalmos,  a  disease  characterized 
by  an  extrusion  of  the  ball,  the  more  marked  the  displacement  the  less 
mobile  the  eye  becomes. 

Weiss 1  has  made  a  series  of  investigations  concerning  the  frequency  of 
these  curves  and  their  effect  upon  the  condition  of  the  eye.  In  sixty  sub- 
jects he  found  the  average  length  of  the  nerve  from  the  optic  foramen  to  its 
penetration  of  the  sclera  to  be  twenty-four  millimetres,  the  greatest  length 
found  being  thirty  millimetres,  the  least  twenty  millimetres.  The  direct 
distance,  measured  from  the  foramen  to  the  sclera,  averaged,  however, 
eighteen  millimetres,  there  being  a  maximum  of  twenty-four  millimetres 
and  a  minimum  of  fourteen  millimetres.  There  is,  therefore,  in  all  eyes  a 
surplus  of  length  allowed  to  the  optic  nerve  which  admits  a  certain  amount 
of  play  of  the  ball.  This  surplus,  or  slack  of  the  nerve  (Abrollungsstrecke 
of  Weiss),  averages  five  and  six-tenths  millimetres,  having  a  maximum  of 
twelve  millimetres  and  a  minimum  of  three  millimetres. 

The  importance  of  this  extra  length  in  the  optic  nerve  will  be  appreci- 
ated when  the  peculiarity  of  the  eye-movements  is  considered.  The  ocular 
muscles  are  so  inserted  upon  the  ball  that  they  turn  it  about  a  centre  that 
coincides  very  nearly  with  the  centre  of  figure.  Consequently,  in  direct- 
ing the  vision  towards  any  object,  the  posterior  pole  passes  through  the 
same  arc  as  the  anterior  one,  and  if  it  is  not  freely  movable  the  optic  nerve 
will  suffer  more  or  less  traction.  In  movements  of  extreme  convergence 
the  nerve  becomes  nearly  straight,  and  may  in  some  cases  be  stretched 
enough  to  affect  the  disk.  Weiss  found  that  when  the  amount  of  slack  is 
above  seven  and  a  half  millimetres,  the  nerve  is  never  stretched,  no  matter 
how  great  the  movements  of  the  eye  may  be ;  when  from  seven  and  a  half 
to  five  and  a  half  millimetres,  the  nerve  is  dragged  in  excesssive  move- 
ments and  a  transverse  deformity  of  the  disk  sometimes  occurs  ;  in  case  the 

1  Weiss  (Leopold).     Beitrage  zur  Anatomic  der  Orbita,  ii.  Theil.     Tubingen,  1890. 


THE   ANATOMY    OF   THE    EYEBALL.  215 

slack  is  less  than  five  and  a  half  millimetres,  the  nerve  is  dragged,  the  disk 
deformed,  and  the  alterations  known  as  conus  appear. 

The  orbital  fat  surrounds  the  nerve  throughout  its  course.  The  oph- 
thalmic artery,  as  it  passes  forward,  ascends  towards  the  roof  of  the  orbit, 
and  lies  between  the  nerve  and  the  superior  rectus  muscle.  The  central 
artery  of  the  retina,  accompanied  by  the  central  vein  and  an  extremely 
delicate  plexus  derived  from  the  ciliary  nerves,  penetrates  the  optic  nerve 
about  fifteen  millimetres  behind  the  ball.  The  vein  usually  lies  behind 
the  artery,  while  the  plexus  surrounds  it.1  They  soon  gain  an  axial  posi- 
tion within  the  nerve,  being  there  surrounded  by  a  sheath  of  connective 
tissue.  Deyl 2  has  recently  made  a  series  of  investigations  to  determine 
the  correctness  of  the  observations  of  Vossius  with  regard  to  the  entrance 
of  the  vessels.  His  examinations  were  made  with  special  care,  the  orbit 
being  usually  opened  from  below.  He  found  that  in  each  of  twen'ty-one 
eyes  examined  by  him  the  entrance  was  in  the  inferior  nasal  quadrant  of 
the  nerve.  As  this  is  the  situation  of  the  optic  fissure  of  the  embryo,  it 
appears  that  the  vessel  maintains  its  primitive  position,  and  that  the  suppo- 
sition that  the  nerve  and  the  eyeball  have  turned  through  an  angle  of  ninety 
degrees  during  development  is  without  foundation. 

At  its  entrance  into  the  orbit  the  nerve  is  surrounded  by  the  four  recti 
muscles.  The  rectus  superior  and  medialis  have,  in  fact,  regular  insertions 
upon  its  sheath,  and  it  is  probably  because  of  this  attachment  that  move- 
ments involving  these  two  muscles  are  especially  painful  during  inflamma- 
tion of  the  optic-nerve  sheath.  As  the  nerve  passes  forward,  the  muscles 
diverge  to  their  insertions  upon  the  globe  of  the  eye.  The  nerves  of  the 
orbit  that  have  entered  at  the  sphenoidal  fissure  likewise  attend  the  optic 
nerve  in  its  passage,  the  lower  branch  of  the  oculo-motor  nerve,  accom- 
panied by  the  ciliary  ganglion,  lying  nearest  and  along  its  lateral  side.  At 
its  entrance  into  the  globe  the  ciliary  arteries  and  nerves  surround  the 
nerve. 

Immediately  before  piercing  the  ball  the  nerve  becomes  contracted,  for 
the  reason  that  its  fibres  here  lose  their  perineural  sheaths. 

The  fibres  of  which  the  optic  nerve  is  composed  differ  considerably  in 
size,  and  this  is  probably  connected  with  differences  in  function.  They  are 
arranged  in  parallel  bundles  separated  from  each  other  by  septa  derived 
from  the  pial  sheath.  The  bundles  in  front  of  the  entrance  of  the  optic 
nerve  are  smaller  than  those  behind  it,  and,  as  they  here  admit  the  cross- 
bands  of  the  lamina  cribrosa,  they  present  the  peculiar  "  rush-pith"  appear- 
ance characteristic  of  the  nerve. 

It  is  important  for  us  to  understand  the  general  grouping  of  the  fibres, 
with  reference  both  to  their  distribution  upon  the  retina  and  to  their  intra- 
crauial  connections.  This  arises  from  the  fact  that  they  must  be  con- 

1  Testut  (Anatomic  humaine,  vol.  ii.)  speaks  of  this  plexus  as  «  Tiedemann's  nerve." 

2  Deyl  (J.).     Uelber  den   Eintritt  der  Arteria  centralis  retinae  in  den  Sehnerv  beim 
Menschen.     Anatomiscber  Anzeiger,  20  Marz,  1896,  xi.  687. 


216  THE   ANATOMY    OF   THE   EYEBALL. 

sidered  as  avenues  of  conduction,  and  that  a  lesion  of  any  portion  is  accom- 
panied by  disorders  in  corresponding  areas  of  the  retina  or  of  the  nerve- 
tracts  within  the  cranium.  A  tumor  or  a  piece  of  bone  pressing  upon  the 
nerve  may  cause  disturbances  of  vision  that  indicate  accurately  the  situation 
of  the  lesion.  As  direct  anatomical  investigation  gives  no  reliable  infor- 
mation as  to  the  course  of  particular  fibre-bundles,  it  is  necessary  in  such 
cases  to  have  recourse  to  other  methods.  The  discovery  that  the  axis-cylin- 
der processes  of  nerve-cells  atrophy  when  the  cells  to  which  they  belong 
are  destroyed,  assists  us  in  this  matter,  as  it  is  known  that  the  nerve-cells 
whose  axis-cylinder  processes  appear  in  the  optic  nerve  are  mainly  situated 
in  the  retina.  By  observing  the  atrophied  portions  of  the  nerve  in  persons 
who  have  lost  an  eye  when  young,  or  by  noting  the  appearances  in  the 
nerves  of  those  who  have  lost  vision  in  some  restricted  part  of  the  retina, 
we  can  determine  with  some  accuracy  the  general  features  of  distribution. 

First,  we  may  ask  the  situation  of  the  fibres  that  serve  for  most  distinct 
vision.  In  the  retina  these  are  distributed  to  the  fovea  centralis  and  macula 
lutea,  and  a  bundle  of  them  that  passes  from  the  disk  to  the  macula  is 
known  as  the  papillo-maeular  bundle.  (Bunge.)  In  cases  where  vision  is 
lost  in  this  region  (central  scotoma),  an  atrophied  bundle  is  found  in  the 
nerve,  occupying  a  wedge-shaped  area  having  its  point  at  the  central  ves- 
sels and  lying  in  the  lower  temporal  sector.  Behind  the  vessels  it  seeks  a 
more  central  situation  and  has  an  oval  form.  At  the  chiasma  it  lies  in  the 
dorsal  half  of  that  structure.  The  area  of  this  bundle  is  proportionately 
very  large,  amounting  to  one-third  the  area  of  the  nerve. 

It  is  well  established  that  in  man  and  other  animals  having  the  faculty 
of  seeing  objects  with  both  eyes  at  once  (binocular  vision)  the  optic  nerve 
contains  fibres  not  only  from  the  opposite  side  of  the  brain  (crossed)  but 
also  from  the  same  side  (direct  or  uncrossed).  An  examination  of  animals 
in  whom  a  single  eye  has  been  extirpated  shows  that  the  smaller,  direct 
bundle  lies  at  the  temporal  side  of  the  disk  and  of  the  optic  nerve,  being 
surrounded,  however,  in  the  latter  situation  by  crossed  fibres  which  sepa- 
rate it  from  the  sheath.  In  front  of  the  entrance  of  the  central  vessels 
the  crescent-shaped  area  of  the  direct  fibres  is  divided  into  two  portions 
separated  by  a  transverse  area  of  crossed  fibres.1 

Those  fibres  that  are  situated  nearest  the  margin  of  the  nerve  and  those 
found  near  the  central  vessels  usually  atrophy  during  old  age. 

1  Schmidt-Kimpler.     Archiv  f.  Augenheilkunde,  xiv.,  Abth.  iii. 


THE  MICROSCOPICAL  ANATOMY  OF 
THE  EYEBALL. 

BY  GEORGE  A.   PIERSOL,   M.D., 
Professor  of  Anatomy  in  the  University  of  Pennsylvania,  Philadelphia,  Pa.,  U.S.A. 


THE  bulbus  oculi  consists  of  three  coats : 

1.  The  External  Fibrous  Tunic,  comprising  the  sclerotic  and  the  cornea, 
upon  which  devolve  the  maintenance  of  the  form  of  the  organ  and  the 
protection  of  the  more  delicate  structures  enclosed. 

2.  The  Middle   Vascular    Tunic,   including   the   choroid,  the  ciliary 
body,  and  the  iris,  parts  lo  which  the  principal  vascular  supply  of  the 
eye  is  distributed,  with  the  exception  of  the  vessels  ramifying  within  the 
retina. 

3.  The  Inner  Nervous  Tunic,  the  retina,  which  receives  the  terminal 
expansion  of  the  optic  nerve  and  contains  the  specialized  neuroepithelium 
concerned  in  the  perception  of  the  visual  stimulus.     The  aqueous  humor, 
the  crystalline  lens,  and  the  vitreous  body  are  enclosed  by  these  coats,  and 
represent  the  refractive  media  of  the  eye. 

An  important  division  of  the  structures  composing  the  visual  organ  is 
that  based  upon  their  embryonic  origin,  since  the  several  parts  of  the  eye 
may  be  grouped  under  two  headings : 

1 .  Those  parts  developed  from  the  ectoderm. 

2.  Those  parts  developed  from  the  mesoderm. 

The  members  of  the  first  group  may  be  subdivided  into : 

a.  Structures  derived  directly  from  the  ectoderm,  including  the  lens 
and  its  anterior  epithelium,  and  the  epithelium  of  the  cornea  and  of  the 
adjacent  scleral  surface. 

6.  Structures  derived  secondarily  from  the  ectoderm  through  the  optic 
vesicles  protruded  from  the  involuted  ectoderm  of  the  primary  cerebral 
vesicles.  To  this  group  belong  the  primary  retinal  tissues,  including  the 
pigment-layer  as  well  as  the  atrophic  retinal  layers  continued  over  the 
posterior  surface  of  the  ciliary  body  and  the  iris. 

All  other  parts  of  the  eyeball,  comprising  the  remaining  portions  of  the 
sclera,  the  cornea,  the  iris,  the  ciliary  body,  the  choroid,  and  the  vitreous 
body,  as  well  as  the  connective-tissue  ingrowths  of  the  retina,  are  developed 

from  the  mesoderm. 

217 


218 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


FIG.  1. 


THE   CORNEA. 

The  human  cornea,  which  varies  in  thickness  from  .08  millimetre  at 
the  centre  to  1.1  millimetres  at  the  periphery,  when  viewed  in  vertical 
section  in  suitably  prepared  specimens  exhibits  five  layers.  (Fig.  1.) 
From  without  inward  these  are  : 

1.  The  anterior  epithelium. 

2.  The  anterior  limiting  membrane. 

3.  The  corneal  substance  proper. 

4.  The  posterior  limiting  membrane. 

5.  The  posterior  endothelium. 

Referred  to  its  development,  the  anterior  epithelium  alone  is  ectodermic 
in  origin,  the  remaining  layers  being  all  the  direct  products  of  the  meso- 

derm.  At  an  early  stage  in 
the  formation  of  the  eye  the 
lens  is  separated  from  the 
external  ectodermic  epithe- 
lium by  a  sheet  of  mesoder- 
mic  tissue  the  inner  surface 
01  which  is  closely  applied 
to  the  primitive  lens  and  the 
outer  to  the  ectoderm.  This 
mesodermic  tract  differen- 
tiates into  an  outer  thicker 
and  more  compact  stratum 
and  an  inner  much  thinner 
and  looser  lamella ;  the  inner 
layer  becomes  highly  vascu- 
lar, while  the  outer  contains 
no  blood-vessels.  These 
strata  are  separated,  after  a 
time,  by  the  formation  of  a 
distinct  cleft,  which  repre- 
sents the  earliest  appearance 
of  the  anterior  chamber.  The 
cleavage  of  the  mesodermic 
tract  thus  effected  defines  the 
tissues  of  the  primitive  cornea 
from  those  of  the  vascular 
pupillary  membrane.  The 
corneal  lamella  is  directly 
continuous  with  the  scleral 
tissue ;  the  pupillary,  with 
the  primitive  choroidal  tract.  The  innermost  layers  of  the  cornea — the 
posterior  limiting  membrane  and  the  endothelium — bear,  however,  a  par- 


vertical  section  of  the  cornea,  magnified  115  diameters. 
— a,  the  anterior  epithelium ;  b,  the  anterior  limiting  mem- 
brane; c,  the  corneal  substance  proper;  d,  the  posterior 
limiting  membrane ;  e,  the  posterior  endothelium. 


THE   MICROSCOPICAL   ANATOMY  OF  THE   EYEBALL.  219 

tioularly  close  relation  to  the  choroidal  tract,  the  mesodermic  layers  of  the 
cornea  thus  manifesting  variation  in  their  genetic  affinities. 

In  view  of  these  embryological  data,  Waldeyer1  and,  later,  Schwalbe2 
have  adopted  a  division  of  the  cornea  into  three  primary  lamellae : 

1.  Pars  conjunctivalis,  including  the  anterior  epithelium. 

2.  Pars  scleralis,  including  the  substantia  propria  and  the  anterior  limit- 
ing membrane. 

3.  Pars  choroidalis,  including  the  posterior  limiting  membrane  and  the 
endothelium. 

The  anterior  epithelium  covers  the  outer  surface  of  the  cornea,  and  is 
directly  continuous  with  that  covering  the  adjacent  parts  of  the  sclera  and 
lining  the  conjunctival  sac ;  it  is,  therefore,  in  continuity  with  the  epidermis 
of  the  surrounding  skin,  a  relation  emphasized  by  the  primary  unbroken 
extension  of  the  ectoderm  over  the  corneal  area  before  the  appearance  of  the 
eyelids.  This  epithelial  layer  consequently  is  homologous  with  the  epi- 
dermis, but  differs  from  the  latter  structure  in  remaining  permanently  less 
highly  developed,  retaining  the  amphibian  type  (Minot),3  as  characterized 
by  fewer  cell-layers  than  usually  compose  the  surface  covering  and  by  the 
absence  of  the  stratum  corneum. 

The  corneal  epithelium  belongs  to  the  stratified  squamous  group,  and 
consists  of  from  six  to  eight  layers  of  cells  whose  combined  height  consti- 
tutes an  epithelial  stratum  which  measures  about  .045  millimetre  in  thick- 
ness in  the  vicinity  of  the  centre,  and  nearly  double  as  much  (.08  milli- 
metre) at  the  periphery  of  the  cornea. 

In  common  with  other  epithelial  lamella  of  the  stratified  squamous 
variety,  the  corneal  epithelium  is  made  up  of  cells  presenting  marked 
differences  in  the  various  layers.  Three  principal  types  of  elements  are  seen  : 
1,  the  columnar  cells  of  the  deepest  stratum ;  2,  the  polyhedral  cells  of  the 
middle  layers ;  and,  3,  the  thin  expanded  plates  of  the  superficial  planes. 

The  cells  of  the  deepest  stratum  constitute  a  single  row  of  irregular 
columnar  elements,  which,  while  presenting  considerable  variation  in  their 
individual  outlines,  possess  in  common  a  more  or  less  expanded  basal  sur- 
face, resting  upon  the  subjacent  anterior  limiting  membrane,  and  a  rounded 
outer  end  received  among  the  elements  of  the  superimposed  layer.  The 
general  form  of  these  columnar  cells  (the  "  Fusszellen"  of  Rollett 4  and  of 
Lott 5)  is  somewhat  club-shaped,  their  broad  inner  ends  being  often  separated 
from  the  enlarged  outer  extremities  by  an  intervening  constriction,  although 
the  latter  is  sometimes  so  slightly  marked  that  the  typical  cylindrical  form 

1  Waldeyer :  Mikroscopische  Anatomic  der  Cornea,  Sklera,  Lider  und  Conjunctiva, 
Handbuch  der  gesammten  Augenheilkunde  von  Graefe  und  Saemisch,  Bd.  I.,  1874. 

2  Schwalbe  :  Lehrbuch  der  Anatomic  der  Sinnesorgane,  1887. 

3  Minot :  Human  Embryology,  1892. 

*  Kollett :  Ueber  die  Hornhaut,  Strieker's  Handbuch  der  Lehre  von  den  Geweben,  18 
5  Lott :   Ueber  den  feineren  Bau  und  die  physiologische  Regeneration  der  Epithelien, 
insbesondere  d.  Cornealepithels,  Centralblatt  f.  d.  med.  Wissensch.,  No.  37,  1871. 


220 


THE    MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


is  preserved.  The  presence  of  a  distinct,  highly  refracting  basal  plate,  as 
described  by  Rollett  and  Lott,  is  not  to  be  demonstrated  in  accurately  fixed 
and  stained  preparations,  the  expanded  inner  borders  of  the  cells  appearing 
as  sharply  defined  smooth  boundaries  as  figured  by  Waldeyer.  Indications 
of  the  serrations  pictured  by  Langerhans l  are  to  be  seen  in  tissue  fixed  in 
osmic  acid,  but  it  is  probable  that  the  methods  of  isolation  formerly  em- 
ployed were  responsible  in  no  slight  degree  for  the  appearance  of  the  toothed 
basal  border.  The  columnar  cells  possess  round  or  oval  nuclei  which 
occupy  the  outer  ends  of  the  cells.  In  the  tissue  of  young  animals  par- 
ticularly, but  also  occasionally  in  that  of  the  adult,  the  presence  of  nuclei 
in  various  stages  of  division,  as  shown  by  the  karyomitotic  figures  (Fig. 
2),  is  not  uncommon,  since  the  formation  of  the  new  cells  necessary  for  the 
replacement  of  the  older  surface-elements  devolves  upon  the  deepest  layer 
of  the  epithelium,  which  possesses  the  greatest  vital  activity.  The  results 

of  such  cell -multiplication 

FIG.  2.  are  seen  as  smaller  irregu- 

lar elements  lying  between 
the  usual  columnar  cells. 

The  middle  epithelial 
layers  are  made  up  of 
polyhedral  cells  whose  de- 
tails vary  with  their  posi- 
tion. Those  immediately 
succeeding  the  columnar 
elements  are  modified  by 
the  outer  rounded  ends  of 
the  latter,  so  that  their 
deeper  surfaces  exhibit  al- 
ternating depressions  and 
ridges,  which  respectively 
receive  the  extremities  of 
the  lower  cells  and  fill  the 
interspaces  between  them. 

The  elements  of  the  succeeding  middle  layers  assume  a  more  regular 
polyhedral  form  and  possess  conspicuous  nuclei.  Their  outlines  are  not 
smooth,  but  broken  by  minute  projections,  which,  as  is  common  in  similarly 
situated  cells  of  the  epidermis,  bridge  over  the  intercellular  clefts  and  estab- 
lish continuity  between  the  protoplasm  of  the  adjacent  elements.  When 
these  are  isolated  the  torn  connecting  threads  produce  the  appearance  of 
"  prickle-cells." 

The  superficial  layers  of  the  corneal  epithelium  are  composed  of  cells 
which  have  undergone  gradual  diiferentiation  into  the  flattened,  plate-like 


Vertical  section  of  the  anterior  epithelium  (a),  the  anterior 
limiting  membrane  (6),  and  the  superficial  lamellae  of  the  sub- 
stantia  propria  of  the  cornea :  magnified  500  diameters. 


1  Langerhans :   Ueber  mehrschichtige   Epithelien,   Virchow's  Archiv  fur  patholog. 
Anatomie,  Bd.  LVIII.,  1873. 


THE   MICROSCOPICAL   ANATOMY  OF  THE   EYEBALL.  221 

elements  of  the  free  surface  closely  resembling  those  covering  many  mucous 
surfaces.  They  differ  from  the  elements  composing  the  stratum  corneum 
of  the  epidermis  in  possessing  nuclei  and  in  not  being  keratose.  In  pass- 
ing towards  the  free  surface  the  diameter  of  the  cells  increases  as  the  thick- 
ness diminishes,  so  that  the  long  axis  of  the  surface  plates  is  directed 
parallel  with  the  anterior  corneal  border  and  at  right  angles  to  the  longest 
diameter  of  the  cells  of  the  deepest  stratum. 

The  elements  of  the  anterior  epithelium  are  united  by  a  small  amount 
of  semi-fluid  intercellular  cement-substance  which  is  traversed  in  the  mid- 
dle layers  by  the  connecting  threads  uniting  the  adjacent  cells.  The  inter- 
cellular clefts  represent  channels  by  which  nutrient  juices  may  reach  the 
individual  epithelial  elements.  Occasional  migratory  leucocytes  are  ob- 
served within  these  intra-epithelial  spaces,  as  pointed  out  by  Engelinann.1 
In  the  cornea  of  young  animals,  or  in  tissue  subjected  to  irritative  stimuli, 
cells  exhibiting  the  nuclear  manifestations  of  division  are  not  uncommon. 

The  anterior  limiting  membrane  (anterior  basement  membrane,  lamina 
elastica  anterior,  anterior  boundary  layer,  subepithelial  stratum)  is  a  special- 
ized portion  of  the  principal  connective-tissue  layer  of  the  cornea,  the 
outermost  lamella  of  which  appears  as  a  distinct  membrane  in  consequence 
of  its  unusual  condensation. 

This  structure,  which  is  very  conspicuous  in  the  human  eye,  varies 
greatly  in  its  development  and  consequent  prominence  in  different  animals ; 
while  well  developed  in  the  rabbit,  the  guinea-pig,  and  many  ruminants,  it 
is  less  conspicuous  in  other  mammals,  as  the  pig,  the  cat,  the  horse,  and 
the  goat. 

In  the  human  cornea  this  membrane  appears  as  a  homogeneous,  highly 
refracting  band  immediately  beneath  the  anterior  epithelium.  The  seem- 
ingly structureless  layer  measures  from  .018  to  .020  millimetre  in  thickness 
in  the  centre  of  the  cornea,  and  gradually  decreases  towards  the  periphery. 
Within  one  or  two  millimetres  of  the  corneal  limbus  the  anterior  elastic 
membrane  becomes  greatly  attenuated,  and  finally  passes  into  the  membrana 
propria  beneath  the  conjunctival  epithelium. 

The  true  nature  of  this  lamina  as  the  differentiated  outermost  part  of 
the  substantia  propria  may  be  demonstrated  by  subjecting  the  corneal  tissue 
to  the  action  of  potassium  permanganate  solution.  By  means  of  this  re- 
agent Rollett  has  shown  that  the  seemingly  structureless  layer  is  resolvable 
into  bundles  of  white  fibrous  tissue  identical  with  those  composing  the  bulk 
of  the  cornea.  The  assumed  identity  of  the  homogeneous  tissue  composing 
this  layer  with  elastic  tissue — emphasized  by  the  misleading  name  u  ante- 
rior elastic  membrane"  so  often  employed — is  demonstrated  to  be  erroneous 
by  the  solution  of  the  anterior  limiting  membrane  effected  by  prolonged 
boiling  and  the  extended  action  of  mineral  acids.  The  usual  cellular  ele- 
ments of  the  cornea  are  wanting  within  this  stratum.  The  anterior  limiting 

1  Engelmann  :  Ueber  die  Hornhaut  des  Auges,  1867. 


222 


THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 


FlG.  3. 


membrane  must  probably  be  regarded  as  the  homologue  of  the  membrana 
propria  supporting  the  epithelium  of  many  mucous  tracts.  This  relation 
is  further  emphasized  by  the  presence  of  endothelioid  plates,  which  may  be 
demonstrated  in  certain  animals,  as  the  calf,  lying  upon  the  subepithelial 
surface  of  the  substantia  propria.  (Fig.  3.) 

The  corneal  substance  proper  (substantia  propria,  corneal  stroma,  ground- 
substance),   notwithstanding   its   remarkable   transparency   and    apparent 

homogeneity  during  life, 
possesses  histological  de- 
tails of  considerable  com- 
plexity. The  substantia 
propria  consists  of  two 
principal  parts : 

(a)  A  ground-sub- 
stance, composed  of  bundles 
and  lamella?  of  fibrous  tis- 
sue closely  united  by  an 
interfibrillar  cement-sub- 
stance. 

(6)  Numerous  connec- 
tive-tissue cells,  known  as 
the  corneal  corpuscles,  oc- 
cupying interstices  within 
the  ground-substance. 

The  true  character  of 
the  ground-substance  is  not 
suggested  by  its  usual 
homogeneous  appearance  presented  in  ordinary  preparations  of  the  cornea, 
but  is  satisfactorily  displayed  only  after  the  tissue  has  been  subjected  to  the 
action  of  reagents  capable  of  attacking  the  interfibrillar  cement-substance 
which  holds  together  and  optically  fuses  the  constituent  fibres.  One  of  the 
most  important  means  of  demonstrating  the  fibrous  structure  of  the  sub- 
stantia propria  is  its  treatment  by  a  solution  of  potassium  permanganate, 
successfully  applied  by  Rollett  in  his  classical  investigations  of  this  sub- 
ject. Prolonged  maceration  in  ten  per  cent,  solution  of  sodium  chloride 
(Schweigger-Seidel) 1  also  effects  dissociation  of  the  fibrous  tissue.  Osmic 
acid,  likewise,  is  of  value  in  displaying  the  fibrous  structure  of  the  sub- 
stantia propria,  but  for  the  demonstration  of  the  form  and  disposition  of 
the  fibrous  bundles  no  method  yields  more  striking  results  than  that  of  an 
interstitial  injection  of  a  one-fourth  to  one-half  per  cent,  solution  of  silver 
nitrate  into  the  substantia  propria,  with  subsequent  reduction  of  the  silver 
by  exposure  to  daylight.  In  such  preparations  (Fig.  4)  the  interfibrillar 


Endothelioid  markings  on  the  anterior  limiting  membrane 
after  treatment  with  silver:  cornea  of  calf.  Magnified  200 
diameters.  These  markings  do  not  correspond  to  those  of 
the  anterior  epithelium,  being  much  larger  and  less  regular. 


1  Schweigger-Seidel :  TJeber  die  Grundsubstanz  und  die  Zellen  der  Hornhaut  des  Auges, 
Berichte  d.  sachs.  Gesellsch.  d.  Wissensch.,  1869. 


THE   MICROSCOPICAL   ANATOMY  OF  THE   EYEBALL.  223 

cement-substance  becomes  more  or  less  darkly  stained,  while  the  fibrous 
tissue  remains  uncolored  and  contrasts  as  light  lines  against  the  darker 
surrounding  ground. 

The  individual  bundles  of  fibrous  tissue  cross  one  another  at  various 
angles,  and  are  felted  together  into  lamellae  from  .008  to  .010  millimetre  in 
thickness ;  the  bundles  are  disposed  with  considerable  regularity  with  regard 
to  the  corneal  free  surface,  sixty  to  sixty-five  of  such  parallel  strata  usually 
being  present  in  the  human  cornea.  The  separation  of  the  cornea  into  dis- 
tinct lamellae,  sometimes  undertaken,  must  be  regarded  as  artificial,  since 
the  number  of  the  resulting  isolated  strata  depends  largely  upon  the  skill 
with  which  the  dissection  is  performed. 

FIG.  4. 


Interlacing  lamellae  composing  the  substantia  propria  of  calf  s  cornea,  after  interstitial  injection  of 
argentic  nitrate  solution  and  subsequent  exposure  to  sunlight.    Magnified  200  diameters. 

The  individual  lamellae  are  composed  of  the  interlacing  bundles  of 
fibrous  tissue,  which  in  the  human  eye  are  irregularly  disposed,  in  contrast 
to  the  definite  arrangement  of  the  bundles,  almost  at  right  angles,  in  the 
cornea  in  certain  animals,  as  the  frog.  While  the  majority  of  the  bundles 
extend  in  the  general  plane  of  the  lamellae,  others  pass  obliquely,  and  serve 
to  unite  still  more  intimately  the  fibrous  constituents  of  the  stratum. 
Additional  bundles  pass  beyond  the  limits  of  the  individual  layer  and  con- 
nect the  adjacent  surfaces  of  different  lamellae,  thus  materially  contributing 
to  the  firmness  of  their  union. 

The  anterior  layers  of  the   cornea,  immediately  beneath  the  limiting 


224  THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 

membrane,  contain  oblique  or  vertical  bundles  in  unusual  numbers  ;  these 
delicate  bands  extend  through  the  superficial  strata  towards  the  anterior 
limiting  membrane,  in  the  compact  structure  of  which  they  fade  away. 
Such  curved  bands  constitute  the  fibrce  arcuatce,  or  supporting  fibres  ;  they 
are,  however,  only  unusually  oblique  interlacing  bundles  of  the  fibrous 
tissue,  and  not  independent  structures. 

In  addition  to  the  longitudinally  and  obliquely  cut  bundles  constituting 
the  corneal  lamellae,  sections  of  tissue  preserved  in  dilute  spirit  (forty-five 
per  cent.)  display  punctations  occupying  the  intervals  between  the  bundles  : 
these  markings  represent  the  transverse  sections  of  the  individual  fibres 
composing  the  bundles  extending  approximately  at  right  angles. 

When  corneal  sections  are  examined  by  polarized  light,  the  transversely 
cut  bands  remain  constantly  dark  when  viewed  between  the  crossed  Nicol's 
prisms  ;  those  cut  longitudinally  or  obliquely,  on  the  contrary,  appear  alter- 
nately dark  and  light  (His).1  Experiments  with  the  eritire  cornea  under 
polarized  light  indicate  a  preponderance  of  meridionally  coursing  fibres  over 
those  running  in  other  directions  (Rollett).2 

The  fibres  composing  the  bundles  and  lamellae  of  the  ground-substance 
are  intimately  united  by  means  of  an  interfibrillar  albuminous  cement-sub- 
stance. The  refractive  indices  of  the  latter  and  the  fibrous  tissue  during  life 
are  identical,  the  cornea  apparently  being  homogeneous  and  without  structure. 

The  results  obtained  by  boiling  the  substantia  propria  differ  somewhat 
from  those  following  like  treatment  of  ordinary  connective  tissue,  since  the 
substance  so  secured  resembles  chondrin  obtained  from  cartilage  rather  than 
gelatin  yielded  by  connective  tissue.  Both  gelatin  and  chondrin,  however, 
as  shown  by  Morochowetz,3  consist  of  collagen  and  mucin,  derived  respec- 
tively from  the  fibrous  elements  and  the  cement-substance  of  the  tissues. 
The  peculiarity  of  the  corneal  tissue  depends  upon  the  relatively  large  pro- 
portion of  interfibrillar  cement-substance  present,  which  yields  a  greater 
amount  of  mucin  than  is  usually  found  in  connective  tissue,  therein  resem- 
bling the  composition  of  chondrin. 

The  Corneal  Spaces. — Examination  of  vertical  sections  of  the  cornea 
shows  that  the  lamella?  of  fibrous  tissue  are  not  everywhere  in  close  contact, 
but  are  separated  in  many  places  by  intervening  clefts  ;  the  latter  constitute 
the  corneal  spaces.  Seen  in  section,  these  appear  as  narrow  fusiform  cavi- 
ties the  tapering  ends  of  which  fade  away  between  the  opposed  laminae, 
or,  at  best,  are  traceable  as  minute  crevices  bet\veen  the  fibrous  bundles, 
sometimes  as  far  as  the  neighboring  dilated  spaces  into  which  they  open. 

A  satisfactory  display  of  the  arrangement  of  the  corneal  spaces,  how- 
ever, is  had  only  after  the  treatment  of  the  tissue  by  silver  staining ;  by 
means  of  this  reagent  the  generously  distributed  cement-substance  is  colored 

1  His :  Beitrage  zur  nonnalen  und  pathologischen  Histologie  der  Cornea. 

2  Kollett :  loc.  cit. 

3  Morochowetz :  quoted  by  Halliburton,  Chemical  Physiology  and  Pathology,  1891, 
p.  484. 


THE   MICROSCOPICAL   ANATOMY  OP  THE   EYEBALL. 


225 


FIG.  5. 


brown,  while  the  spaces,  remaining  almost  unaffected,  appear  as  light  figures 
on  the  dark  background.    Seen  in  successful  silver  preparations,  the  corneal 
spaces  appear  as  light  irregularly  stellate  areas,  as  shown  in  Fig.  5,  from 
which    minute    ramifications, 
the  corneal  canals,  extend  in 
various  directions  and  estab- 
lish communication  with  the 
adjacent  corneal  spaces. 

These  spaces  and  their  ex- 
tensions, while  constituting  a 
system  of  intercommunicating 
channels  throughout  the  sub- 
stantia  propria,  are  not  pecu- 
liar to  the  cornea,  but  represent 
the  system  of  lymph-spaces 
present  in  other  dense  con- 
nective tissues,  conspicuous 
examples  of  which  are  seen 
in  the  central  tendon  of  the 
diaphragm,  and  particularly 

in  bone.      Just  as   the  lacunge  Surface  view  of  silvered  cornea,  showing  the  inter- 

i.i  i .      -,.     f  ,1      1  communicating  corneal  spaces  («)  lying  within  the  deeply 

ana  tne  CanallCUll  OI   tne  lat-      stained  ground-substance  (g).    Magnified  480  diameters. 

ter  tissue  form  lymph-tracts 

through  osseous  structures,  so  the  spaces  and  canals  of  the  cornea  afford 
channels  for  the  conveyance  of  nutritive  tissue-fluids  throughout  the  non- 
vascular  substantia  propria.  These  spaces  or  "juice-channels,"  then,  must 
be  regarded  as  parts  of  the  interstitial  lymphatics  of  connective  tissue. 

In  addition  to  the  "  negative"  pictures  of  the  corneal  spaces  obtained  by 
the  now  classic  silver  method  of  v.  Recklinghausen,  according  to  which  the 
fresh  tissue  is  immersed  in  .5  to  1  per  cent,  solution  of  argentic  nitrate 
and  subsequently  exposed  to  sunlight  until  the  silver  albuminate  formed  by 
the  cement-substance  has  been  darkened,  silver  staining  may  be  employed 
to  yield  "  positive"  views  of  the  spaces,  as  shown  in  the  accompanying 
Fig.  6.  Positive  pictures  of  the  corneal  lacunae  and  canaliculi  of  great 
beauty  may  often  be  secured  by  an  interstitial  injection  of  the  fresh  cornea 
with  .2  to  .25  per  cent,  solution  of  silver  nitrate  and  subsequent  reduction 
in  sunlight.  When  successful,  the  silver  deposit  outlines  with  remarkable 
clearness  the  spaces,  and  fills  more  or  less  completely  the  delicate  radiating 
canals.  The  exhibition  of  the  spaces  thus  obtained  probably  differs  from 
that  by  the  usual  silver  impregnation  in  depending  upon  the  reduction 
effected  by  the  albuminous  contents  of  the  spaces  rather  than  upon  that 
induced  by  the  surrounding  cement-substance. 

Other  methods  of  demonstrating  the  existence  of  the  corueal  juice- 
channels  include  the  interstitial  injection  of  mercury  (Bowman) ;  of  oily 
mixtures,  especially  turpentine  and  olive  oils  colored  with  alkauet  root 

VOL.  I.— 15 


226 


THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 


(Rollett) ;  of  olive  oil  followed  by  osmic  acid  (Schwalbe) ;  of  a  solution  of 
asphalt  in  chloroform  (Retzius) ;  and  the  impregnation  of  the  tissues  with 
solutions  of  potassium  ferrocyanide  and  their  subsequent  reduction  by 
means  of  ferric  salts,  as  suggested  by  v.  Wittich  and  v.  Recklinghausen. 
The  ingenious  experiment  of  Genersich x  proved  the  existence  of  the  inter- 
communicating channels  by  the  invasion  of  leucocytes  secured  by  exposing 
the  corneal  tissue  within  the  dorsal  lymph-sac  of  living  frogs ;  subsequent 
examination  showed  the  presence  of  the  migratory  cells  in  large  numbers 
within  the  spaces  and  the  canals  of  the  corneal  tissue.  The  impregnation 

Fio.  6. 


Corneal  spaces  of  ox  after  interstitial  injection  of  argentic  nitrate ;  "  positive"  picture.    Magnified 

450  diameters. 

methods  are  to  be  preferred  to  injections,  as  yielding  trustworthy  pictures 
of  the  form  and  arrangement  of  the  lacunae  and  canaliculi,  since,  with  the 
exception  of  slight  shrinkage,  the  appearance  of  these  spaces  in  prepara- 
tions so  made  may  be  accepted  as  representing  the  actual  relations  within 
the  living  tissue  :  such  pictures  are  in  marked  contrast  to  the  abnormally 
distended  and  exaggerated  condition  of  the  spaces  usually  encountered  after 
artificial  injections.  The  presence  of  rows  of  closely  placed  minute  lanceo- 
late or  irregularly  oval  figures,  the  so-called  "  corneal  tubes,"  must  be 
regarded  as  an  artificial  detail  resulting  from  the  interstitial  injections 
employed  for  their  demonstration. 

The  corneal  spaces  are  found  throughout  all  parts  of  the  substantia 
propria,  although  some  variation  exists  as  to  size  and  number.     The  super- 


1  Genersich. :  Zur  Lehre  von  den  Saftkanalchen  in  der  Cornea,  Wiener  medicin.  Jahr- 
biicher,  1871. 


THE   MICROSCOPICAL   ANATOMY  OF  THE  EYEBALL. 


227 


ficial  lamellae,  immediately  beneath  the  anterior  limiting  membrane,  con- 
tain the  smallest  but  most  closely  placed  spaces ;  the  deep  lamella  adjo'ining 
the  posterior  limiting  membrane,  on  the  contrary,  have  the  largest  but  most 
widely  separated  lacuna?.  The  corneal  spaces  and  canals  do  not  possess 
distinct  walls  of  their  own,  but  are  defined  by  the  surrounding  fibrous 
tissue  and  cement-substance,  the  latter  particularly  bearing  a  close  relation 
to  the  clefts.  (Fig.  7.) 

Corneal  Cells. — In  addition  to  the  tissue-juices  contained  within  the 
system  of  spaces  throughout  the  substantia  propria  as  part  of  the  usual 
contents   of   the    lymph- 
radicles,  the  corneal  spaces  ^IQ-  ?• 
are  occupied  by  conspic- 
uous   morphological    ele- 
ments,  the    corneal    cor- 
puscles, together  with   a 
variable,  and  usually  very 
limited,    number   of   mi- 
gratory    leucocytes,     the 
wandering  cells. 

The  corneal  corpuscles 
are  connective-tissue  cells 
lodged  within  the  minute 
lymphatic  lacuna?  of  the 
substantia  propria.  The 

arrangement   here    found  Nr*\\£» 

is  by  no  means  peculiar 
to  the  cornea,  but  corre- 
sponds with  that  seen  in 

many  other  localities,  repeating  the  general  relation  existing  in  other  organs 
between  the  cells  and  ground-substance  of  dense  connective  tissues. 

The  presence  of  these  elements  may  be  appreciated  in  well-stained  sec- 
tions of  the  cornea,  the  cells  appearing  in  profile  as  thin  fusiform  proto- 
plasmic masses  which  usually  adhere  to  some  portion  of  the  lenticular 
cavity  into  which  they  project  and  which  they  partially  occlude.  The 
completeness  with  which  the  cells  fill  the  spaces  in  which  they  lie  depends 
much  upon  the  condition  of  the  protoplasm,  since  when  the  latter  has 
suffered  shrinkage  in  consequence  of  the  action  of  reagents  the  dispropor- 
tion between  the  extent  of  the  space  and  the  size  of  the  corneal  corpuscle 
is  exaggerated.  The  most  careful  fixation  of  the  tissue  points  to  the  fact 
that  during  life  the  corneal  cell  almost  completely  occupies  one  of  the 
broader  walls  of  the  space,  with  the  outlines  of  which  it  often  closely 
corresponds ;  the  cell-body,  however,  does  not  equal  in  thickness  the  width 
of  the  lacuna,  so  that  often  a  considerable  cleft  remains  for  the  passage  of 
the  tissue-juices. 

In  many  places,  particularly  in  the  course  of  the  canaliculi  connecting 


Surface  view  of  silver  preparation  of  corneal  stroma  of  ox. 
showing  relation  of  spaces  to  fibrous  tissue.  Magnified  450 
diameters. 


228 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


the  adjacent  lacunae,  the  protoplasmic  net-work  formed  by  the  union  of  the 
processes  of  the  corneal  cells  by  no  means  accurately  corresponds  to  the  out- 
lines of  the  boundaries  of  the  lymph-channels.  Not  infrequently  two  or 
more  of  the  interlacing  protoplasmic  processes  lie  within  a  single  canal, 
thus  forming  a  net-work  of  much  greater  richness  and  complexity  than 
that  formed  by  the  canaliculi  and  spaces.  This  discrepancy  may  be  de- 
monstrated in  human  cornese,  as  suggested  by  v.  Recklinghausen,1  without 
reagents,  by  simply  guarding  against  too  rapid  loss  of  the  watery  constitu* 
ents,  the  cells  and  the  spaces  then  appearing  with  great  clearness. 

FIG.  8. 


Corneal  corpuscles  as  seen  in  surface  view  in  gold  preparations.    Magnified  480  diameters. 

While  the  relation  between  the  extent  of  the  corneal  spaces  and  the 
bulk  of  the  enclosed  cells  is  best  appreciated,  probably,  in  vertical  sections, 
an  adequate  picture  of  the  corneal  corpuscles,  as  to  their  form  and  connec- 
tions, is  furnished  by  surface  views  alone. 

Such  preparations  may  be  secured  by  several  methods  of  staining, 
including  those  by  carmine  and  haBmatoxylin,  but  for  the  exhibition  of 
the  corneal  corpuscles  impregnation  with  .5  per  cent,  solution  of  gold  chlo- 
ride and  subsequent  reduction  have  long  been  acknowledged  as  the  classic 
method  and  one  capable  of  yielding  admirable  although  uncertain  results. 
Silver  staining,  while  ordinarily  producing  negative  pictures,  may  be  modi- 
fied in  its  application  so  as  to  furnish  admirable  preparations  of  the  corneal 
cells.  Strong  applications  of  lunar  caustic  to  the  living  tissue  and  subse- 


iii. 


1  Recklinghausen :  Ueber  die  Saftkanalchen  der  Hornhaut,  Anatom.  Anzeiger,  Bd. 
1888. 


THE   MICROSCOPICAL   ANATOMY  OF  THE  EYEBALL.  229 

quent  exposure  to  light,  as  shown  by  Strieker,1  often  result  in  beautiful 
demonstrations  of  the  corpuscles  stained,  a  peculiar  purplish-brown  tint  of 
the  protoplasm  being  indicative  of  the  action  of  the  silver  upon  the  living 
cells. 

When  the  perfectly  fresh  transparent  cornea  is  examined,  no  trace  of  the 
elaborate  net-work  of  protoplasmic  processes  of  the  corneal  corpuscles  is 
visible,  and  not  until  the  tissue  begins  to  lose  its  homogeneity  of  refraction 
are  its  histological  details  suggested.  The  cells  subsequently  become  ap- 
parent as  slightly  opaque,  delicately  granular  bodies  within  the  still  trans- 
parent ground-substance.  The  vapor  of  iodine  applied  to  the  corneal 
tissue  while  under  observation  brings  to  view  the  presence  of  the  corpuscles 
in  a  very  striking  manner,  the  cells  seemingly  gradually  growing  into  ex- 
istence beneath  the  eye  in  a  way  to  recall  the  formation  of  crystals.  Iodine 
preparations  of  these  elements,  however,  possess  the  disadvantage  of  not 
being  permanent,  since,  notwithstanding  their  crisply  defined  pictures  at 
the  time,  a  few  hours  suffice  for  their  complete  disappearance. 

Successful  preparations  of  the  cornea  by  either  of  the  approved  methods 
of  staining  display  the  same  general  picture  of  the  corpuscles  and  their 
processes.  Viewed  from  the  surface,  these  cells  appear  as  irregularly  stel- 
late, flattened  bodies  within  the  protoplasmic  masses  of  which  lie  conspicu- 
ous oval  nuclei.  Numerous  processes  extend  in  various  directions,  often  in 
different  planes,  from  the  body  of  the  cell,  and  join,  either  directly  or  by 
means  of  secondary  branches,  similar  prolongations  from  the  neighboring 
corpuscles.  The  union  thus  effected  establishes  a  more  or  less  continuous 
protoplasmic  net-work  throughout  the  various  corneal  lamellae,  the  cells 
lying  within  the  same  stratum  being  most  closely  associated. 

The  nuclei  of  the  corneal  corpuscles  are  very  distinct,  being  limited  by 
a  nuclear  membrane  and  traversed  by  numerous  branching  threads  of 
deeply  staining  chromatin,  which  contrast  sharply  with  the  faintly  colored 
intervening  nuclear  matrix.  A  nucleolus,  and  not  infrequently  two  or 
even  three  pseudo-nucleoli,  are  usually  to  be  distinguished,  although  in 
gold  preparations  the  presence  of  these  bodies  is  not  always  capable  of  satis- 
factory demonstration. 

The  relations  between  the  corneal  corpuscles  and  the  spaces  are  inter- 
esting, since  the  disposition  and  limits  of  the  system  of  intercommunicating 
juice-channels  in  a  general  way  determine  the  arrangement  of  the  proto- 
plasmic cells  and  processes  which  they  contain.  In  cornese  in  which  the 
component  bundles  of  the  substantia  propria  are  grouped  with  great  regu- 
larity, as  in  the  frog,  the  intervening  lymph-clefts  are  disposed  with  much 
greater  regularity  than  in  those  cases,  as  in  the  higher  mammals,  where  the 
interlacing  of  the  corneal  lamellae  is  more  intimate  and  less  definite.  This 
difference  is  very  manifest  on  comparing  gold  pictures  of  amphibian  and 
mammalian  come®,  in  the  first  of  which  the  cell  processes  extend  as  long, 

1  Strieker :  Studien  aus  dem  Institut  f.  experimentelle  Pathologic  fn  Wien. 


230  THE   MICROSCOPICAL   ANATOMY  OF   THE   EYEBALL. 

straight,  protoplasmic  threads  running  parallel  with  one  another  or  cross- 
ing those  of  different  planes  almost  at  right  angles.  The  cells  of  the 
mammalian  cornese,  on  the  contrary,  send  processes  in  various  directions 
and  planes  as  branching  and  tortuous  outrunners,  the  protoplasmic  figures 
following  the  more  complicated  course  of  the  lymph-channels  between  the 
irregularly  disposed  corneal  bundles. 

The  corneal  corpuscles  are  applied  to  one  wall  of  the  spaces,  and  their 
protoplasmic  processes  extend  into  the  canaliculi,  thus  repeating,  more  or 
less  accurately,  the  outlines  of  the  system  of  lymphatic  channels ;  the  cells 
and  their  processes,  however,  probably  never  completely  fill  the  lymph- 
canals,  but  are  separated  from  the  unoccupied  wall  by  clefts,  thereby  afford- 
ing a  path  for  the  tissue-juices.  As  already  pointed  out,  the  protoplasmic 
net-work  is  usually  of  greater  delicacy  and  complexity  than  the  lymph- 
channels,  owing  to  the  fact  that  not  infrequently  two  or  more  processes  lie 
within  a  single  canal.  In  the  cornese  of  young  mammals,  as  the  kitten  or 
the  puppy,  the  larger  spaces  often  contain  several  corpuscles  which  form  a 
continuous  more  or  less  perfect  lining  applied  to  the  walls  of  the  cavities. 
Silver  preparations  of  such  tissue  display  the  outlines  of  these  cell-groups 
by  markings  closely  resembling  limited  endothelial  areas,  which,  in  princi- 
ple at  least,  these  flattened  adjoining  connective-tissue  cells  really  repre- 
sent. The  surfaces  of  contact  between  these  elements  are  indicated  by  lines 
of  deeply  stained  cement-substance  just  as  in  silver  pictures  of  ordinary 
endothelium,  the  nuclei  being  brought  to  view  by  additional  coloring  with 
haematoxylin  or  carmine. 

The  Wandering  Cells. — In  addition  to  the  corneal  corpuscles  proper,  or 
the  "  fixed"  corneal  cells,  careful  examination  sometimes  reveals  the  pres- 
ence of  minute  irregular  or  ovoid  masses  of  granular  protaplasm  within 
the  crevices  between  the  corneal  corpuscles  and  the  walls  of  the  spaces  or 
the  narrower  lymph-canals ;  these  bodies  are  the  so-called  wandering  cells 
of  the  cornea. 

Critical  inspection  of  these  elements  shows  them  to  be  migratoiy  leuco- 
cytes which  have  extended  their  journeys  within  the  lymph-channels  of  the 
cornea,  just  as  they  invade  the  lymphatic  spaces  of  other  connective  tissues. 
They  are,  therefore,  not  peculiar  to  the  cornea,  but  are  the  representatives  of 
widely  distributed  elements.  The  presence  of  leucocytes  within  the  normal 
corneal  tissue  has  been  questioned,  their  advent  being  regarded  as  evidence 
of  pathological  change.  There  seems,  however,  to  be  ample  reason  for  con- 
sidering these  migratory  cells  as  transient  guests  of  perfectly  normal  struc- 
tures. The  appellation  "fixed"  cells  applied  to  the  corneal  corpuscles 
proper,  as  distinguished  from  the  migratory  elements,  is  not  entirely  accu- 
rate, since  these  bodies,  in  common  with  many  other  active  connective-tissue 
cells,  may  display  changes  of  form  in  response  to  the  irritation  of  electricity, 
inflammation,  or  other  stimuli.  The  notable  increase  in  the  number  of  the 
wandering  cells  witnessed  during  inflammatory  processes  affecting  the  cornea 
is  probably  due  not  only  to  congregation  of  the  migratory  leucocytes  in 


THE   MICROSCOPICAL   ANATOMY  OF  THE   EYEBALL. 


231 


response  to  the  unusual  stimulus,  but  also  to  the  production  of  new  cells 
from  the  division  of  some  of  the  fixed  corneal  corpuscles,  as  long  ago 
convincingly  demonstrated  by  Strieker  and  Norris.1 

The  form  of  the  interfibrillar  cleft  in  which  the  migratory  cells  lie  has 
much  to  do  with  the  contour  of  the  individual  elements  ;  since,  it  is  evident, 
the  same  protoplasmic  mass  may  become  greatly  elongated  or  assume  a  more 
spherical  outline  according  to  the  limits  of  the  lymph-channel  in  which  it 
is  confined. 

The  Posterior  Limiting  Membrane  (the  membrane  of  Descemet,  the  pos- 
terior elastic  lamina,  the  membrane  of  Demours,  the  inner  basement  mem- 
brane) constitutes  an  apparently  homogeneous  elastic  lamella  which  separates 
the  inner  border  of  the  substantia  propria  from  the  endothelium  covering  the 
posterior  surface  of  the  cornea.  This  layer  differs  from  the  anterior  limit- 
ing membrane  in  its  physical,  chemical,  and  morphological  characteristics. 

It  is  much  less  intimately  attached  to  the  substantia  propria  than  is  the 
anterior  membrane,  as  explained  by  the  developmental  relations  of  the  cor- 
ueal  layers.  Prolonged 

maceration    in     ten    per  FlQ-  9- 

cent,  solution  of  sodium 
chloride  effects  its  com- 
plete isolation  from  the 
"fibrous  tissue  of  the  cor- 
nea. The  separated  mem- 
brane displays  a  marked 
tendency  to  roll  together, 
with  the  anterior  surface 
innermost,  and  resembles 
closely  elastic  tissue  in 
its  general  appearance 

and  behavior.  Its  marked  resistance  to  the  action  of  acids,  alkalines,  boil- 
ing water,  and  other  reagents  further  distinguishes  it  from  the  anterior 
limiting  membrane,  while  its  identity  with  elastic  tissue  is  called  in  ques- 
tion by  its  more  rapid  digestion  in  solutions  of  trypsin  (Sasse).2 

The  posterior  limiting  membrane  contains  no  cells,  and,  ordinarily, 
presents  no  indications  of  being  composed  of  secondary  lamellae  j  after  pro- 
longed boiling  in  water,  however,  it  is  sometimes  possible  to  resolve  the 
membrane  into  a  number  of  delicate  structureless  strata.  Exceptionally, 
sections  of  silvered  cornea  under  high  amplification  show  traces  of  these 
component  lamella.  The  membrane  also  differs  from  the  anterior  limiting 
layer  in  being  thinnest  at  the  centre  and  thickest  at  the  periphery  of  th 
,  its  thickness  at  these  points  being  respectively  .OC 


Vertical  section  of  the  posterior  lamellae  of  the  cornea  (a),  to- 
gether with  the  posterior  limiting  membrane  (c)  and  endothelium 
(d);  b,  corneal  cell  in  profile  within  a  space.  Magnified  500 
diameters. 


cornea 


i  Strieker  und  Norris  :  Versuche  iiber  Hornhautentzundung,  Studien  aus  dem  Institut 
f.  exper.  Pathologic  in  Wien,  I.,  1870. 

*  Sasse:  Zur  Chemie  der  Descemet  'schen  Membran,  Untersuch.  d.  physic 

d.  Univ.  Heidelberg,  n.,  1879. 


232  THE  MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL. 

metre  and  .010  to  .012  millimetre.  After  middle  life  the  membrane  be- 
comes somewhat  thicker,  in  some  instances  measuring  .020  millimetre ;  at 
the  same  time,  not  infrequently  low  papilliform  elevations  appear  on  the 
inner  surface  of  the  membrane  and  increase  with  age  both  in  size  and  in 
number.  At  the  periphery  the  posterior  limiting  membrane  breaks  up  into 
numerous  bands,  which  are  continued  from  the  sclero-corneal  junction  and 
become  the  structures  occupying  the  anterior  angle  of  the  ciliary  region  ;  in 
this  situation  they  are  known  as  the  ligamentum  pectinatum  iridis,  of  which 
a  fuller  description  is  given  in  connection  with  the  spaces  of  Fontana. 

The  Posterior  Endothelium,  the  endothelium  of  Descemet's  membrane, 
covers  the  inner  surface  of  the  cornea  and  forms  part  of  the  lining  of  the 
anterior  chamber  of  the  eye.  In  view  of  its  direct  derivation  from  meso- 
dermic  tissue,  this  layer  is  properly  regarded  as  endothelium,  and  not  as 
epithelium. 

The  posterior  covering  of  the  cornea  is  composed  of  a  single  layer  of 
thin  polyhedral  plates,  the  outlines  of  which,  as  displayed  by  silver-staining, 
present  a  mosaic  of  considerable  regularity  when  the  normal  curvature  and 
tension  of  the  tissue  are  maintained.  The  individual  cells  possess  the  usual 
characteristics  of  endothelial  plates,  having  an  oval,  sometimes  reniform, 
nucleus  situated  somewhat  eccentrically  within  the  faintly  granular  cell- 
protoplasm.  The  nuclei  are  sometimes  thicker  than  the  remaining  portions 
of  the  plates  and  project  beyond  the  general  level  of  the  cell-surface ;  in 
such  cases  the  line  of  the  inner  border  of  the  endothelial  layer,  as  seen  in 

vertical  sections,  is  not  straight,  but  sinuous,  the 
position  of  the  nuclei  then  being  indicated  by 
slight  elevations. 

These  cells,  being  subject  to  variations  in 
tension  due  to  relaxation  of  the  cornea  in  the 
j  course  of  preparation,  not  infrequently  present 
unusual  pictures.  Among  such  are  cells  with 
sinuous  outlines  or  stellate  forms^  the  project- 
ing rays  of  the  adjacent  elements  alone  being  in 
normal  contact.  The  production  of  such  star- 
like  figures  has  been  regarded  by  Schwalbe  as 

Endothelium  covenng  the  pos-  * 

terfor  limiting  membrane  seen     depending   upon    the   appearance   of  vacuoles 

SZe^"Urftee'    Magnlfled  "°     within  the  intercellular  cement;  by  others,  as 

Klebs,  and  Strieker  and  Norris,  the  endothelial 

cells  are  regarded  as  possessing,  especially  under  the  influence  of  irritation, 
the  power  of  contracting  and  of  changing  shape. 

The  remarkable  details  of  the  cells  of  Descemet's  membrane  described 
and  pictured  by  Smirnow,1  and  later  by  Nuel  and  Cornil,2  in  the  cornea  of 

1  Smirnow :  Ueber  die  Zellen  der  Descemet'schen  Haut  bei  Vogeln,  Internal.  Monatsbl. 
f.  Anat.  u.  Phys.,  Bd.  vn.,  1890. 

1  Nuel  et  Cornil:  De  Pendothelium  de  la  chambre  anterieure  de  1'ceil,  particuliere- 
ment  de  celui  de  la  cornee,  Archives  de  Biologic,  t.  x.,  1890. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL.  233 

the  pigeon,  duck,  chicken,  etc.,  include  the  presence  of  radiating  bundles 
of  fibrilhe  which  pass  from  near  the  nucleus  of  the  endothelial  cell  across 
the  intercellular  space  to  the  vicinity  of  the  nuclei  of  the  adjacent  plates. 
The  number  of  bundles  of  fibrilla}  in  general  corresponds  with  the  number 
of  sides  of  the  polyhedral  elements,  the  bundles  radiating  and  often  over- 
lapping those  from  neighboring  cells.  The  fibrilhe  rapidly  disappear  after 
death,  accurate  fixation  of  the  living  tissue  being  indispensable  for  their 
satisfactory  demonstration.  These  interesting  details  appear  to  be  limited 
to  the  cells  of  the  cornese  of  birds,  since  they  are  not  found  in  the  corre- 
sponding mammalian  structures. 

In  common  with  those  of  other  serous  surfaces,  the  endothelial  plates 
lining  the  anterior  chamber  are  continually  undergoing  destruction,  the  effete 
cells  being  replaced  by  new  elements,  as  indicated  by  the  karyokinetic  figures 
observed  within  the  endothelium  in  suitably  fixed  tissue.  The  presence  of 
minute  openings  between  the  endothelial  cells  along  their  lines  of  juncture, 
particularly  at  their  angles,  is  demonstrated  by  silver  stainings.  These 
apertures  or  stomata  are  regarded  by  Ciaccio l  as  directly  communicating 
with  the  system  of  lymph-channels  situated  within  the  ground-substance 
between  the  endothelium  and  the  posterior  elastic  membrane. 

The  Blood-Vessels  of  the  Cornea. — The  normal  cornea  is  non-vascular, 
except  within  a  very  limited  zone  at  the  extreme  periphery.  At  the  cor- 
neal  margin  the  anterior  limiting  membrane  fades  away  in  a  delicate  layer 
of  loose  connective  tissue  lodged  between  the  epithelium  and  the  substantia 
propria.  This  narrow  marginal  zone  contains  the  terminal  vascular  loops 
which  encircle  the  cornea  and  alone  represent  its  supply  of  blood-vessels. 
The  vascular  area,  although  very  narrow,  is  not  quite  uniformly  devel- 
oped, being  broadest  above  and  below,  where  it  measures  1-1.5  millimetres 
(at  most  2.0  millimetres),  and  narrowest  on  each  side,  where  it  attains  a 
width  of  .5-1.0  millimetre.  The  capillary  loops  composing  this  peripheral 
net- work  are  derived  from  delicate  arterial  stems  continued  from  the  anterior 
ciliary  arteries  through  the  episcleral  branches.  After  passing  the  linibus 
the  arterioles  rapidly  break  up  by  dichotomous  division  into  twigs  of  great 
delicacy,  .005-.006  millimetre  in  width,  which  communicate  with  one 
another  by  numerous  anastomoses  and  terminate  in  capillary  loops  forming 
a  series  encircling  the  cornea.  The  capillaries  give  origin  to  the  wider 
venous  radicles,  the  diameter  of  which,  as  determined  by  Leber,2  is  about 
double  that  of  the  arteries.  The  veins  join  the  episcleral  net-work  and 
become  tributaries  to  the  anterior  ciliary  trunks.  During  foetal  life  the 
peripheral  parts  of  the  cornea  are  invaded  by  the  precorneal  vascular  net- 
work. This  normally  diminishes  before  birth  to  the  limits  already  de- 
scribed, and  probably  at  no  time  extends  completely  over  the  cornea,  the 

1  Ciaccio :  Osservazioni  intorno  alia  membrana  del  Descemet  e  al  suo  endothelio,  Me- 
naorie  dell'  accad.  di  Bologna,  ser.  in.  t.  v.,  1875. 

2  Leber :    Die   Circulations-  und    Ernahrungsverhaltnisse    des  Auges,   Graefe  und 
Saemisch's  Handbuch,  Bd.  u.,  p.  334. 


234  THE   MICROSCOPICAL  ANATOMY  OF  THE   EYEBALL. 

central  portion  remaining  uninvaded.  The  more  deeply  situated  vascular 
loops  occasionally  observed  in  the  human  cornea  are  probably  to  be  re- 
ferred to  pathological  processes,  although  in  some  animals,  as  the  ox  and 
the  sheep,  according  to  Richiardi,1  such  vessels  normally  exist. 

FIQ.  11. 


Vascular  net  work  at  the  corneal  limbus.  (After  Leber.)— A,  A,  episcleral  branches  of  the  anterior 
ciliary  arteries;  V,  V,  anterior  ciliary  veins,  united  by  means  of  the  episcleral  venous  plexus;  a,  a,  v,  v, 
the  arterial  and  venous  limbs  of  the  marginal  loops ;  ac,  vc,  anterior  conjunctival  arteries  and  veins 
given  off  from  the  episcleral  vessels.  Low  magnification. 

The  Lymphatics  of  the  Cornea. — The  elaborate  system  of  intercommu- 
nicating corneal  spaces,  as  already  pointed  out,  must  be  regarded  as  repre- 
senting the  lymph-tracts  of  the  dense  connective  tissue  of  which  the  cornea 
is  composed.  In  addition  to  these  interstitial  clefts,  the  larger  nerve-trunks 
on  entering  the  cornea  are  accompanied  for  a  short  distance — about  one 
millimetre — by  distinct  perineural  sheaths  which  are  to  be  considered  as 
lymph-channels  into  which  the  adjacent  corneal  spaces  often  directly  open. 
These  perineural  canals  number  between  sixty  and  eighty,  and  are  lined 
with  a  more  or  less  perfect  covering  of  flattened  eudothelial  plates. 

The  fluids  occupying  the  corneal  spaces  are  probably  derived  from  two 
sources, — from  the  marginal  vascular  loops  and  from  the  diffusion  from  the 
anterior  chamber.  (Schwalbe.)  The  nutritive  juices  obtained  from  the 
peripheral  vessels  seem  to  be  devoted  particularly  to  the  anterior  epithelium 
and  the  more  superficial  layers  of  the  substantia  propria,  while  the  fluids 
derived  from  the  anterior  chamber  are  especially  destined  for  the  nourish- 
ment of  the  deeper  portions  of  the  cornea. 

1  Richiardi :  Sui  vasi  sanguiferi  della  cornea,  Zoologisch.  Anzeiger,  No.  7G,  1881. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL.  235 

The  presence  of  the  posterior  endothelium  offers  an  insurmountable 
barrier  to  the  loss  of  the  aqueous  humor  by  filtration  into  the  cornea,  since 
as  long  as  the  endothelial  elements  are  intact,  as  pointed  out  by  Leber, 
the  escape  of  the  aqueous  humor  is  extremely  insignificant.  When,  on  the 
contrary,  the  cells  covering  the  membrane  of  Descemet  are  removed,  as 
after  death,  the  corneal  tissue  rapidly  becomes  filled  with  the  watery  fluids, 
and,  consequently,  opaque.  The  possibility,  however,  of  diffusion  taking 
place,  at  least  to  a  very  limited  extent,  through  the  posterior  layers  of  the 


.-.,_       w- 

Perineural  lymph-sheath  lined  by  an  imperfect  endothelial  layer ;  in  two  places  corneal  spaces  directly 
communicate  with  the  lymph-channels.    Magnified  200  diameters. 

cornea  seems  to  be  demonstrated  by  the  experiments  of  Leber  and  Kriikow1 
and  of  Knies,2  in  which,  after  injection  of  potassium  ferrocyanide  and  sub- 
sequent treatment  with  iron  chloride  solution,  not  only  the  membrane  of 
Descemet  and  the  adjacent  lamellae  of  the  substantia  propria  were  found 
tinged,  but  also  the  intercellular  cement-substance  of  the  endothelium  ap- 
peared as  blue  lines. 

While  limited  diffusion  may  directly  affect  the  lamellae  contiguous  with 
the  membrane  of  Descemet,  the  more  usual  path  is  by  the  circuitous  route 
through  the  spaces  of  Fontana  at  the  angle  of  the  anterior  chamber,  and 
thence  into  the  lymph-channels  of  the  sclera  which  directly  communicate 

1  Quoted  by  Schwalbe :  Anatomic  der  Sinnesorgane,  p.  164. 

2  Knies :  Ueber  die  vorderen  Abflusswege  des  Auges  und  die  kunstliche  Erzeugung 
von  Glaukom,  Archiv  f.  Augenheilkunde,  Bd.  xxvin.,  1894. 


236  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

with  those  of  the  cornea.  The  importance  of  the  indirect  lateral  path  from 
the  anterior  chamber  into  the  corneal  tissue  has  been  strongly  emphasized 
by  the  recent  investigations  of  Staderini,1  which  conclusively  demonstrate 
the  conspicuous  r6le  played  by  the  spaces  of  Foutana  and  the  scleral  tissue 
in  the  escape  of  the  contents  of  the  anterior  chamber. 

Fluids  conveyed  within  the  lymph-channels  pass  into  the  surrounding 
corneal  stroma  through  the  especial  agency  of  the  interfibrillar  ground- 
substance  ;  the  absorption  thus  effected  results  in  the  complete  transference 
of  the  contents  of  the  lymph-canals  to  the  substantia  propria,  as  demon- 
strated in  cases  where  the  corneal  spaces  are  originally  filled  with  colored 
fluids,  the  spaces  after  a  time  appearing  empty  and  the  ground-substance 
deeply  tinged.  The  views  of  Straub,2  that  the  contents  of  the  lymph- 
channels  are  not  uniformly  distributed,  but  are  directed  under  usual  condi- 
tions particularly  to  those  parts  of  the  intercommunicating  interfascicular 
clefts  in  which  the  corneal  corpuscles  are  lodged,  are  at  variance  with  the 
testimony  of  other  observers,  and  are  directly  opposed  by  the  later  investi- 
gations of  v.  Recklinghausen3  and  of  Gutmann,4  which  sustain  the  ac- 
cepted doctrine  of  the  preformation  of  definite  paths  for  the  uniform  diffu- 
sion of  the  tissue-juices  throughout  the  corneal  stroma.  Gutmann  further 
succeeded  in  demonstrating  a  communication  between  the  intercellular  clefts 
within  the  anterior  epithelium  and  the  corneal  spaces  by  means  of  injections 
of  a  solution  of  asphalt  in  chloroform,  an  observation  of  interest  in  view 
of  the  previous  statements  of  Pfliiger5  maintaining  the  absorption  through 
the  anterior  surface  of  the  cornea. 

Although  diffusion  may  take  place  to  a  limited  extent  through  the  pos- 
terior surface  of  the  cornea,  as  already  pointed  out,  filtration  of  the  aqueous 
humor  into  the  corneal  strata  during  life  is  prevented,  probably  to  an 
absolute  degree,  by  the  presence  of  the  endothelium  of  the  membrane  of 
Descemet.  The  impermeability  of  this  layer  is  maintained  even  when 
subjected  to  unusual  pressure,  the  fluid  contents  of  the  anterior  chamber 
escaping  by  means  of  the  passage-ways  existing  at  the  sclero-iridial  angle. 

The  fluids  collected  within  the  system  of  corneal  spaces  are  carried  off 
by  two  principal  outlets :  (1)  by  the  lymphatics  of  the  conjunctiva;  (2)  by 
the  perineural  lymph-tracts.  The  first  of  these — the  conjunct! val  lymph- 
tracts — constitute  the  most  important  channels  for  the  escape  of  the  contents 
of  the  intra-corneal  lymph-paths ;  the  spaces  surrounding  the  nerve-trunks 


1  Staderini :  Ueber  die  Abflusswege  des  Humor  Aqueus,  Archiv  f.  Ophthalmologie, 
Bd.  xxxvu.,  Abth.  3,  1891. 

*  Straub:  Die  Lympbbahnen  der  Hornhaut,  Archiv  f.  Anat.  u.  Phys.,  Anat.  Abth., 
1887. 

3  v.  Recklinghausen :  Ueber  die  Saftkanalchen  der  Hornhaut,  Anatom.   Anzeiger, 
Bd.  in.,  No.  19,  1888. 

4  Gutmann :  Ueber  die  Lymphbahnen  der  Cornea,  Archiv  f.  mik.  Anat.,  Bd.  XXXII., 
1888. 

5  Pfluger :  Zur  Ernahrung  der  Cornea,  Klin.  Monatsblatt  f.  Augenheilkunde,  Bd. 
xx.,  1882. 


FIG.  13. 


Lymph-spaces  of  the  pig's  cornea,  injected  with  asphalt-chloroform,  showing  communication  between 
the  corneal  spaces  (a)  and  the  intra-epithelial  clefts  (6).    (Gutmann.) 


Lymph-spaces  of  the  sclerotic  of  pig  injected  with  asphalt-chloroform.    (Qutmann.) 


THE   MICROSCOPICAL   ANATOMY   OP   THE   EYEBALL.  237 

also  afford  no  insignificant  exit,  and  serve  to  convey  the  lymphatic  fluid 
within  the  perineural  sheaths  along  the  course  of  the  anterior  ciliary  nerves. 
While  the  lymph-channels  of  the  cornea  and  the  sclera  directly  communi- 
cate, as  well  demonstrated  by  the  investigations  of  Gutmanu,1  the  distinctly 
smaller  size  of  the  spaces  within  the  scleral  tissue  renders  it  improbable  that 
the  corneal  juices  pass  into  the  sclera  to  any  extent ;  on  the  other  hand, 
direct  observations  show  that  the  cornea  receives  no  inconsiderable  part  of 
its  tissue-juices  by  means  of  the  sclerotic  spaces. 

The  Nemes  of  the  Cornea. — The  nerves  of  the  cornea,  together  with 
those  of  the  ciliary  muscle  and  the  iris,  are  derived  from  the  ciliary  plexus 
formed  by  the  long  and  short  ciliary  nerves.  Those  destined  for  the  supply 
of  the  cornea  proceed  through  the  sclera,  on  the  outer  side  of  Schlemm's 
canal,  to  the  vicinity  of  the  corneal  margin,  where  they  unite  to  form  an 
encircling  net- work,  the  plexus  annulaiis. 

Two  sets  of  fibres  are  given  off  from  this  plexus  :  1.  Twigs  which,  after 
obliquely  piercing  the  sclera,  pass  to  the  conjunctiva  and  there  join  the 
conjunctival  nerves  in  the  formation  of  net- works  supplying  that  structure ; 
from  the  conjunctival  plexus  thus  constituted  a  number  of  nerve-trunks  are 
given  off  at  the  limbus,  which  enter  the  cornea  and  are  especially  destined 
for  the  supply  of  the  anterior  layers  of  that  structure.  2.  Additional  and 
far  more  numerous  corneal  branches  which  are  given  off  from  the  annular 
plexus  and  proceed  directly,  more  or  less  radially  arranged,  to  the  substantia 
propria  cornea?,  which  they  enter  well  towards  the  membrane  of  Descemet. 

The  entire  number  of  nerve-trunks  thus  entering  at  the  corneal  margin 
aggregates  between  sixty  and  eighty.  The  nerve-bundles  are  accompanied, 
as  already  mentioned,  by  enveloping  perineural  lymph-sheaths  for  a  short 
distance — about  .75  to  1  millimetre — along  their  course  within  the  corneal 
stroma.  These  trunks,  which  vary  greatly  in  thickness, — some  containing 
two  or  three  nerve-fibres,  others  ten  to  twelve, — soon  lose  their  medullary 
substance,  becoming  non-medullated  within  .5  to  1.5  millimetres  after 
entering  the  cornea ;  they  soon  break  up  within  the  substantia  propria  into 
delicate  fibrillaj,  which  take  part  in  the  formation  of  numerous  plexuses. 

Our  knowledge  concerning  the  arrangement  and  the  ultimate  distribu- 
tion of  the  nerves  within  the  cornea,  derived  from  the  investigations  of 
Ciaccio,2  Kolliker,3  Hoyer,4  Konigstein,5  Klein,6  Waldeyer,7  Ranvier,8  and 

1  Gutmann :  loc.  cit. 

2  Ciaccio :  On  the  Nerves  of  the  Cornea,  etc.,  Quart.  Journal  of  Mic.  Science,  1868. 

3  Kolliker:    Ueber  die   Nervenendigungen  in  der  Hornhaut,  Wiireburg.  naturwis. 
Zeitschr  ,  Bd.  vi.,  1866. 

*  Hoyer:  Ueber  die  Nerven  der  Hornhaut,  Archiv  f.  mik.  Anatomie,  Bd.  IX.,  187 

5  Konigstein  :  Beobachtungen  ueber  die  Nerven  der  Cornea  u.  s.  w.,  Wiener  Sitzungs- 
ber.,  1877. 

6  Klein :  The  Termination  of  the  Nerves  in  the  Mammalian  Cornea,  Quart.  Journal 
of  Mic.  Science,  1880. 

7  Waldeyer:  Ueber  die  Endungsweise  der  sensiblen  Nerven,  Archiv  f.  mik.  Anat.,  B 
xvii.,  1880. 

8  Ranvier:  Traite  technique  d'histologie,  1888. 


238 


THE   MICROSCOPICAL  ANATOMY   OF   THE   EYEBALL. 


FIG.  14. 


others,  has  been  extended  by  the  more  recent  observations  of  Dogiel,1  to 
whom  we  are  indebted  for  many  additional  accurate  details  regarding  the 
disposition  of  the  nerves  within  the  human  cornea.  Dogiel  has  sought  the 
assistance  of  the  newer  technique  of  nerve-staining,  and  has  depended  largely 
upon  preparations  colored  with  methyleue-blue,  which  method  has  yielded 
such  brilliant  results  in  displaying  the  nerve-endings  in  other  structures. 

According  to  this  investigator,  about  two-thirds  of  the  entire  number 
of  nerve-trunks  passing  into  the  human  cornea,  or  between  forty  and  fifty, 

are  devoted  to  the  supply  of 
the  more  anteriorly  situated 
lamellae,  the  remainder,  or 
from  twenty  to  thirty,  being 
distributed  to  the  posterior 
layers.  An  additional  distinc- 
tion in  the  distribution  of  the 
fibres  occurs  in  the  constitution 
of  the  primary  or  fundamental 
plexus  formed  by  the  nerve- 
trunks  shortly  after  entering 
the  substantia  propria,  since  the 
fundamental  plexus  situated 
within  the  peripheral  part  of 
the  cornea  is  derived  from  the 
more  anteriorly  placed  nerves, 
and  that  of  the  central  area  is 
contributed  by  the  posterior 
twigs. 

The  constituents  of  the 
nerve-trunks  during  their 
course  through  the  corneal  tis- 
sue, before  losing  their  medul- 
lary substance,  give  off  delicate 
non-medullated  fibres  at  the 
constrictions  marking  the 
nodes  of  Ranvier ;  the  fibres 
so  given  off  soon  break  up  into  their  component  varicose  fibrillse,  and  are 
distributed  to  the  various  lamellae  of  the  substantia  propria  through  which 
the  larger  nerve- trunk  passes. 

In  addition  to  the  lateral  twigs  which  spring  from  the  main  nerve-stem 
at  various  levels,  perforating  branches  ascend  through  the  anterior  corneal 
lamellae  as  far  as  the  epithelium,  beneath  which  tissue  they  form  the  sub- 
epithelial  plexus.  Since  the  portions  of  the  subepithelial  plexus  which  these 


A,  B.  medullated  nerve-fibres  from  an  anterior  corneal 
nerve-trunk ;  a,  the  peripheral  part ;  b,  the  central  fibre  of 
the  axis-cylinder ;  c,  the  medullary  substance ;  d,  d',  nerve- 
fibrils  which  pass  oft'  from  the  larger  fibre  at  a  node  of 
Ranvier  and  break  up  into  ultimate  fibrillse ;  at  e  the 
central  fibre  of  the  axis-cylinder  loses  its  medullary  coat 
and  proceeds  as  minute  varicose  fibrillse.  Methylene-blue 
staining.  (Dogiel.) 


1  Dogiel:  Die  Nerven  der  Cornea  des  Menschen,  Anatom.  Anzeiger,  Bd.  v.,  No.  16, 


1890. 


THE    MICROSCOPICAL  ANATOMY  OF  THE   EYEBALL. 


FIG.  15. 


239 

ascending   branches  contribute  correspond  to  the  portions  of  the  funda 
mental  plexus  from  which  they  spring,  it  follows  that  the  nerve-fibres  com 
posing  the  peripheral  parts  of  the  subepithelial  net- work  are  derived  from  the 
anterior  trunks,  while  those  of  the  central  part  proceed  from  the  posterior 
stems. 

The  terminations  of  the  perforating  twigs  composing  the  subepithelial 
plexus  bear  a  twofold  relation  to  the  anterior  corneal-  epithelium.     A  few 
fibres  directly  pass  towards  the  epithelium  and  break  up  into  fibrillae  which 
penetrate    between     the 
cells  and   bear   conspic- 
uous round  or  pyriform 
terminal  end-bulbs.  (Fig. 
15.)      A  second  set  of 
fibres  ascend    from   the 
subepithelial  plexus  into 
the    epithelium,    among 
the   elements   of  which 
they  ramify  as  terminal 
fibrillae,   and    constitute 
the  so-called  intra-epithe- 
lial  plexus.     The  older 
descriptions   of  a  dense 
net-work     of     commu- 
nicating     nerve-fibrillse 
existing      between     the 

•  u   v          n    /•  j  i.  Termination  of  the  nerves  in  the  anterior  part  of  the  cornea.— 

epithelial  Cells  formed  by  a,  twig  from  an  anterior  corneal  nerve-trunk ;  6,  perforating  branch  ; 

HIP  nninn  nf  rhp  iil+imnte  c>  P*"  of  subepithelial  plexus!  d,  intra-epithelial  fibrillae  ending 

'  in  terminal  plates.    Methylene-blue  preparation.    (Dogiel.) 

nervous  threads  are  not 

substantiated  by  the  more  accurate  recent  investigations,  the  presence  of 
such  terminal  net-works  being  not  only  doubtful  in  regard  to  the  corneal 
tissue,  but  likewise  in  opposition  to  the  conclusions  concerning  nerve- 
terminations  in  general,  as  based  upon  the  advances  made  in  the  more 
intimate  knowledge  of  the  final  disposition  of  nerve-filaments. 

The  observations  by  Dogiel  on  nerves  stained  with  methylene-blue  agree 
with  those  of  Feist,1  and  show  that  the  axis-cylinders  of  the  corneal  fibres 
consist  of  an  intensely  colored  central  fibre  surrounded  by  a  less  deeply 
tinged  envelope, — Feist's  peripheral  axis-cylinder  substance.  The  ultimate 
elements  of  the  axis-cylinders  of  the  more  robust  fibres  are  traced  as  deli- 
cate varicose  fibrilla?,  which  pass  with  many  tortuosities  to  their  final  dis- 
tribution. 

The  terminal  fibres  of  the  corneal  nerves  are  connected  with  various 
forms  of  end-organs,  among  which  intricately  wound  ball-like  convolutions, 
less  closely  contorted  hooks  and  loops,  and  irregularly  quadrate  plates  are 


1  Feist :  Ueber  die  vitale  Metbylenblaufarbung  markhaltiger  Nervenstamme,  1889. 


240 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


conspicuous.  The  spherical  convolutions  consist  of  aggregations  of  the 
greatly  contorted  and  closely  intertwined  non-medullated  terminal  fibrillse, 
which  thus  form  conspicuous  objects  at  the  extreme  periphery  of  the  cornea 
within  a  zone  from  .5  to  1.5  millimetres  in  width  next  the  liinbus.  The 
terminal  hooks,  which  occur  within  the  same  area,  are  formed  by  less  closely 
wound  fibrillse  derived  by  the  repeated  division  of  the  larger  bundles  ;  they 
present  variations  in  their  details,  some  being  sharply  turned,  while  others 

are  bent  in  more  gradual  curves. 
In  addition  to  terminal  convolutions 
and  hooks  which  are  found  espe- 
cially within  the  periphery  beneath 
the  anterior  limiting  membrane,  the 

FIG.  17. 


FIG.  16. 


Special  endings  of  corneal  nerves.—^.,  medul- 
lated  nerve-fibres  dividing  into  two  branches,  one 
of  which  terminates  in  the  convolution  B,  the 
other  breaks  up  into  three  twigs,///' ;  /  subdivides 
into  h  and  i;  h  ends  in  a  loop  (D)  and  a  hook  (E) ; 
f  passes  into  the  terminal  convolution  B;  b" 
splits  into  smaller  fibrillse ;  g,  terminal  fibrillae  of 
neighboring  twig,  which  take  part  in  forming  the 
convolution ;  a,  6,  parts  of  the  axis-cylinder ;  C, 
medullary  substance.  Methylene-blue  preparation. 
(Dogiel.) 


Portion  of  fundamental  plexus  from  the 
periphery  of  the  anterior  layers  of  the  cornea.— 
a,  b,  twigs  derived  from  different  corneal  nerves; 
c,  <?,  areas  in  which  a  and  b  form  an  especially  rich 
plexus.  Methylene-blue  preparation.  (Dogiel.) 


terminal  fibrillse  within  this  zone  are  associated  with  special  endings  in  the 
form  of  irregularly  quadrate  or  spade-like  plates;  these  are  usually  con- 
nected with  varicose  fibrillae  which  pass  off  as  non-medullated  twigs,  de- 
rived from  the  fundamental  plexus  or  from  the  perforating  branches.  The 
existence  of  the  peculiar  club-shaped  and  forked  nerve-endings  described 
by  Brand1  in  the  human  cornea  is  very  doubtful,  and,  indeed,  has  been 
directly  denied  by  Dogiel ;  the  pictures  obtained  by  Brand  seem  to  be  due 


1  Brand  :  Die  Nervenendigungen  in  der  Hornhaut,  Arckiv  f.  Augenheilkunde,  Bd. 
.,  1888. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL. 


241 


to  imperfectly  stained  preparations,  since  he  declares  the  rami  perforantes 
to  be  the  terminal  fibrils  of*  the  corueal  nerves,  and  denies  the  presence  of 
nervous  filaments  both  beneath  and  within  the  epithelium. 

The  plexuses  within  the  substantia  propria  are  formed  by  the  ramifica- 
tions of  the  twigs  given  off  at  various  levels  from  the  perforating  trunks 
and  of  the  terminal  pencils  of  fibrillae  into  which  the  nerve-trunks  break 
up.  In  successful  preparations,  five  or  six  distinct  layers  of  interwoven 
fibrils  occur  within  the  substantia  propria  in  different  planes,  the  plexuses 
spreading  out  between  the  strata  of  fibrous  tissue. 

Tlie  points  of  meeting  of  the  fibres  supplying  the  corneal  stroma  are 
usually  marked  by  angular  areas  outlined  as  well  as  traversed  by  the  in- 
terlacing fibrils  which  join  one  another  at  acute  angles  of  varying  magni- 
tude. Within  these  nodal  points  nuclei  are  often  to  be  distinguished, 
which,  however,  belong  to  the  delicate  investing  sheaths,  and  not  to  gan- 
glion-cells, as  formerly  maintained  by  some  authorities. 

FIG.  18. 


Portion  of  fundamental  plexus  of  anterior  layers  of  the  cornea.  Gold  preparation.— n,  n,  nodal  points. 

Magnified  120  diameterS. 

The  fibrillje  constituting  the  fundamental  plexuses  of  the  substantia 
propria  are  distinguished  by  the  remarkable  zigzag  course  which  they  pur- 
sue after  their  liberation  from  the  larger  nerve-fibres  traversing  the  corneal 
tissue.  The  distribution  of  the  fibrill®,  furthermore,  is  not  uniform,  since 
at  certain  points  they  become  tortuous  to  an  unusual  degree,  two,  three,  or 
even  more  fibrillffi  taking  part  in  the  production  of  an  area  which  becomes 
conspicuous  on  account  of  the  elaborate  windings  of  the  terminal  nervous 
threads.  (Fig.  17,  c,  c'.) 

VOL.  I.— 16 


242  THE    MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 

The  question  as  to  the  relations  of  the  terminal  uerve-fibrillse  to  the  cor- 
neal  corpuscles  has  claimed  much  attention  and  elicited  no  little  disagree- 
ment of  opinion  among  observers.  Kiihne,  Konigstein,  Ciaccio,  Waldeyer, 
and  others  have  accepted  a  more  or  less  direct  connection  between  the 
fibrillse  and  the  protoplasm  of  the  corneal  cells,  while  Hoyer,  Klein, 
Schwalbe,  and  others  regard  such  continuity  as  only  apparent.  Dogiel 
gives  very  positive  testimony  in  opposition  to  the  assumed  continuity  of 
these  structures,  which  conclusion  fully  accords  with  the  views  of  the 
writer.  The  fact  that  the  nerve-fibrillse  forming  the  various  interlacements, 
which  collectively  constitute  the  fundamental  plexus,  lie  within  the  inter- 
fascicular  clefts,  and  hence  closely  associated  in  position  with  the  corueal 
corpuscles,  renders  the  determination  of  the  exact  relation  between  the  cells 
and  fibrillae  often  a  matter  of  difficulty  and  uncertainty,  but  critical  exam- 
ination usually  convinces  that  contiguity  and  not  continuity  exists  between 
these  structures,  and  that  the  relations  of  the  nerve-fibrillae  to  the  connective- 
tissue  elements  within  the  corneal  stroma  do  not  present  a  remarkable 
exception  to  the  relations  of  such  structures  elsewhere. 

THE   SCLERA. 

The  sclerotic  coat  contributes  the  posterior  four-fifths  of  the  dense 
fibrous  tunic,  and  is  largely  instrumental  in  maintaining  the  form  of  the 
eyeball ;  at  the  sclero-corneal  juncture  the  sclerotic  tissue  is  directly  contin- 
uous with  that  of  the  cornea  in  front.  This  coat  is  imperfect  in  the  vicinity 
of  the  posterior  pole,  owing  to  the  passage  of  the  optic  nerve  through  the 
fibrous  tunic,  at  this  point  the  sclera  being  represented  by  the  net-work  of 
interlacing  fibrous  bundles  which  constitutes  the  lamina  cribrosa,  through 
the  interstices  of  which  the  optic  fibres  escape  in  their  course  from  the  inte- 
rior of  the  eye  to  the  brain.  The  sclerotic  is  additionally  pierced  by  blood- 
vessels and  nerves  ;  those  constituting  the  short  ciliary  group  enter  close  to 
the  optic  nerve,  those  forming  the  long  ciliary  group  passing  obliquely 
through  the  sclera  a  few  millimetres  farther  forward,  while  the  large  vorti- 
cose venous  trunks  make  their  exit  near,  but  slightly  behind,  the  equator. 
The  anterior  ciliary  vessels  traverse  the  scleral  tissue  a  short  distance  pos- 
terior to  the  corneal  limbus. 

The  sclera  is  thickest  in  the  immediate  neighborhood  of  the  optic  nerve, 
at  a  point  about  two  and  a  half  to  three  millimetres  within  and  about  one 
millimetre  below  the  posterior  pole  of  the  eyeball.  In  this  locality  the 
fibrous  tunic  measures  about  one  millimetre  in  thickness,  gradually  be- 
coming thinner  towards  its  anterior  boundary.  The  thinnest  part  of  the 
sclera  corresponds  to  the  zone  covered  by  the  tendons  of  the  ocular  muscles, 
which  lies  about  seven  millimetres  behind  the  limbus  ;  beneath  the  tendons 
the  coat  measures  only  about  .35  millimetre.  After  receiving  the  expanded 
insertions  of  the  recti  muscles,  the  anterior  segment  of  the  sclerotic  increases 
slightly  in  thickness,  and  at  the  junction  with  the  cornea  measures  approxi- 
mately .6  millimetre.  The  bluish  tint  of  the  sclerotic  coat  in  children  de- 


THE   MICROSCOPICAL   ANATOMY  OP  THE   EYEBALL.  243 

pends  upon  the  pigmented  tissue  of  the  underlying  choroid  showing  through 
the  partially  translucent,  thin,  immature  fibrous  tunic  of  the  young  eye ;  the 
yellowish  tint  observed  in  old  age,  on  the  contrary,  is  due  to  accumulations 
of  adipose  tissue. 

The  sclera  resembles  the  cornea  in  its  general  structure  in  being  made 
up  of  a  framework  of  closely  united  interlacing  bundles  of  white  fibrous 
tissue,  throughout  which  extends  a  system  of  intercommunicating  lymph- 
spaces  in  which  lie  the  connective-tissue  cells,  the  sclerotic  corpuscles. 

The  white  fibrous  tissue  of  the  sclera  yields  gelatin  on  boiling,  and  is 

FIG.  19. 


Section  of  sclera  showing  the  component  fibrous  tissue  (<t)  arranged  as  Interlacing  lamellae ;  in  the 
clefts  between  the  bundles  lie  the  scleral  cells ;  b,  b,  obliquely  cut  bundles  of  circularly  disposed  fibres. 
Magnified  480  diameters. 

arranged  in  bundles  disposed  principally  in  two  general  directions,  equa- 
torially  and  meridionally,  although  the  individual  fibrous  bands  interlace 
with  one  another  at  all  angles.  In  the  vicinity  of  the  corneo-scleral  junc- 
ture the  equatorial  fibres  are  unusually  well  developed,  while  posteriorly 
the  meridional  ones  preponderate.  The  scleral  bundles  contain  numerous 
elastic  fibres,  which  are  particularly  rich  within  the  inner  strata  of  the  coat, 
especially  around  the  points  of  passage  of  the  vessels  and  nerves. 

The  fibrous  bundles  received  from  the  insertions  of  the  straight  ocular 
muscles  blend  principally  with  the  meridional  fibres  of  the  sclerotic ;  those 
from  the  oblique  muscles  mingle  particularly  with  the  equatorial  bundles ; 
in  both  cases,  however,  they  soon  leave  the  outer  surface  of  the  sclera  to 
dip  deeply  into  and  become  intimately  united  with  the  scleral  tissue. 
(LSwig.1) 

The  tissue-spaces  within  the  sclerotic  coat  resemble  those  of  the  cornea, 
but  are  smaller,  less  regularly  disposed,  and  less  generously  provided  with 
an  elaborate  system  of  minute  connecting  canals,  the  canaliculi.  In  profile 

1  Lowig :  Beitrage  zur  Morphologic  des  Auges,  Leipzig,  1858. 


244 


THE   MICROSCOPICAL   ANATOMY   OP   THE    EYEBALL. 


they  appear  as  lenticular  clefts  between  the  interlacing  bundles  of  fibrous 
ground-substance. 

The  cells  occupying  these  spaces — the  scleral  corpuscles — in  general  re- 
semble those  of  the  cornea,  and  occupy  the  walls  of  the  spaces ;  they  do 
not,  however,  present  the  extended  and  richly  branched  outlines  of  the 
corneal  cells.  Migratory  leucocytes — the  ivandering  cells — are  also  occasion- 
ally observed  within  the  juice-canals  of  the  sclera.  The  system  of  lymph- 
spaces  of  the  sclerotic  coat  is,  as  a  whole,  less  well  developed  than  that  of 
the  cornea,  with  which  it  stands  in  free  and  direct  communication  at  the 
limbus. 

In  addition  to  the  usual  plate-like  corpuscles  and  the  occasional  wander- 
ing cells  included  within  the  lymph-spaces,  the  sclerotic  coat  contains  pig- 
mented  connective  tissue  cells.  These  occur  in  profusion  within  the  inner- 

FIG.  21. 


Section  through  the  adjacent  parts  of  the  sclera  and  the  choroid,  with  the  intervening  supra- 
choroidal  space. — s,  fibrous  tissue  of  the  sclera,  the  innermost  pigmented  layer  of  which  constitutes 
the  lamina  fusca,  f;  p,  the  supra-choroidal  space  traversed  by  the  loose  reticuluin  formed  by  the  tra- 
beculse ;  the  latter  are  invested  in  places  by  the  pigmented  cells ;  c,  outer  zone  of  the  choroid ;  v,  partial 
section  of  a  choroidal  vessel.  Magnified  480  diameters. 

most  stratum,  where  they  contribute  the  color  which  distinguishes  the  lamina 
fusca  as  a  dark-brown  layer  immediately  next  the  supra-choroidal  space, 
the  cleft  separating  the  sclera  from  the  choroid.  The  inner  surface  of  the 
pigmented  layer,  which  constitutes  the  outer  wall  of  the  space  just  men- 
tioned, is  clothed  with  endothelium  continuous  with  the  covering  of  the 
fibrous  trabeculse  traversing  the  cleft  as  well  as  the  outer  surface  of  the 
choroid. 

*  Pigmented  cells  also  occur  within  the  sclera  in  the  vicinity  of  the  corneal 
margin,  around  the  optic  entrance,  and  along  the  canals  transmitting  the 
perforating  blood-vessels  and  nervous  trunks.  The  pigmented  elements 
occurring  in  these  localities  present  the  usual  appearances  of  irregularly 
stellate  connective-tissue  cells  in  which  the  protoplasm  has  become  more 
or  less  filled  with  the  colored  particles.  The  nuclei  generally  remain 


THE    MICROSCOPICAL   ANATOMY   OP   THE   EYEBALL.  245 

uninvaded,  and  appear  as  light  ovoid  areas,  in  sharp  contrast  to  the 
surrounding  deeply  colored  cell-bodies. 

The  outer  surface  of  the  sclerotic  coat  is  also  covered  throughout  its 
greater  extent,  from  the  insertion  of  the  ocular  muscles  to  the  sheath  of 
the  optic  nerve,  with  a  layer  of  endothelium  which  constitutes  the  lining 
of  the  inner  wall  of  the  episcleral  lymph-sac,  or  space  of  Tenon,  this  latter 
space  being  included  between  the  capsule  of  Tenon  without  and  the  sclerotic 
within. 

At  the  position  of  the  optic  nerve,  the  sclerotic  tissue  becomes  'directly 
continuous  with  the  fibrous  tissue  composing  the  external  or  dural  sheath 
of  the  nerve ;  upon  the  outer  surface  of  this  sheath  the  endothelium  covering 
the  eyeball  is  reflected  for  some  distance  to  form  the  inner  lining  of  the 
supra- vaginal  lymph-space.  The  external  surface  of  the  sclera  is  roughened 
by  the  attachment  of  numerous  fibrous  trabeeulse  which  traverse  the  peri- 
ocular  space  to  reach  the  capsule  of  Tenon,  which  forms  its  outer  wall. 

The  Blood-  Vessels  of  the  8dera. — The  vascular  structures  of  the  sclerotic 
coat  naturally  fall  within  two  groups  : 

1.  The  blood-vessels  which  perforate  the  sclera  in  their  passage  to  the 
more  deeply  lying  uveal  tract ; 

2.  The  blood-vessels  of  the  sclera  which  are  concerned  in  supplying  its 
tissue. 

The  vessels  constituting  the  first  group,  which  far  exceed  in  size  and  im- 
portance those  distributed  directly  to  the  fibrous  tunic,  are  represented  by  three 
sets, — (a)  those  piercing  the  anterior  portion  of  the  sclera,  as  the  branches 
of  the  anterior  ciliary  vessels ;  (6)  those  passing  out  near  the  equator  of 
the  eyeball,  as  the  large  venae  vorticosae ;  and  (c)  those  penetrating  the  pos- 
terior region  of  the  sclerotic  coat  in  the  vicinity  of  the  optic  entrance,  as 
the  long  and  the  short  posterior  ciliary  arteries.  These  vessels  during  their 
passage  through  the  fibrous  coat  give  off  no  branches  directly  distributed  to 
the  sclera. 

The  especial  vascular  supply  of  the  sclera  is  exceedingly  meagre,  con- 
sisting, for  the  most  part,  of  a  few  twigs  given  off  from  the  wide-meshed 
episcleral  net-work,  which  ramify  within  the  more  superficial  strata  of  the 
tunic.  The  episcleral  net-work  itself  is  formed  by  the  delicate  branches 
contributed  by  both  the  anterior  and  the  posterior  ciliary  arteries,  those 
from  the  former  being  of  most  importance. 

In  the  immediate  vicinity  of  the  position  at  which  the  optic  nerve 
pierces  the  fibrous  tunic  of  the  eyeball  the  short  ciliary  arteries  give  off 
a  few  small  twigs  to  the  sclera ;  these  vessels  unite  within  this  tunic  to 
form  au  encircling  anastomosis, — the  circuhis  Zwrm,— from  which  minut.- 
branches  extend  towards  the  dural  sheath  of  the  nerve  and  join  with  the 
arterioles  supplying  that  envelope,  thus  establishing  a  communication 
between  the  branches  of  the  central  retinal  and  the  ciliary  vessels. 

The  small  veins  which  collect  the  blood  from  the  anterior  part  of  the 
extended  tract  provided  for  by  the  episcleral  net-work  become  tributaries 


246  THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 

of  the  anterior  ciliary  veins ;  those  draining  the  equatorial  zone  of  the 
sclera  empty  into  the  large  venae  vorticosse,  while  posteriorly  the  minute 
venous  radicles  from  the  sclera  and  the  optic  nerve  join  to  form  the  short 
ciliary  veins. 

The  lymphatics  of  the  sclera  are  represented  by  the  intercommunicating 
cell-spaces  alone,  distinct  lymph-vessels  being  wanting.  The  system  of 
juice-channels  being  less  developed  than  that  of  the  cornea,  the  fluids  within 
the  former  pass  freely  into  the  corneal  spaces.  The  lymph-clefts  within 
the  sclera  in  the  immediate  neighborhood  of  the  angle  of  the  anterior 
chamber  stand  in  closest  relation  with  the  spaces  of  Fontana,  as  emphasized 
by  the  investigations  of  Staderini ;  through  their  agency,  therefore,  it  is 
probable  that  lymph-streams  are  established  towards  the  more  capacious 
channels  within  the  corneal  stroma,  as  well  as  towards  the  lumen  of  the 
contiguous  veins. 

The  nerves  of  the  sclera  may  be  grouped,  like  its  blood-vessels,  into 
perforating  trunks  destined  for  the  supply  of  other  structures,  and  those 
distributed  to  the  scleral  tissue  itself.  These  latter — by  no  means  numer- 
ous— ramify  principally  within  the  more  superficial  layers  of  the  coat,  being 
derived,  according  to  the  observations  of  Konigstein,1  as  minute  twigs  from 
the  ciliary  nerves  as  these  pass  forward  between  the  sclerotic  and  the  choroid 
coat.  The  delicate  branches  so  given  off  soon  break  up  into  finer  bundles 
of  axis-cylinders,  which  terminate  in  highly  tortuous  and  intricately  coursing 
ultimate  fibrillae  between  the  fibrous  fasciculi,  in  a  manner  somewhat  re- 
sembling the  nerve-endings  within  the  central  parts  of  the  corneal  stroma. 

THE   SCLERO-CORNEAL   JUNCTURE. 

The  transition  of  the  dense  opaque  tissue  of  the  sclera  into  the  beauti- 
fully transparent  structure  of  the  cornea  marks  the  outer  boundary  of  a 
region  of  especial  interest,  in  which  four  important  portions  of  the  eye — the 
cornea,  the  sclera,  the  iris,  and  the  ciliary  body — meet.  The  conspicuous 
differences  between  the  scleral  and  the  corneal  portions  of  the  fibrous  tunic 
depend  upon  physical  rather  than  morphological  variations,  since,  as  already 
seen,  the  structural  elements  and  their  general  plan  of  arrangement  closely 
correspond. 

The  tissue  of  the  sclera  and  the  contained  lymph-spaces  are  directly 
continuous  with  those  of  the  cornea,  and  while  the  scleral  spaces  are  smaller 
and  the  circularly  disposed  bundles  of  fibrous  tissue  are  particularly  well 
developed,  there  is  much  less  histological  differentiation  at  the  zone  of  tran- 
sition than  might  be  expected  from  the  sharply  defined  macroscopic  changes 
marking  the  juncture. 

Examination  of  meridional  sections  shows  that  the  scleral  tissue  extends 
somewhat  farther  forward  at  the  anterior  and  posterior  borders  of  the  coat 

1  Konigstein :  Ueber  die  Nerven  der  Sclera,  Archiv  f.  Ophthalmologie,  Bd.  xvn., 
1881. 


THE    MICROSCOPICAL    ANATOMY   OF   THE   EYEBALL. 


247 


than  at  the  intervening  levels,  the  projection  of  the  outer  boundary  passing 
farther  towards  the  anterior  pole  than  the  inner  process.  In  consequence 
of  this  arrangement  the  corneal  margin  is  received  within  the  recess  between 
the  external  and  internal  scleral  processes  in  a  manner  suggesting  the  often 
quoted  relation  of  a  watch-crystal  to  its  frame. 


FIG.  22. 


Meridional  section  of  ciliary  region. — a,  cornea;  6,  sclera;  c.  conjunctiva;  d,  iris;  «,  angle  of  ante- 
rior chamber;  /,  pupillary  margin  of  iris;  i,  ciliary  processes;  k,  ciliary  ring;  /,  artificial  separation 
between  choroid  and  sclera ;  m,  ciliary  muscle.  Magnified  23  diameters. 

The  connections  of  the  inner  scleral  process  are  of  especial  interest  on 
account  of  their  important  relations  to  the  structures  lying  at  the  anterior 
angle  of  the  ciliary  region  where  the  cornea,  the  iris,  and  the  ciliary  muscle 
meet.  The  anterior  and  external  border  of  the  inner  scleral  process  marks 

FIG.  23. 


Section  through  the  lateral  wall  of  the  anterior  chamber.— o,  angle  of  anterior  chamber.  .S. 
Schlemm's  canal ;  v,  one  of  the  adjacent  scleral  veins ;  c,  corneal  tissue  ;  »,  scleral  tissue;  i.  beginning 
of  the  pecti nate  ligament ;  /,  spaces  of  Fontana ;  i,  iris ;  m,  ciliary  muscle.  Magnified  7( 

the  position  of  a  conspicuous  circular  channel  which  surrounds  the  limbus 
cornese  and  lies  close  to  the  angle  of  the  anterior  chamber ;  this  is  the  canal 
of  Schlemm,  an  annular  venous  sinus  possessing  intimate  relations  to  the 
anterior  lymph-spaces  of  the  eye. 


248  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

The  canal  of  Schlemm  (sinus  venosus  iridis,  sinus  venosus  Schlemmii, 
circulus  venosus  ciliaris),  as  seen  in  meridional  sections,  usually  presents  a 
single  lumen,  elliptical  or  pyriform  in  outline,  which  measures  .30  to  .34 
millimetre  antf  .045  to  .050  millimetre  in  its  longest  and  shortest  diameters 
respectively ;  not  infrequently,  however,  the  lumen  of  the  canal  is  broken 
up  into  two,  or  even  three,  compartments  by  the  presence  of  trabeculee. 
This  subdivision,  however,  is  usually  limited  to  small  portions  of  the  tube, 
the  lumen  becoming  again  single  beyond  the  position  of  the  bridging  bands. 

The  outer  wall  of  Schlemm's  canal  is  formed  by  the  dense  scleral  stroma ; 
its  inner  boundary,  on  the  contrary,  is  composed  of  a  peculiar  spongy  reticu- 
lated nbro-elastic  lamella,  which  appears  to  be  the  anterior  continuation  of 
the  inner  scleral  process,  but  which  really  is  derived,  as  shown  by  Angelucci,1 
as  an  extension  of  the  choroidal  tract.  It  is  intimately  united  with  the 
membrane  of  Descemet  anteriorly,  and  closely  related  to  the  ligamentum 
pectinatum  iridis  and  to  the  outer  or  meridional  fibres  of  the  ciliary  muscle 
internally. 

The  ligamentum  pectinatum  iridis  consists  of  an  annular  prismoidal  mass 
of  spongy  tissue,  composed  of  interlacing  trabeculse  derived  from  the  split- 
ting up  of  the  periphery  of  the  membrane  of  Descemet ;  it  occupies  the 
angle  of  the  anterior  chamber,  and  unites  the  iris,  the  ciliary  muscle,  and 
the  inner  corneal  border.  Its  inner  surface,  next  the  anterior  chamber,  is 
concave,  and  stretches  between  the  membrane  of  Descemet  and  the  ciliary 
border  of  the  iris.  The  pectinate  ligament  of  man  represents,  according  to 
Iwanoff  and  Rollett,2  a  structure  which  in  many  ruminants  appears  as  a 
series  of  conical  processes  extending  from  the  iris  towards  the  cornea.  The 
posterior  border  of  the  pectinate  mass  within  the  human  eye  abuts  against 
the  root  of  the  iris ;  its  external  and  longest  side  is  intimately  associated 
with  the  ciliary  muscle,  the  posterior  part  of  this  surface  being  in  close 
relation  with  the  origin  of  the  meridionally  disposed  muscular  fibres.  The 
posterior  layer  of  the  substantia  propria  at  the  corneal  margin  leaves  the 
membrane  of  Descemet  to  become  directly  continuous  with  the  scleral 
stroma  constituting  the  outer  wall  of  Schlemm's  canal,  the  interspace  re- 
sulting from  this  divergence  being  occupied  by  the  anterior  extremity  of 
the  reticulated  continuation  of  the  inner  scleral  process.  This  latter  tissue, 
while  closely  joined  to  the  scleral  coat,  is  genetically  a  part  of  the  uveal 
tract,  and  must  be  regarded  as  belonging  to  the  choroid  rather  than  to  the 
sclerotic  coat.  The  intimate  relations  existing  between  the  membrane  of 
Descemet  and  its  endothelium  and  the  anterior  limits  of  the  vascular  tunic 
of  the  eye  are  founded  on  their  primary  common  genesis  and  early  con- 
tinuity, in  recognition  of  which,  as  already  stated,  these  strata  are  regarded 
as  constituting  the  pars  uvealis  cornece. 

1  Angelucci :  Ueber  Entwickelung  und    Ban  des  vorderen  Uvealtractus  der  Verte- 
braten,  Archiv  f.  mik.  Anatom.,  Bd.  xix.,  1881. 

2  Iwanoff  und  Rollett :  Bemerkungen  zur  Anatomie  der  Irisanheftung  und  des  Annulus 
ciliaris,  Archiv  f  O-phthalmol. ,  Bd.  xv.,  1869. 


THE   MICROSCOPICAL   ANATOMY   OP  THE   EYEBALL.  249 

The  peripheral  portion  of  this  structure,  which  the  apparent  continuation 
of  the  inner  scleral  process  may  be  regarded  as  forming,  sometimes  presents 
a  thickened  anterior  edge,  described  by  Schwalbe l  as  the  boundary  ring  of 
Descemet's  membrane.  This  ring,  which  in  many  lower  mammals  is  repre- 
sented by  a  well-developed  annular  bundle  of  elastic  fibres,  gives  origin  to  the 
numerous  bands  constituting  the  reticulum  of  the  inner  wall  of  Schlemm's 
canal,  as  well  as  attachment  to  many  trabeculse  taking  part  in  the  formation 
of  the  pectinate  ligament. 

The  tissue  composing  the  ligamentum  possesses  the  homogeneous  elastic 
character  of  the  posterior  limiting  membrane  of  the  cornea,  from  which  the 
trabeculse  are  directly  continued.  As  pointed  out  by  Straub,2  the  mem- 
brane of  Descemet  gives  origin  to  two  layers,  an  inner  and  an  outer.  The 

FIG.  24. 


Section  through  sclero-corneal  junction. — S,  Schlemm's  canal ;  I,  the  relaxed  tissue  constituting  the 
pectinate  ligament,  including  the  interfascicular  spaces  of  Fontana  (a, «) ;  c,  corneal  attachment  of 
trabeculse  of  pectinate  ligament ;  m,  several  bundles  of  the  ciliary  muscle.  Magnified  100  diameters. 

former  breaks  up  into  an  open  mesh-work  of  delicate  trabeculae,  from  .002 
to  .007  millimetre  in  thickness,  imperfectly  invested  by  endothelial  plates, 
which  pass  towards  the  iris,  with  which  structure  they  finally  blend.  The 
behavior  of  this  tissue  when  viewed  by  polarized  light  demonstrates  the 
identity  of  its  nature  with  that  of  the  membrane  of  Descemet,  of  which  it 
is  a  part,  or  at  least  the  direct  extension,  and  not  merely  ordinary  connective 
tissue.  The  deeper  layer  forms  connected  plates  or  lamellae  which  constitute 
a  cavernous  tissue  of  elastic  character :  while  a  small  portion  of  this  struc- 
ture may  be  traced  into  the  iris,  the  greater  part  affords  attachment  to  the 
bundles  composing  the  ciliary  muscle.  The  interlacing  of  the  bands  so 
derived  produces  a  sponge-like  framework  which  encloses  numerous  inter- 
communicating clefts,  the  spaces  of  Fontana.  These  spaces  are  covered 
with  an  imperfect  endothelial  investment,  formed  by  the  plate-like  elements 

1  Schwalbe :  Untersuchungen  iiber  die  Lymphbahnen  des  Auges  und  ihre  Begren- 
zungen,  Archiv  f.  mik.  Anat.,  Bd.  vi.,  1870. 

2  Straub :  Notiz  ueber  die  Ligamentum   pectinatum   und  die  Endigung  d.   Memb. 
Descemeti,  Archiv  f.  Ophthalmol.,  Bd.  xxxiii.,  1887. 


250  THE   MICROSCOPICAL    ANATOMY   OF   THE    EYEBALL. 

directly  continued  over  the  individual  trabeculae  from  the  endothelium  of 
Descemet's  membrane. 

The  spaces  of  Fontana  of  the  human  subject  are  relatively  much  less 
developed  than  in  the  eyes  of  many  lower  mammals,  as  the  horse,  ox,  or 
pig,  in  which  they  are  of  greater  size  and  form  a  more  elaborate  system 
of  intercommunicating  lacunae.  The  spaces  in  man  are  mere  interfibrillar 
crevices,  small  and  narrow  near  the  corneal  margin,  but  of  larger  dimen- 
sions in  the  vicinity  of  the  iris ;  from  the  nature  of  their  boundaries  they 
are  not  completely  isolated,  but  constitute  a  system  of  imperfectly  walled 
irregular  channels  which,  in  addition  to  freely  communicating  with  one 
another,  allow  the  ready  entrance  of  the  fluid  contents  of  the  anterior 
chamber.  The  aqueous  humor  consequently  enters  and  fills  these  inter- 
fascicular  spaces  within  the  tissue  of  the  pectinate  ligament.  The  close 
contact  of  the  latter  with  the  reticulated  inner  wall  of  Schlemm's  canal 
brings  the  clefts  contained  within  the  septal  tissue  into  intimate  relations 
with  the  spaces  of  Fontana,  an  important  passage-way  for  the  escape  of 
the  intra-bulbar  lymph  being  thus  suggested. 

The  nature  of  the  canal  of  Schlemm  and  of  the  relations  of  this 
channel  to  the  anterior  chamber  has  been  the  subject  of  repeated  investi- 
gations which  have  led  to  divergent  opinions. 

In  1869  Schwalbe1  observed  that  on  injecting  Berlin  blue  into  the 
anterior  chamber  not  only  the  spaces  of  Fontana  and  the  adjacent  inter- 
fascicular  clefts  became  filled  with  the  coloring  matter,  but  likewise  the 
canal  of  Schlemm,  and  secondarily  the  neighboring  scleral  veins,  the 
injected  substance  finally  reaching  the  anterior  ciliary  veins.  Schwalbe, 
therefore,  upon  the  evidence  of  his  experiments,  announced  the  existence 
of  a  free  communication  between  the  anterior  chamber  and  the  anterior 
ciliary  veins,  maintaining  the  connection  of  the  canal  of  Schlemm  with  the 
anterior  chamber  through  the  interfascicular  and  Fontana's  spaces  on  the 
one  hand,  and  with  the  scleral  and  anterior  ciliary  veins,  by  means  of  lateral 
channels  uniting  Schlemm's  canal  with  the  adjacent  scleral  veins,  on  the 
other.  This  authority  originally  regarded  Schlemm's  canal  as  an  annular 
lymph-sinus  which  discharged  the  fluids  received  from  the  anterior  chamber 
into  the  tributaries  of  the  anterior  ciliary  veins. 

Leber,2  while  admitting  the  close  relation  between  the  anterior  chamber 
and  the  canal  of  Schlemm,  denied  the  possibility  of  injecting  non-diffusible 
substances  from  the  anterior  chamber  into  the  canal  unless  sufficient  pressure 
were  employed  to  rupture  the  delicate  endothelial  partition  which  normally 
closes  the  canal.  This  investigator  further  declared  that  diffusible  sub- 
stances alone  passed  into  Schlemm's  canal  and  the  ciliary  veins  when  all 
mutilations  were  avoided.  Leber  also  combated  the  opinion  that  the  canal 

1  Schwalbe  :  Untersuchungen  iiber  die  Lymphbahnen  des  Auges  und  ihre  Begren- 
zungen,  Archiv  f.  mik.  Anat.,  Bd.  vi.,  1870. 

2  Leber :   Studien  ueber  den  Fliissigkeitswechsel  im  Auge,  Archiv  f.  Ophthalmol., 
Bd.  xix.,  1873. 


THE   MICROSCOPICAL    ANATOMY   OF  THE   EYEBALL.  251 

was  a  lymph-space,  maintaining  that  its  true  nature  was  that  of  an  annular 
venous  sinus,  often  represented  by  a  plexus  of  smaller  radicles  rather  than 
by  a  single  trunk. 

The  controversy  thus  initiated,  based  as  it  was  upon  the  directly  con- 
tradictory results  obtained  by  two  eminently  skilful  observers,  naturally 
attracted  much  attention ;  subsequently  the  desirability  of  possessing  accu- 
rate information  concerning  the  nature  and  relations  of  Schlemm's  canal 
incited  investigators  from  time  to  time  to  renewed  studies  of  the  subject, 
with  a  view  of  determining  which  of  the  conflicting  opinions  was  correct. 
Among  those  accepting  Schwalbe's  views  and  supporting  them  by  their 
experiments,  Waldeyer l  and  Heisrath 2  are  conspicuous.  It  is  to  be  noted, 
however,  that  the  last-named  observer,  in  his  later  paper,3  gives  only  partial 
allegiance  to  his  older  views,  and  partially,  at  least,  inclines  to  those  of 
Leber  ;  indeed,  Schwalbe 4  himself,  apparently,  later  modified  his  teaching 
regarding  the  nature  of  Schlemm's  canal  so  far  that  he  accepted  its  charac- 
ter as  a  venous  channel  and  no  longer  maintained  that  it  was  purely  a 
lymph- vessel.  Rochon-Duvigneaud 5  and  Gifford6  are  also  investigators 
who  succeeded  in  filling  the  canal  of  Schlemm  and  its  neighboring  veins 
by  injection  introduced  into  the  anterior  chaml>er. 

Those  supporting  Leber's  views,  that  Schlemm's  canal  is  a  venous  sinus, 
and  that  the  anterior  chamber  does  not  stand  in  open  communication  with 
the  ciliary  veins  by  means  of  the  canal,  include  Brugsch,7  Konigstein,8 
Angelucci,9  Morf,10  Staderini,11  Merian,12  and  Merkel.13 

Until  very  recently,  therefore,  notwithstanding   repeated   painstaking 

1  Waldeyer:  in  Graefe  und  Saetnisch's  Handbuch. 

2  Heisrath  :   Ueber  den  Zusammenhang  der  vorderen  Augenkammer  mit  den  vorderen 
Ciliarvenen,  Archiv  f.  mik.  Anat.,  Bd.  xv.,  1878. 

3  Heisrath  :  Ueber  die  Abflusswegedes  Humor  aqueus,  mit  besonderer  Beriicksichtigung 
des  sogennanten  Fontana'schen  und  Schlemm'schen  Canales,  Archiv  f.  Ophtbalmol.,  Bd. 
xxvi.,  1880. 

4 Schwalbe:  Anatomic  der  Sinnesorgane,  1887,  p.  176. 

5  Kochon-Duvigneaud:  Eecherches  anatomiques  sur  Tangle  de  la  chambre  anterieure 
etle  canal  de  Schlemm,  Archives  d'ophthalmol.,  t.  xn.,  xin.,  1892-93. 

6  Gifford :  Weitere  Untersuche  iiber  die  Lymphstrome  und  Lymphwege  des  Augee, 
Archiv  f.  Augenheilkunde,  Bd.  xxvi.,  1893. 

7  Brugsch  :   Ueber  die  Kesorption  kornigen  Farbstoffs  aus  der  vorderen  Augenkam- 
mer, Archiv  f.  Ophtbalmol.,  Bd.  xxiu.,  1887. 

8  Konigstein:    Ueber  den   Canalis  Schlemmii,  Archiv  f.  Ophthalmol.,  Bd.  xxvi., 

1880. 

»  Angelucci :  Ueber  Entwickelung  und  Bau  des  vorderen  Uvealtractus  der  Ver 

Archiv  f.  mik.  Anat.,  Bd.  xix.,  1881. 

10  Morf:    Experimented  Beitrage  zur  Lehre  von  dem  Abflusswegen  d 
Augenkammer,  Inaug.  Dissertation,  "Winterthur,  1888. 

"  Staderini :  Ueber  die  Abflusswege  des  Humor  aqueus,  Archiv  f.  Ophthalm 

xxx  vi  i    1891 . 

"  Merian  :  Versuche  ueber  die  Lymphwege  des  Auges,  Archiv  f.  Anat.  und 

logie,  1891. 

"  Merkel :  Ergebnisse  der  Anatomie  und  Entwickelungsgeschicnte,  E 

240. 


252  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

investigations  concerning  the  behavior  of  injection-fluids  forced  into  the 
anterior  chamber,  no  definite  conclusions  were  possible,  since  the  same 
conflicting  testimony  existed  as  did  a  decade  before.  It  will  be  noticed, 
however,  that  the  majority  of  these  investigators  gave  weight  to  the 
view  opposing  an  open  communication  between  the  anterior  chamber  and 
Schlemm's  canal,  most  of  these  observers  declaring  that  it  was  impossible 
to  introduce  substances  injected  into  the  anterior  chamber  into  the  scleral 
veins  by  way  of  the  canal. 

The  appearance  of  the  paper  of  Gutmann l  in  1895  gave  new  and  very 
strong  additional  support  to  Schwalbe's  claim  that  it  is  possible  to  inject  a 
non-diffusible  substance,  as  Berlin  blue,  into  the  scleral  veins  by  way  of 
the  canal  of  Schlemm,  since  Gutmann's  experiments  with  a  large  series 
of  human  eyes  conclusively  show  that  such  filling  takes  place  even  when 
the  injections  are  performed  under  conditions  which  preclude  the  occurrence 
of  any  mutilation  from  the  employment  of  undue  force  or  pressure.  Gut- 
mann's investigations  were  so  carefully  carried  out  that  it  is  impossible  to 
doubt  the  conclusive  character  of  their  evidence. 

Stimulated  by  these  results,  which  threatened  the  overthrow  of  his  long- 
defended  views  as  to  the  impossibility  of  injecting  the  anterior  ciliary  veins 
from  the  anterior  chamber,  Leber,2  with  the  assistance  of  Beutzen,  under- 
took a  series  of  renewed  investigations  to  furnish  additional  support  of  his 
opinion  and  to  meet  the  results  of  Gutmann's  observations.  Leber  and 
Beutzen  avoided  the  one  vulnerable  feature  of  Gutmann's  work — the  em- 
ployment of  eyes  derived  from  human  cadavers,  which  were  therefore 
not  absolutely  fresh — by  carrying  out  the  injections  on  eyes  which  were 
entirely  free  from  post-mortem  change. 

His  former  experiments  with  carmine  and  Berlin  blue  Leber  repeated 
under  conditions  similar  to  those  originally  observed,  with  almost  identical 
results, — namely,  that  the  carmine  passed  into  the  veins  by  diffusion  while 
the  Berlin  blue  remained  behind.  On  performing  injections  in  repetition 
of  Gutmaun's  experiments  under  the  conditions  emphasized  by  this  writer, 
— after  the  partial  escape  of  the  aqueous  humor, — Leber  was  astonished  to 
find  that  the  canal  of  Schlemm  and  the  anterior  ciliary  veins  almost  at 
once  became  filled  with  the  Berlin  blue  injected  into  the  anterior  chamber 
under  low  pressure  and  with  the  least  possible  force.  Careful  and  repeated 
experiments  conclusively  demonstrated  that  the  conflicting  results  of  the 
various  investigations  during  the  last  fifteen  years  depended  not  upon  inac- 
curate observations,  but  largely  upon  the  degree  to  which  the  aqueous  humor 
had  escaped  before  the  injection  was  undertaken.  To  Leber,  therefore, 
belongs  the  satisfaction  of  having  thus  discovered  the  principal  source  of 
the  perplexing  discrepancies  which  have  so  long  existed  between  the  results 

1  Gutmann  :  Ueber  die  Natur  des  Schlemm 'schen  Sinus  und  seine.Beziehungen  zur 
vorderen  Augenkammer,  Archiv  f.  Ophthalmol.,  Bd.  XLI.,  1895. 

2  Leber :  Die  Circulus  venosus  Schlemmii  steht  nicht  in  offener  Verbindung  mit  der 
vorderen  Augenkammer,  Archiv  f.  Ophthalmol.,  Bd.  XLT.,  1896. 


THE   MICROSCOPICAL    ANATOMY   OF   THE   EYEBALL.  253 

of  the  many  able  investigators  who  have  busied  themselves  with  the  rela- 
tions of  the  anterior  lymph-paths  of  the  eye. 

The  exact  nature  of  the  communication  between  the  canal  of  Schlcmiii 
and  the  adjacent  interfascicular  clefts  still  remains  to  be  settled,  but  it  may 
be  assumed,  as  accepted  by  Leber  and  Merkel,  that  intra-ocular  fluids  anil 
injected  substances  find  a  passage  into  the  canal  of  Schlemm  through  the 
minute  clefts  and  stomata  between  the  elements  of  the  endothelial  wall  of 
the  channel;  that  an  "  open  communication,"  in  the  sense  originally  main- 
tained by  Schwalbe,  does  not  exist  will  be,  we  believe,  the  conclusion  of 
most  observers  who  have  carefully  studied  the  histological  characteristics 
of  the  inner  wall  of  Schlemm's  canal. 

The  relations  of  the  lymph-filled  spongy  tissue  interposed  between  the 
anterior  chamber  and  the  canal  of  Schlemm  find  a  close  parallel  in  the 
arrangement  of  the  tissue  of  the  arachnoidal  villi  and  the  sinuses  of  the 
dura  mater,  as  seen  in  the  Pacchionian  bodies.  Just  as  these  structures 
serve  as  points  of  escape  for  the  lymph-fluid  of  the  subarachnoidal  space 
into  the  venous  channels,  whereby  the  equilibrium  of  the  intra-cranial 
pressure  is  maintained,  so  the  constant  escape  of  the  aqueous  humor  into 
the  venous  channels,  by  means  of  the  paths  afforded  by  the  interfascicular 
clefts  situated  at  the  angle  of  the  anterior  chamber,  is  an  important  means 
of  maintaining  the  normal  intra-ocular  tension. 

The  venous  nature  of  Schlemm's  canal,  as  constantly  maintained  by 
Leber,  is  now  almost  universally  accepted.  Waldeyer,  who  long  held  to 
the  lymphatic  character  of  the  canal,  has  recently1  indirectly  announced  the 
relinquishment  of  his  former  views  in  favor  of  the  venous  nature  of  the 
sinus.  The  evident  correspondence  between  the  relations  of  the  arachuoidal 
villi  and  the  dural  venous  sinuses  was  a  potent  argument  in  inducing 
Waldeyer  to  accept  the  venous  nature  of  Schlemm's  canal. 

The  seemingly  strong  argument  which  has  been  so  often  advanced 
against  considering  the  canal  a  venous  channel — namely,  that  the  lumen 
of  the  canal  is  usually  empty,  or  at  least  devoid  of  blood-corpuscles — loses 
much  of  its  force  when  we  appreciate  the  fact,  as  emphasized  by  Leber,  that 
the  filtration-current  continues  for  some  time  after  death,  and,  consequently, 
tends  to  remove  what  blood-cells  were  within  the  canal  during  life.  It  k 
however,  by  no  means  rare  to  observe  blood-cells  still  within  the  canal,  such 
elements  being  especially  present  in  eyes  subjected  to  fixing  solutions  imme- 
diately after  death.  Positive  testimony  as  to  the  occurrence  of  blood-cells 
within  the  canal  has  been  given  by  a  number  of  observers,  among  whom 
are  Leber,  Heisrath,  Konigstein,  and  Rochon-Duvigueaud.  Leber  found 
no  difficulty  in  injecting  the  canal  of  Schlemm  from  the  scleral  veins,  but 
records  his  failure  to  cause  the  injection  to  pass  into  the  spaces  of  Fontana 
or  the  anterior  chamber. 


1  Gutmann,  in  the  above-cited  paper,  states  his  distinct  authorization  to  announce  this 

change  in  Waldeyer's  views. 


254 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


The  canal  of  Schlemm  may  therefore  be  regarded  as  closely  connected 
with  the  anterior  chamber  on  the  one  hand,  and  in  direct  communication 
with  the  anterior  ciliary  veins  on  the  other,  probably  forming,  as  suggested 
by  Schwalbe,  an  annular  reserve  diverticulum  for  the  reception  and  tempo- 
rary storage  of  venous  blood  when  the  usual  escape  of  the  latter,  as  afforded 
by  the  anterior  ciliary  veins,  is  for  any  reason  unduly  retarded.  The  ex- 
planation of  the  fact  that  the  canal  of  Schlemm,  under  ordinary  conditions, 
contains  little  blood  is  to  be  sought,  according  to  Schwalbe,  in  the  narrow- 
ness (.024  millimetre  in  diameter)  of  the  vessels  connecting  the  canal  with  the 
anterior  scleral  veins,  which  renders  the  entrance  of  blood  into  the  lateral 
diverticulum,  while  passing  along  these  vessels,  much  less  easy  than  the 
direct  continuance  along  tne  veins. 

THE   VASCULAR  TUNIC   OF   THE   EYE. 

The  middle  or  vascular  coat,  frequently  also  called  the  uveal  tract,  is 
distinguished  by  the  number  of  its  blood-vessels  and  the  dark  color  im- 
parted to  it  by  the  pigmented  cells  which  it  contains.  The  tunic  is  com- 

FIG.  25. 


Section  of  the  choroid  with  portions  of  the  adjacent  coats. — a,  retinal  pigment;  b.  vitreous 
membrane ;  c,  chorio-capillaris ;  d,  layer  containing  large  blood-vessels ;  e,  pigmented  stroma ;  /, 
supra-choroidal  space;  g,  lamina  fusca  of  the  sclerotic.  Magnified  335  diameters. 

posed  of  two  portions,  the  choroidal  tract,  and  the  iris.  The  former  lines 
the  sclerotic  coat  from  the  position  of  the  optic-nerve  entrance  to  the  sclero- 
corneal  juncture ;  the  latter  diverges  a  short  distance  behind  the  corneal 
margin  to  form  the  conspicuous  diaphragm  which  stretches  across  the  front 
of  the  crystalline  lens  and  behind  the  cornea.  The  iris  divides  the  space 
occupied  by  the  aqueous  humor  into  the  posterior  and  the  anterior  chamber, 
communication  between  the  two  being  established  through  the  circular 
opening  occupying  the  central  area  of  the  iris, — the  pupil. 

The  anterior  portion  of  the  choroidal  tract,  extending  from  the  anterior 


THE   MICROSCOPICAL   ANATOMY   OF  THE    EYEBALL.  255 

margin  of  the  visual  part  of  the  retina,  or  the  ora  serrata,  to  the  sclero- 
corneal  juncture,  presents  remarkable  specializations  which  result  in  the 
production  of  a  greatly  thickened  layer ;  on  the  outer  surface  of  the  latter 
an  important  triangular  muscular  ring — the  ciliary  muscle — is  developed, 
and  on  the  inner  a  series  of  prominent  radial  projections — the  ciliary 
processes — is  formed.  It  is  usual,  therefore,  to  designate  the  anterior  part 
of  the  choroidal  tract — which  includes  the  triangular  area  bounded  by  the 
sclero-corneal  juncture  and  the  sclera  externally,  the  iris  internally,  and 
the  ora  serrata  posteriorly — collectively  as  the  ciliary  body ;  the  entire 
choroidal  tract  may  be  regarded  as  thus  composed  of  two  segments,  the 
choroid  proper,  lying  behind  the  ora  serrata,  and  the  ciliary  body,  in  front 

THE  CHOROld. 

While  closely  applied  to  the  inner  surface  of  the  fibrous  tunic,  the 
choroid  proper  is  less  intimately  united  to  the  sclera  than  might  at  first 
sight  be  supposed,  since  only  at  two  points — namely,  around  the  optic  en- 
trance and  at  the  inner  scleral  process — are  the  tissues  of  the  two  coats 
firmly  united.  The  perforating  blood-vessels  and  nerve-trunks  passing 
between  the  sclera  and  the  choroid  afford  important  additional  points  of 
fixation  between  the  two  coats.  Throughout  the  remaining  extent  of  the 
choroid  the  sclerotic  and  vascular  tunics  are  separated  by  the  supra- 
choroidal  space,  the  two  being  held  together  by  the  intervening  bands  of 
fibrous  tissue  which  bridge  this  cleft  and  form  a  loose  mesh-work  of  tra- 
beculse  attached  to  its  inner  and  outer  walls.  The  outer  surface  of  the 
choroid  is  roughened  by  the  attachment  of  these  fibrous  trabeculse ;  its 
inner  surface,  on  the  contrary,  is  smooth  and  so  intimately  related  to  the 
external  pigmented  layer  of  the  retina  that  the  latter  very  frequently 
adheres  to  the  choroid  rather  than  to  the  remaining  parts  of  the  nervous 
tunic  when  the  latter  is  removed  from  the  eye. 

The  color  of  the  choroid  after  death  varies  from  a  reddish  to  a  dark 
brown,  depending  upon  the  amount  of  pigment  contained  within  its  cells. 
In  thickness,  the  coat  presents  a  gradual  reduction  from  the  posterior  pole, 
where  in  the  vicinity  of  the  optic  entrance  it  measures  from  .05  to  .08  milli- 
metre, to  the  neighborhood  of  the  ora  serrata,  at  which  point  it  is  thinnest, 
being  little  more  than  half  as  broad  as  in  its  posterior  segment  At  the 
equator  of  the  eyeball  the  relative  thickness  of  the  sclera,  the  choroid,  and 
the  retina  is  respectively  about,  as  sixteen,  five,  and  seven. 

The  structure  of  the  choroid  includes,  essentially,  a  more  or  less  compact 
connective-tissue  stroma  supporting  numerous  blood-channels  which  vary 
in  size  from  the  large  and  conspicuous  emergent  veins  to  minute  capillaries. 
The  arrangement  of  the  blood-vessels  largely  accounts  for  the  peculiariti 
which  distinguish  the  layers  into  which  the  choroid  is  conventionally 
divided.  These  are  three  in  number : 

1.  The  layer  of  choroidal  stroma  containing  large  blood-vessels. 

2.  The  layer  of  dense  capillary  net-works,— the  chorio-capillaris. 


256 


THE    MICROSCOPICAL    ANATOMY    OF   THE    EYEBALL. 


PlQ.  26. 


3.  The  homogeneous  glassy  lamina,  or  vitreous  membrane. 
The  delicate  stratum  of  spongy  tissue  connecting  the  outer  surface  of 
the  choroid  and  the  inner  aspect  of  the  sclera,  the  lamina  supra-choroidea, 
by  some  authorities  is  regarded  as  a  fourth,  and  external,  layer. 

The  lamina  supra-choroidea  consists  of  some  half-dozen  irregular  sheets 
of  broad  trabeculse  which  join  one  another  at  various  acute  angles  to  pro- 
duce a  delicate  reticulum,  the 
contained  meshes  of  which 
form  a  system  of  intercom- 
municating lymphatic  clefts 
collectively  known  as  the 
supra-choroidal  lymph-space. 
The  propriety  of  regard- 
ing the  supra-choroidal  cleft 
as  a  lymph-space  has  been 
questioned  by  Langer,1  who 
considers  it  the  result  of  the 
necessity  of  a  loose  connec- 
tion between  the  adjacent 
surfaces  of  the  sclera  and  the 
choroid  in  order  to  admit  of 
the  play  of  the  choroid  in  re- 
sponse to  the  contractions  of 
the  meridional  fibres  of  the 

Surface  view  of  a  fragment  of  the  lamina  supra-cho-  ciliary  muscle.  While  ad- 
roidea:  the  flat .plgmented  •co^ectlve.tiMue  celta  lie  upon  mitting  thig  purpose  of  the 
the  elastic  lamellae.  Magnified  335  diameters. 

loose  connection  between  the 

two  tunics,  there  seems  no  adequate  reason  to  ignore  the  direct  testimony 
of  Schwalbe 2  and  of  Michel 3  as  to  the  communication  of  the  supra-cho- 
roidal space  with  other  lymph-tracts.  The  arrangement  of  the  lymphatic 
capillaries  within  the  choroidal  stroma,  as  described  by  Alexander,4  and 
the  well-recognized  universal  close  relations  of  interfascicular  clefts  to  the 
lymphatic  system  in  other  structures,  are  additional  considerations  for  re- 
garding the  perichoroidal  space  as  a  lymph-channel. 

The  membrane-like  trabeculse  or  partitions  are  made  up  of  a  framework 
composed  of  interlacing  elastic  fibres,  upon  the  surface  of  which  lie  numer- 
ous flattened,  irregularly  branched,  pigmented  connective-tissue  cells.  These 
elements  appear  as  conspicuous  stellate  or  plate-like  bodies,  the  deeply  colored 


1  Langer  :  Beitrage  zur  normalen  Anatomie  des  menscblichen  Auges,  Sitzungs- 
berichte  d.  k.  Akad.  d.  Wissensch.  in  Wien,  Bd.  xcix.,  1890. 

1  Schwalbe  :  loc.  cit. 

8  Michel :  Beitrage  zur  Kenntnisse  der  hinteren  Lymphbahnen  des  Auges,  Arcbiv  f. 
Ophthalmol.,  Bd.  xvin.,  1872. 

*  Alexander:  Ueber  die  Lymphcapillaren  der  Choroidea,  Archiv  f.  Anat.  u.  Physiol., 
Anat.  Abth.,  1889. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL. 


257 


protoplasm  of  which  is  relieved  by  clearer  ovoid  areas  denoting  the  position 
of  the  uuiuvaded  nuclei.  In  places  where  the  cells  are  grouped  their  form 
approaches  more  closely  that  of  the  typical  endothelial  plate,  possessing  a 
distinctly  polyhedral  outline. 

These  plates  not  only  cover  the  surfaces  of  the  membranous  trabeculse, 
but  provide  a  more  or  less  perfect  eudothelial  investment  for  the  inner 
choroidal  and  the  outer  scleral  wall  of  the  space,  as  satisfactorily  demon- 
strated by  the  application  of  silver  staining.  As  pointed  out  by  Hache,1 
however,  the  cells  covering  the  scleral  surface  of  the  space  much  more 
closely  approach  the  endothelial  type  than  those  occupying  the  inner  wall, 

FIG.  27. 


Surface  view  of  a  portion  of  the  stroma  of  the  choroid  containing  branched  pigmented  cells  between 
which  lie  the  non-pigmented  connective-tissue  elements.    Magnified  35 

the  latter  retaining  to  a  greater  extent  their  characteristics  as  isolated  con- 
nective-tissue corpuscles.  At  those  points  where  the  perforating  vessels 
and  nerves  cross  the  supra-choroidal  space  the  reticulated  tissue  becomes 
condensed  and  contributes  sheaths  which  surround  the  trunks  during  their 
passage,  the  pigment-cells  sometimes  forming  accompanying  chains  of  col- 
ored protoplasmic  figures. 

The  choroidal  stroma  or  ground-substance  consists  of  connective- 
lamellae  closely  interwoven  with  one  another  and  intimately  related  1 
blood-vessels  which  they  support.  The  structural  elements 

i  Hache  :  Sur  la  structure  de  la  chorioide  et  sur  1'analogie  des  espaces  conjonctifs  et  de. 
cavites  lymphatiques,  Compt.  rend.  hebd.  d.  s.  de  1'acad.  d.  sciences,  to 
VOL.  I.— 17 


258 


THE    MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL,. 


FIG.  28. 


include  delicate  fibrous  bundles,  numerous  elastic  fibres,  stellate  pigmented 
and  endothelioid  cells.  These  are  so  closely  united  with  the  walls  of  the 
blood-channels  that  considerable  firmness  is  given  to  the  choroid,  especially 
its  outer  zone. 

The  stroma  layer,  with  the  large  blood-vessels,  constitutes  the  greater 
part  of  the  choroid  ;  the  freely  branching  arterial  and  venous  trunks,  taking 
their  course  within  the  supporting  tissue  made  up  of  closely  united  connec- 
tive-tissue lamella?,  elastic  fibres,  and  pigment-cells,  appear  as  lighter-colored 
channels  within  the  darker  surrounding  stroma. 

The  largest  vessels  occupy  the  most  superficial  or  outer  stratum  of  the 
choroidal  stroma,  those  next  in  size  the  middle  layer,  while  the  smallest 
approach  the  inner  boundary  of  the  choroid,  where  they  constitute  the  dense 
capillary  net-work  known  as  the  chorio-capillaris. 

The  most  conspicuous  of  the  large  superficial  blood-channels  are  the 
four  great  venous  trunks,  the  vence  vortieosce,  which  mark  upon  the  outer 

surface  of  the  choroid,  at 
points  about  equidistant 
within  the  equatorial 
plane,  the  foci  towards 
which  the  smaller  tribu- 
taries of  each  quadrant 
converge  to  form  the  re- 
markable venous  whorls 
occupying  the  outer  layers 
of  the  choroidal  stroma. 
Not  infrequently  one  or 
more  of  these  large  re- 
ceiving veins  is  repre- 
sented by  two  vessels 
separated  often  by  some 
distance,  this  disposition 
resulting  in  the  presence 
of  five  or  six  vena?  vorti- 
cosa3. 

The  blood  carried  off 
by  these  vessels  is  col- 
lected not  only  from  the 
choroid  proper,  but  also  from  the  ciliary  body  and  the  iris.  The  radially 
disposed  and  but  slightly  tortuous  tributaries  coming  from  the  latter  sources 
pass  backward  in  their  course  to  join  the  large  collecting  veins.  On  ap- 
proaching the  equator  of  the  eyeball,  and  still  farther  towards  the  posterior 
pole,  the  venous  radicles  assume  a  progressively  more  and  more  archea 
course  in  order  to  reach  the  vorticose  veins,  in  consequence  of  which  dis- 
position the  characteristic  whorled  arrangements  are  produced.  After  re- 
ceiving their  tributaries,  the  vena3  vorticosa?  cross  the  supra-choroidal 


Surface  view  of  injected  choroid.  (Arnold-Sappey.)— The 
smaller  venous  radicles  converge  in  the  peculiar  manner  to  form 
the  larger  trunks  (3,  3),  which  in  turn  are  tributaries  of  still 
larger  veins  (2,  2) ;  1, 1,  veins  of  greater  diameter. 


THE   MICROSCOPICAL   ANATOMY   OP   THE    EYEBALL. 


259 


lymph-space  and  pierce  the  sclera  obliquely  and  backward,  accompanied  by 
an  imperfect  envelope  continued  from  the  lamina  supra-choroidea. 

The  veins  of  the  choroid  are  usually  provided  with  a  perivascular 
lymph-sheath  formed  by  the  addition  of  an  enveloping  layer  of  endothe- 
lial  plates ;  this  sheath  is  strengthened  externally  by  an  adventitious  coat 
composed  of  concentrically  disposed  connective-tissue  lamellae  in  which 
longitudinal  fibres  are  well  marked.  The  adventitia  is  relatively  better 
developed  on  the  smaller  than  on  the  larger  veins. 

The  arteries  of  the  choroid,  in  addition  to  the  well-marked  circularly 
disposed  muscle,  possess  longitudinal  bundles  of  muscle-fibres ;  these  axial 
bands  in  some  instances,  particularly  in  the  vessels  supply  ing  the  posterior 
segment,  are  connected  by  a  net-work  of  smaller  fasciculi. 

The  layer  containing  the  larger  veins  is  separated  from  that  supporting 
the  capillary  reticulum  by  means  of  a  narrow  boundary  zone,  about  .010 
millimetre  in  thickness, 
consisting  of  a  close  felt- 
work  of  elastic  fibres 
and  sparingly  distributed 

connective  -  tissue    cells. 

The   latter    are    usually 

entirely  devoid  of  pig- 
ment, or,  at    most,  but 

slightly      tinted.       The 

boundary  zone,  therefore,  -5^ 

is  to  be  regarded  not  as 

a  distinct  portion  of  the 

choroid,  but  rather  as  the 

innermost    part    of   the 

stroma   layer,   which    is 

unusually  condensed  and 

slightly  pigmented. 

In  the  eyes  of  many 

animals      (horse,      cow, 

sheep)  the  boundary  zone 

possesses  wavy   bundles 

of  connective  tissue,  to 


Surface  view  of  the  injected  choroid,  showing  the  dense  net 
work  of  the  chorio-capillaris.    (Sappey.)-In  the  centre  ol 
field  the  converging  capillaries  form  one  of  the  venom 
lying  within  an  external  plane. 


the  pe"cuHar  "arrangement  of   which  is  due  the  metallic  reflex  sometimes 
seen  in  such  eyes  ;  this  shining  structure  constitutes  the  tapdumfib,      tm, 
as  distinguished  from  the  iridescent  tapetum  cdlulosum  of  the  carnivora, 
which  latter   depends  upon  the  presence  of  several   layers 
cells  containing  innumerable  small  crystals. 

The   chorio-capilla™  (membrane  of  Ruysch),  the  capillary  zone 
choroid,  occupies  the  inner  portion  of  the  vascular  coat,  being  separated 
from  the  nervous  tunic  by  the  delicate  vitreous  membrane  alo 
capillary  network,  derived  from  numerous  twigs  given  off  from  tl 


260 


THE   MICROSCOPICAL   ANATOMY  OF   THE   EYEBALL. 


posterior  ciliary  arteries,  lies  embedded  within  an  apparently  homogeneous 
ground-substance,  devoid  of  pigmented  elements,  which  fills  the  inter- 
capillary  meshes.  The  exact  nature  of  this  matrix  is  uncertain,  but  it  may 
be  regarded  as  a  modified  connective  tissue  of  soft  consistence,  which  stands, 
probably,  according  to  the  investigations  of  Alexander,1  in  close  relation 
with  the  more  definite  lymph-paths  and  the  perivascular  lymph-sheaths 
surrounding  the  larger  venous  trunks. 

In  extent  the  chorio-capillaris  corresponds  closely  with  the  visual  por- 
tion of  the  retina,  for  the  nutrition  of  the  outer  non-vascular  layers  of 

FIG.  30. 


Surface  view  of  the  choroid  seen  from  the  inner  side.— a,  a,  choroidal  stroma  separating  larger 
blood-vessels  (6,  6) ;  c,  c,  the  more  superficial  capillary  net-work  of  the  chorio-capillaris.  Magnified 
115  diameters. 

which  this  capillary  net-work  seems  to  be  especially  designed.  It  extends 
from  the  optic  entrance,  around  which  anastomosis  between  the  choroidal 
and  the  retinal  system  of  vessels  takes  place,  as  far  forward  as  the  ora 
serrata,  at  which  point  it  abruptly  terminates. 

The  vitreous  membrane,  glassy  lamina,  or  membrane  of  Bruch,  consti- 
tutes the  inner  boundary  of  the  choroid,  separating  the  chorio-capillaris 
from  the  outer  retinal  layer.  This  lamella  appears  as  a  delicate  homo- 
geneous, structureless  zone,  measuring  but  .002  millimetre  in  thickness, 
which  is  intimately  united  with  the  capillary  layer  on  the  one  hand, 
and  supports  the  retinal  pigment  of  the  retina  on  the  other ;  patches  of 
the  pigment  not  infrequently  adhere  to  the  surface  of  the  glassy  lamina, 
thereby  producing  polygonal  tracings.  The  use  of  macerating  reagents 
sometimes  effects  the  separation  of  the  lamella  into  two  layers,  the  outer  of 
which  appears  finely  reticular. 

1  Alexander:  Ueberdie  Lymphcapillaren  der  Chorioidea,  Archiv  f.  Anat.  u.  Physiol., 
Anat.  Abth.,  1889. 


THE    MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL.  261 

Sattler1  regards  this  separation  as  indicating  the  normal  constitution 
of  the  vitreous  membrane  of  two  distinct  lamella?,  an  inner  homogeneous 
and  an  outer  composed  of  a  delicate  reticulation  of  interlacing  trabecul» 
of  varying  thickness.  According  to  this  author,  the  two  layers  are  always 
separable  in  the  eyes  from  young  subjects,  but  very  imperfectly  so  in  those 
from  old  individuals.  Kerschbaumer 2  also  accepts  this  differentiation  of 
the  seemingly  structureless  vitreous  membrane  into  an  outer  and  an  inner 
lamella. 

The  nerves  of  the  choroid  are  derived  from  twigs  which  are  given  off 
from  the  long  and  short  ciliary  nerves  as  these  pass  between  the  fibrous 
and  vascular  tunics  in  their  course  to  the  ciliary  body,  from  which  filaments 
proceed  to  the  cornea  and  the  iris. 

The  especial  branches  destined  for  the  choroid,  consisting  of  both  med- 
ullated  and  non-medullated  fibres,  join  within  the  lamina  supra-choroidea 
to  form  a  wide-meshed  plexus,  at  the  nodal  points  of  which  larger  or  smaller 
groups  of  ganglion-cells  are  situated.  The  plexus  so  formed  contributes 
numerous  fine  non-medullated  fibres,  which  proceed  to  the  arteries,  accom- 
panying them  to  their  finest  ramifications  as  nervous  filaments  of  increasing 
delicacy  ;  ganglion-cells,  isolated  or  in  very  limited  groups,  are  not  infre- 
quently found  along  the  vessels.  Since  the  nervous  supply  of  the  choroid 
is  especially  distributed  to  the  muscular  tissue  of  the  blood-vessels,  the 
component  fibres  may  be  regarded  as  vaso-motor  in  character. 

The  blood-vessels  of  the  choroid  include  the  branches  derived  from  the 
short  posterior  ciliary  arteries  and  the  tributaries  of  the  great  collecting 
veins  ;  the  detailed  disposition  of  these  vessels  has  already  been  considered. 

The  lymphatics  of  the  choroid  are  represented,  according  to  Alexander,5 
by  distinct  capillaries  which  are  intimately  related  to  the  intercapillary 
spaces  within  the  chorio-capillaris  on  the  one  hand,  and  to  the  perivascular 
sheaths  leading  to  the  larger  lymph-channels  on  the  other. 

THE   CILIARY   BODY. 

Under  this  name  is  included  the  specialized  anterior  portion  of  the 
choroidal  tract  extending  from  the  ora  serrata  to  the  sclero-corneal  junc- 
ture. The  general  outline  of  this  region,  as  seen  in  meridional  sections,  is 
triangular,  the  outer  and  longer  side  lying  next  the  sclero-corneal  juncture 
and  the  sclera,  the  short  anterior  side  being  bounded  by  the  pectinate  liga- 
ment, with  the  included  spaces  of  Fontana  and  the  base  of  the  iris,  and 
the  irregular  inner  border  being  covered  by  the  deeply  pigmented  extension 
of  the  retinal  tunic  which  rests  upon  the  anterior  margin  of  the  vitreous 
body. 

1  Sattler :  Ueber  den  feineren-  Bau  der  Chorioidea  u.  s.  w.,  Archiv  f.  Ophthalmol.,  Bd. 

xxii.,  1876. 

2  Kerschbaumer:  t!ber  Altersveranderungen  der  Uvea,  Archiv  f   Ophthalmol.,  B 

xxxviii.,  1892. 

3  Alexander  :  loc.  cit. 


262 


THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 


Fio.  81. 


Posterior  view  of  iris 
and  ciliary  bodies.  (After 
Bappey.)— 1.  sclera ;  2,  cho- 
roid ;  3,  ciliary  ring ;  4, 
position  of  ora  serrata;  5, 
zone  of  ciliary  processes ; 
6,  iris;  7,  pupil.  Natural 


The  structures  included  within  this  area  present  differences  which  at 
once  suggest  a  subdivision  of  the  ciliary  body  into 
three  secondary  portions.  The  first  of  these,  the  cili- 
ary ring,  appears  as  a  slightly  thickened,  smooth  zone, 
about  four  millimetres  in  width,  which  extends  from 
the  anterior  limit  of  the  choroid  proper,  opposite  the 
ora  serrata,  as  far  forward  as  the  base  of  the  irregular 
projections  which  mark  the  inner  surface  of  the  adja- 
cent portion  of  the  ciliary  body. 

The  second  portion,  the  ciliary  processes,  includes 
the  anterior  part  of  the  inner  surface  of  the  body, 
and  is  distinguished  by  the  presence  of  a  series  of 
irregular  projections  or  processes  which,  covered  by 
the  pigmented  retinal  layer,  look  towards  the  adjacent 
vitreous. 

The  third  portion  of  the  ciliary  body,  in  many  respects  its  most  im- 
portant constituent,  is  formed  principally  by  the  fibres  of  the  ciliary  muscle, 

a  triangular  mass  of  muscular  tis- 
sue which  occupies  the  outer  two- 
thirds  of  the  ciliary  body  and 
provides  an  essential  part  of  the  ac- 
commodative apparatus  of  the  eye. 
The  ciliary  ring,  or  orbiculus 
ciliaris,  includes  an  annular  band 
about  four  millimetres  in  width, 
immediately  preceding  the  anterior 
limit  of  the  choroid  proper  as 
marked  by  the  ora  serrata;  its 
inner  surface,  directed  towards  the 
vitreous  body,  is  covered  by  the 
pigmented  cells  of  the  atrophic 
layers  of  the  pars  ciliaris  retinae, 
presently  to  be  described. 

This  portion  of  the  choroidal 
tract  differs  in  its  structure  from 
that  of  the  choroid  proper  chiefly 
in  the  absence  of  the  capillary 
layer,  which  ceases  at  the  ora 
serrata  in  correspondence  with  the 
distribution  of  the  layer  of  rods 
and  cones  of  the  retina,  for  whose 
nutrition  the  chorio-capillaris  is 
particularly  designed.  The  connective-tissue  matrix  of  the  ciliary  ring 
also  varies  from  that  of  the  choroid  proper  in  the  greater  number  of  bundles 
of  fibrous  tissue,  principally  meridionally  disposed,  which  it  contains.  The 


Section  through  the  ciliary  ring  close  behind  the 
ciliary  processes. — a,  6,  the  inner  and  outer  layers  of 
the  pars  ciliaris  retinae ;  c,  the  continuation  of  the 
vitreous  membrane ;  d,  the  continuation  of  the  cho- 
roidal connective-tissue  stroma  containing  muscle- 
cells  (m)  derived  from  the  ciliary  muscle ;  e,  fibres  of 
attachment  of  the  suspensory  ligament  of  the  lens. 
Magnified  335  diameters. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL. 


263 


Fio.  33. 


vitreous  membrane  is  continued  over  the  inner  surface  of  the  ciliary  ring, 
but  in  this  location  presents  numerous  thickenings  which  appear  as  a  re^ 
ticulation  of  low  ridges ;  the  latter  gradually  become  less  conspicuous  as 
they  pass  towards  the  ciliary  processes,  to  which  they  give  origin. 

While  the  capillary  stratum  is  no  longer  present  within  the  ciliary  ring, 
the  larger  venous  trunks  are  represented  by  the  veins  which  return  the  blood 
from  the  iris  and  the  ciliary  processes  as  tributaries  to  the  more  posteriorly 
situated  vente  vorticosae.  The  blood  from  the  greater  part  of  the  ciliarv 
muscle,  on  the  other  hand,  is  carried  off  by  vessels  emptying  into  the 
anterior  ciliary  veins.  The  venous  channels  of  this  region  are  provided 
with  adventitious  coats  and  perivascular  sheaths. 

In  addition  to  the  foregoing  peculiarities,  the  stroma  of  the  ciliary  ring 
is  in  intimate  relation  with  the  contractile  tissue  of  the  ciliary  muscle.     As 
pointed   out   by   Schwalbe,1   the 
latter  structure  must  be  regarded 
as  a  new  formation,  intercalated 
between  the  sclera  and  the  con- 
nective-tissue stratum,  and  con- 
tributed   by  the   matrix   of  the 
choroidal  tract,  rather  than  as  an 
integral  part  of  the  latter.     The 
inner  surface  of  the  ciliary  ring  is  i 
closely  invested,  as  already  noted, 
by  the  pigmented  layers  of  the  2|l 
pars  ciliaris  retinae,  the  relation 
being  especially  intimate  in  the 
recesses  between  the  reticulated 
ridges  on  the  glassy  membrane. 

The  ciliary  processes,  or  plicae 
ciliares,  constitute  an  annular 
series  of  meridionally  directed 
highly  vascular  folds,  about 
seventy  in  number,  which  extend 
from  the  anterior  limit  of  the 
ciliary  ring  forward  and  inward  to 
the  base  of  the  iris.  They  are  be- 
trvveen  two  and  three  millimetres 

in  length,  .12  to  .15  millimetre  in  breadth,  and,  at  their  apices,  attain  a 
height  of  .8  to  one  millimetre.  Seen  from  the  posterior  surface  (Fig.  31), 
they  constitute  a  broken  ring  of  radial  plications  encircling  the  outer 
boundary  of  the  iris.  Each  process  begins  behind  by  the  apparent  fusi..n 
of  several  of  the  ridges  above  mentioned  as  existing  on  the  ciliary  ring, 
and  rapidly  increases  in  breadth  and  height  to  a  point  about  opposite  the 


Injected  ciliary  processes  viewed  from  behind. 
(Sappey.)— 1,  2,  venous  plexuses  of  tortuous  venom 
radicles  composing  the  bulk  of  the  projections ;  8,  8, 
efferent  trunks  which  become  tributaries  of  the  ven» 
vorticosae :  4,  4,  venous  radicles  from  the  iris.  Magni- 
fied 40  diameters. 


Schwalbe:  Anatomic  der  Sinnesorgane,  S.  190,  1887. 


264 


THE   MICROSCOPICAL  ANATOMY   OF  THE    EYEBALL. 


Fio.  34. 


c  /<-~  ^ 


margin  of  the  crystalline  lens ;  the  process  then  abruptly  diminishes  to  the 

level  of  the  posterior  surface  of  the  iris. 

Examined  in  meridional  section,  each  process  is  represented  by  a  series 

of  irregular  projections  which  vary  greatly  in  size  and  exact  arrangement  j 

they  gradually  increase  in  height  towards  the  iris,  the  maximum  elevation 

being  reached  in  the  last  pro- 
jection, which  corresponds  to 
the  inner  angle  of  the  macro- 
scopic structure. 

The  general  mass  of  the 
ciliary  process  consists  of  the 
thickened  stratum  of  fibro- 
elastic  connective  tissue  di- 
rectly continued  from  the  ma- 
trix of  the  ciliary  ring.  In 
addition  to  this  tissue,  which 
constitutes  the  supporting 
framework  of  the  process,  a 
rich  convolution  of  capillary 
blood-vessels  still  further 
contributes  to  the  bulk  of  the 

Meridional  section  of  the  ciliary  processes.— a.  the  outer 
pigmented  layer  of  the  pars  ciliaris  retinae;  b,  the  unpig- 
mented  layer  of  the  same ;  c,  connective  tissue  of  choroidal 
tract ;  p,  p,  anterior  surface  of  the  processes.  Magnified  160 


diameters. 


projections.  The  inner  sur- 
face of  the  ciliary  processes  is 
invested  by  the  continuation 
of  the  vitreous  membrane, 

which  here  is  somewhat  thickened,  attaining  a  thickness  of  from  .003  to 
.004  millimetre,  and  separates  the  connective-tissue  stratum  from  the  pig- 
mented covering  derived  from  the  atrophic  pars  ciliaris  retina?. 

FIG.  35. 


Section  through  the  posterior  part  of  the  ciliary  processes  near  their  termination.— a,  a,  narrow, 
gland-like  recesses  lined  by  c,  b,  the  inner  and  outer  layers  of  the  pars  ciliaris  retinae.  Magnified  155 
diameters 

The  arteries  distributed  to  the  ciliary  processes  proceed  from  the  greater 
drculus  arteriosus,  situated  at  the  periphery  of  the  iris,  to  the  anterior  end 
of  the  plications,  the  usual  arrangement,  according  to  Leber,  being  such 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL.  265 

that  the  smaller  arterioles  supply  each  a  single  process,  while  the  larger 
branches  suffice  for  several  folds.1  After  attaining  the  anterior  end  of  the 
pro<  ss,  the  artenole  breaks  up  into  numerous  capillary  vessels  the  tor 


tuous  courses  of  which  produce  the  elaborate  convolutions  so  conspicuous 
in  injected  preparations  of  these  structures. 

The  capillary  net-works  gradually  pass  over  into  the  mesh-work  of 

1  Leber :    Die  Circulations-  und  Ernahrungsverhaltnisse  des  Auges,  Graefe  u.  Sae- 
misch's  Handbuch,  Bd.  II.,  1876. 


266  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

venous  radicles,  which  converge  to  form  several  minute  veins  for  each 
process ;  the  latter,  in  turn,  emerge  at  the  posterior  extremity  of  the  fold, 
and,  after  a  meridional  and  posterior  course  across  the  ciliary  ring  towards 
the  great  venous  foci  of  the  vascular  tunic,  finally  become  tributaries  of 
the  large  vena?  vorticosse. 

The  ciliary  muscle  forms  the  most  conspicuous  constituent  of  the  ciliary 
body,  appearing  in  meridional  sections  as  a  triangular  field  of  involuntary 
muscular  and  connective  tissue.  This  area  is  bounded  by  the  sclerotic  coat 
externally,  extending  from  the  sclero-corueal  juncture  anteriorly  as  far  as 
the  ciliary  ring  posteriorly,  and  is  limited  on  its  internal  and  posterior  sur- 
faces respectively  by  the  pectinate  ligament  and  the  connective-tissue  stratum 
contributed  by  the  choroidal  tract.  The  ciliary  muscle  in  its  entirety  forms 
a  prismatic  annular  baud  which  encircles  the  angle  of  the  anterior  chamber 
and  the  root  of  the  iris. 

When  critically  examined,  the  triangular  area  of  cross-sections  of  the 
muscle  is  seen  to  be  composed,  in  addition  to  the  connective  tissue,  of  inter- 
lacing branches  of  involuntary  muscle  which  are  disposed  in  three  principal 
directions, — meridionally,  radially,  and  circularly. 

The  meridionally  disposed  muscular  bundles  are  closely  grouped  and 
separated  by  small  bands  of  interfascicular  connective  tissue,  thus  forming 
a  compact  outer  layer  next  the  sclera,  the  tensor  choroidece,  to  which  the 
trabeculse  of  the  supra-choroidal  space  are  attached.  The  meridional  fibres 
take  origin  especially  from  the  scleral  process  and  the  reticular  tissue  con- 
stituting the  inner  wall  of  Schlemm's  canal ;  posteriorly,  they  fade  away 
into  the  tissue  of  the  choroidal  tract,  to  which  they  are  attached  or  inserted 
by  means  of  delicate  tapering  and  often  interlacing  processes.  The  fibres 
situated  most  externally  pursue  a  typically  meridional  course,  the  tendency 
to  assume  a  radial  direction  becoming  more  and  more  pronounced  as  the 
inner  and  posterior  limits  of  the  muscle  are  approached. 

The  radially  arranged  fibres  are,  therefore,  not  sharply  defined  from 
the  meridional  bundles,  since  the  change  in  disposition  is  very  gradually 
effected,  the  two  sets  of  fibres  blending  towards  the  periphery.  The  radial 
bundles  are  much  more  loosely  arranged,  and  form  a  reticulation  in  which 
the  muscular  bands  are  separated  by  considerable  tracts  of  connective  tissue. 
Anteriorly,  the  radial  fibres  are  attached  somewhat  farther  forward  than 
are  the  meridional  bundles,  their  point  of  origin  being  the  tissue  of  the 
inner  wall  of  Schlemm's  canal  and  the  trabeculse  derived  from  the  periph- 
eral splitting  up  of  the  membrane  of  Descemet.  Beginning  at  their  point 
of  origin,  the  interlacing  muscular  bundles  diverge  posteriorly  and  internally 
in  a  fan-like  manner,  the  innermost  bundles  passing  towards  the  ciliary 
processes,  the  more  externally  situated  reaching  to  and  beyond  the  anterior 
limits  of  the  ciliary  ring.  The  posterior  border  of  the  muscular  reticulum 
formed  by  the  radial  fibres  is  irregular  in  outline,  and  occupied  at  the 
extreme  boundary  by  muscle  bundles  which  bend  sharply  to  assume  a  cir- 
cular course. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL.  267 

The  circularly  disposed  fibres,  in  addition  to  those  just  noted,  are  col- 
lected principally  at  one  poiiit,  where  they  form  an  annular  group  which 
occupies  the  inner  posterior  angle  of  the  general  muscular  triaugle  formed 
by  the  ciliary  muscle,  and  surrounds  the  base  of  the  iris.  These  bundles 
constitute  the  circular  or  ring  muscle  of  Midler,  as  contrasted  with  the  tensor 
choroidece,  composed  of  the  remaining  meridional  and  radial  portions  of  the 
ciliary  muscle. 

In  emmetropic  eyes  the  general  mass  of  the  ciliary  muscle  approaches 
in  outline  a  right-angled  triangle,  the  greatly  elongated  hypothenuse  of 
which  is  represented  by  the  scleral  border,  the  other  sides  being  the  short 

FIG.  37. 


Part  of  a  meridional  section  through  the  ciliary  region,  showing  the  circularly  disposed  bundles 
composing  Miiller's  muscle  iu  transverse  section.— v,  small  vein  within  the  connective-tissue  stroma. 
Magnified  336  diameters. 

iridial  and  long  choroidal  surfaces.  The  apex  of  the  angle  is  somewhat 
rounded  by  the  circular  fibres  of  Miiller.  Pronounced  hypermetropic  and 
myopic  eyes  often  exhibit  marked  variations  from  the  normal  form  of  the 
muscle,  as  described  by  Iwanoff  and  confirmed  by  later  investigations. 
These  changes  depend  largely  upon  the  over-  or  uuder-development  of  the 
circular  fibres.  In  myopia,  Miiller's  bundles  are  more  or  less  atrophic,  the 
angle  of  the  ciliary  muscle  consequently  appearing  relatively  obtuse.  In 
hypermetropia,  on  the  contrary,  the  excessive  demands  made  upon  these 
fibres  result  in  their  unusual  development,  with  corresponding  preponder- 
ance of  this  portion  of  the  mass,  in  consequence  of  which  the  angle  becomes 
acute,  the  including  sides  subtending  less  than  the  approximate  ninety 
degrees  of  the  normal  muscle. 

The  ciliary  processes,  in  addition  to  affording  support  and  partial  attach- 
ment to  the  suspensory  apparatus  of  the  crystalline  lens,  are  undoubtedly 
closely  concerned  in  the  production  of  the  aqueous  humor  which  occupies 
both  the  anterior  and  the  posterior  chamber. 


268  THE   MICROSCOPICAL   ANATOMY   OP   THE    EYEBALL,. 

Deutschmann  l  convincingly  demonstrated  the  active  rdle  played  by  this 
portion  of  the  uveal  tract  in  the  secretion  of  the  aqueous  humor  by  excising 
the  ciliary  body  and  noting  the  subsequent  arrest  of  secretion  of  this  fluid. 
The  later  experiments  and  observations  of  Schoeler,2  Leplat,3  Gifford,4 
Knies,5  and  Greeff6  all  emphasize  the  important  relations  between  the 
ciliary  processes  and  the  secretion  of  the  aqueous  humor,  as  well  as  estab- 
lish the  existence  of  the  current  of  this  fluid  from  the  posterior  chamber 
through  the  pupil  into  the  anterior  chamber. 

While  it  may  be  regarded  as  established  that  the  aqueous  humor  is 
produced  through  the  agency  of  the  ciliary  processes,  the  determination 
of  the  structures  especially  engaged  in  this  secretion  has  been  less  exact. 
The  most  definite  conclusions  concerning  this  point  are  those  advanced  by 
Collins,7  who,  under  the  name  of  "  ciliary  glands,"  describes  epithelial  ex- 
tensions of  the  outer  layer  of  the  pars  ciliaris  retinae,  which  he  regards  as 
the  structures  engaged  in  the  elaboration  of  the  aqueous  fluid. 

The  ciliary  zone  of  the  rudimentary  anterior  segment  of  the  inner  tunic 
of  the  eyeball,  including  the  portion  of  the  tract  extending  from  the  ora 
serrata  to  the  root  of  the  iris,  consists,  as  does,  indeed,  the  entire  tract,  which 
reaches  as  far  as  the  anterior  pupillary  margin,  of  two  layers  of  cells.  These 
strata,  which  represent  the  inner  and  outer  lamellae  of  the  secondary  optic 
vesicle,  differ  in  their  histological  details.  While  a  fuller  account  of  these 
structures  will  be  found  in  connection  with  the  description  of  the  nervous 
tunic,  a  few  facts  regarding  the  constitution  of  the  pars  ciliaris  retinae  must 
here  be  anticipated  for  the  present  purpose. 

The  rudimentary  retinal  expansion  which  covers  the  inner  surface  of  the 
ciliary  ring  and  the  ciliary  processes  consists  of  a  double  layer  of  epithelial 
elements.  The  inner  of  these  is  composed  of  cells  possessing  a  well-pro- 
nounced columnar  form ;  the  outer,  of  elements  which  are  lower  and  assume 
often  a  more  cuboidal  character.  Over  the  posterior  surface  of  the  iris,  as 
well  as  over  the  anterior  and  most  conspicuous  parts  of  the  ciliary  processes, 
the  inner  cells  are  loaded  with  dark  pigment ;  towards  the  posterior  and  less 
projecting  parts  of  the  processes  this  pigment  becomes  much  less  intense, 

1  Deutschmann :  Ueber  die  Quellen  des  Humor  aqueus  im  Auge,  Archiv  f.  Ophthal., 
Bd.  xxvi.,  1880. 

2  Schoeler :  Ueber  das  Fluorescein  in  seiner  Bedeutung  fur  Erforschung  des  Fliissigkeits- 
wechsels  im  Auge,  Archiv  f.  Anat.  u.  Physiolog.,  Physiolog.  Abth.,  1882. 

3  Leplat:  Etudes  sur  la  nutrition  du  corps  vitre,  Annales  d'Oculistique,  t.  xcvm., 
1887. 

4  Gifford :    Weitere  Versuche  uber  die  Lymphstrome  und  Lymphwege  des  Auges, 
Archiv  f.  Augenheilkunde,  Bd.  xxvi.,  1898. 

6  Knies :  Ueber  die  vorderen  Abflusswege  des  Auges  und  die  kiinstliche  Erzeugung 
von  Glaukom,  Archiv  f.  Augenheilkunde,  Bd.  xxvni.,  1894. 

6  Greeff:  Neue  Befunde  zur  Kenntniss  des  Fliissigkeitswechsels  im  Auge  und  zur 
Lehre  von  der  Fibrinbildung  im  Kammerwasser,  Bericht  uber  d.  23.  Versarnml.  d.  Oph- 
thalmol.  Gesellsch.  zu  Heidelberg,  1893. 

7  Collins  :  The  Glands  of  the  Ciliary  Body  in  the  Human  Eye,  Trans,  of  the  Ophthal. 
Society  of  the  United  Kingdom,  vol.  XL,  1891. 


THE   MICROSCOPICAL   ANATOMY  OF  THE   EYEBALL.  269 

and  is  limited  to  the  outer  portions  of  the  cells ;  while  over  the  ciliary  ring 
the  elements  composing  the  inner  layer  are  almost,  if  not  entirely,  devoid 
of  colored  particles.  There  is  thus  a  progressive  decrease  of  the  pigment 
within  the  cells  of  the  inner  lamella  from  the  bases  towards  the  apices  of 
the  ciliary  processes.  The  outer  layer,  on  the  contrary,  remains  more  or 
less  deeply  pigmented  throughout  its  extent,  and  over  the  ciliary  ring  and 
the  apices  of  the  processes  the  cells  contain  a  considerable  amount  of  colored 
particles. 

When  carefully  examined  in  meridional  sections,  the  outer  contours  of 
the  elements  of  the  external  layer  appear  irregular,  and  here  and  there  the 
adjacent  tissue  is  encroached  upon  by  minute  cylindrical  projections.  These 
latter  structures,  upon  examination  after  removal  of  the  obscuring  pigment 
by  bleaching,  Collins  describes  as  composed  of  aggregations  of  epithelial 
cells  arranged  after  the  manner  of  the  elements  composing  tubular  glands 
in  other  situations,  which  he  regards  as  true  secreting  tissue  composing  the 
glands  of  the  ciliary  body.  These  cylindrical  projections  from  the  outer 
cell-stratum  are  most  frequent  and  conspicuous  in  the  plicated  portion  of 
the  ciliary  region,  being  particularly  numerous  and  developed  at  the  junc- 
tion of  the  apices  of  the  ciliary  processes  and  the  smooth  ciliary  ring. 
Collins  regards  these  minute  outgrowths  as  the  glandular  apparatus  by 
means  of  which  the  aqueous  humor  is  secreted.  Additional  weight  in 
support  of  this  view  is  found,  according  to  this  author,  in  the  greatly  ex- 
aggerated and  hypertrophied  condition  of  these  "  glands"  in  pathological 
processes  associated  with  excessive  secretion  of  the  aqueous  humor,  as  in 
serous  iritis.  Sections  of  the  ciliary  region  of  eyes  so  affected  display  the 
presence  of  conspicuous  tubular  proliferations  closely  associated  with  the 
covering  of  this  part  of  the  uveal  tract. 

That  the  ciliary  processes,  at  least  in  part,  if  not  as  a  whole,  are  inti- 
mately and  directly  related  to  the  production  of  the  aqueous  humor  is 
established  beyond  dispute ;  critical  study  of  the  foregoing  structures  de- 
scribed as  the  ciliary  glands,  however,  will  fail  to  convince  many,  and 
among  them  is  Leber,1  that  these  minute  cylindrical  projections,  which  are 
separated  from  the  posterior  chamber  by  means  of  the  unbroken  inner 
layer  of  tall  columnar  cells  of  the  pars  ciliaris  retinae,  suffice  for  the  elabo- 
ration of  the  no  inconsiderable  quantity  of  fluid  continually  escaping  from 
the  eye.  The  broader  view  of  Nicati,2  who  attributes  the  secretion  of  the 
humor  aqueus  to  the  "  uveal  gland,"  the  epithelium  of  which  is  represented 
by  the  pars  ciliaris  retinae,  the  blood-supply  by  the  rich  vascular  distribution 
within  the  adjacent  structures,  and  the  contractile  tissue  by  the  cilio-cho- 
roidal  muscle,  offers  a  more  comprehensive  interpretation. 

The  Blood-  Vessels  of  the  Ciliary  Body.— The  blood-vessels  especially 
supplying  the  ciliary  muscle  are  derived  from  two  sources, — from  the  long 

1  Leber:  Ergebnisse  der  Anatomic  u.  Entwickelungsges.,  Bd.  iv.,  1894,  p.  17-Y 

2  Nicati :  La    glandula  de    1'humeur  aqueuse,   Compt.-rend.  hebd.  de    la  Soc.  de 
Biolog.,  Ser.  9,  t.  in.,  1892. 


270 


THE    MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 


and  the  anterior  ciliary  arteries.  These  vessels,  in  their  course  through  the 
anterior  part  of  the  muscle  to  gain  the  root  of  the  iris,  where  they  form  the 
larger  arterial  circle,  give  off  branches  which  anastomose  with  considerable 
frequency  and  constitute  a  somewhat  incomplete  arterial  circuit,  the  circuhis 
arteriosus  musculi  ciliaris  ;  from  the  latter  minute  twigs  pass  in  various 
directions  into  the  substance  of  the  muscle  for  the  supply  of  the  muscular 
tissue. 

The  arteries  supplying  the  ciliary  processes  are  derived  from  the  circulus 
iridis  major,  as  branches  which  pass  backward,  traversing  the  inner  part  of 
the  ciliary  muscle,  to  reach  the  anterior  extremities  of  the  plications.  On 

FIG.  38. 


Diagram  of  vascular  supply  of  anterior  segment  of  eye.  (Leber.) — c,  c',  anterior  ciliary  artery  and 
vein;  d,  d',  posterior  conjunctival  vessels;  o,  recurrent  twig  to  choroid ;  p,  circulus  arteriosus  iridis 
major  in  section;  (/.vessels  of  iris;  r,  vessels  of  ciliary  processes;  s,  vein  returning  blood  from  iris  and 
ciliary  process ;  t,  tributary  of  anterior  ciliary  vein  from  ciliary  muscle;  u,  circulus  venosus  (canal  of 
Schlemm) ;  v,  vascular  loops  of  corneal  limbus ;  w,  anterior  conjunctival  vessels. 

entering  the  latter — a  single  artery  often  providing  for  more  than  one  process 
— the  main  stem  rapidly  breaks  up  into  secondary  branches,  which  soon  pass 
over  into  the  intricate  convolutions  forming  so  large  a  part  of  the  entire  fold. 

The  venous  radicles  draining  the  ciliary  muscle  become  tributary  to  two 
distinct  groups,  the  anteriorly  and  the  equatorially  situated  veins.  A  por- 
tion of  the  blood  is  carried  inward  and  backward  to  join  that  carried  by  the 
veins  returning  the  blood  from  the  ciliary  processes ;  it  is,  therefore,  finally 
taken  up  and  conveyed  from  the  deeper  structures  by  the  great  equatorial 
trunks,  the  venae  vorticosse ;  another  portion  of  the  blood  from  the  ciliary 
muscle  is  conducted  forward  and  outward  by  means  of  vessels  which  pierce 
obliquely  the  sclerotic  coat  and  empty  into  the  anterior  ciliary  veins. 
During  their  passage  through  the  fibrous  tunic  they  come  into  proximity 
to  the  canal  of  Schlemm  and  receive  the  small  tributaries  proceeding  from 
that  channel,  by  means  of  which,  as  already  noted,  indirect  communication 
is  established  between  the  annular  sinus  and  the  anterior  ciliary  veins. 

The  Nerves  of  the  Ciliary  Body. — The  remarkable  functional  activity  of 
the  structures  included  within  the  ciliary  body  leads  to  the  anticipation  of 
the  existence  of  a  rich  nervous  supply  to  this  region. 


THE   MICROSCOPICAL   ANATOMY   OP  THE   EYEBALL.  271 

The  anterior  ramifications  of  the  long  ciliary  nerves,  together  with  fila- 
ments contributed  by  the  short  ciliary  trunks,  on  entering  the  ciliary  body 
unite  to  form  an  annular  plexus,  the  orbiculus  ganglioms,  within  the  sub- 
stance of  the  ciliary  muscle.  The  varied  character  of  the  component  fibres 
of  these  branches  explains  the  presence  of  motor,  sympathetic,  and  sensory 
nerve-fibres  within  this  plexus. 

FIG.  39. 


Nerve-terminations  within  the  ciliary  muscle.  Methylene-blue  staining.— a,  bundle  of  medullated 
nerve-fibres  giving  off  a  lateral  twig  (6).  which  divides  into  branches  which  break  up  into  the  terminal 
arborizations  (c,  d).  (After  Agababow.) 

The  more  recent  investigations  of  Agababow  and  Arnstein *  have  added 
to  our  definite  knowledge  concerning  the  complex  disposition  of  the  nerves 
of  this  region.  According  to  these  observers,  four  sets  of  nerve-fibres  may 
be  demonstrated  within  the  ciliary  body  of  the  cat,  and,  with  modifications, 
also  within  that  of  man.  These  groups  of  nervous  filaments  comprise: 
1,  the  vaso-motor  fibres  supplying  the  vascular  tissues ;  2,  the  motor  fibres 
ending  within  the  tissue  of  the  ciliary  muscle  ;  3,  the  sensory  fibres,  forming 
a  subscleral  distribution ;  4,  fibres  terminating  in  ramifications  within  the 
intermuscular  tissue  of  the  ciliary  muscle. 

The  vaso-motor  fibres  are  especially  concerned  in  the  innervation  of  the 
walls  of  the  blood-vessels,  within  the  outer  parts  of  which  they  break  up 
into  fibrillae  which  penetrate  the  muscular  tunic.  The  distribution  of  the 
vaso-motor  fibres  to  the  ciliary  processes  has  been  carefully  studied  by 
Meyer 2  and  Griinhagen.3 

The  motor  fibres,  destined  particularly  for  the  tissue  of  the  ciliary  muscle, 
possess  a  characteristic  arrangement,  in  which  the  finer  fibrillffi  pursue  a 
course  largely  corresponding  with  the  disposition  of  the  muscle  elements. 
The  slender  straightly  running  fibrilla3  present  minute  varicosities,  and 
terminate  in  free  endings  of  great  delicacy  between  the  contractile  cells. 

The  sensory  fibres  of  the  ciliary  body,  which  in  the  cat  and  some  other 

1  Agababow  und  Arnstein  :    Die  Innervation  des  Ciliarkorpers,  Anatom.  Anzeiger, 
Bd.  viii.,  No.  17,  1893. 

2  Meyer  :  Die  Nervenendigungen  in  der  Iris,  Archiv  f.  mik.  Anat,  Bd.  XVII.,  1* 

5  Griinhagen:  Die  Nerven  der  Ciliarfortsatze  des  Kaninchens,  Archiv  f.  mik.  Anat., 
Bd.  xxii.,  1883. 


272  THE   MICROSCOPICAL   ANATOMY    OF   THE    EYEBALL. 

animals  are  represented  by  a  superficial  plexus  occupying  the  outer  sub- 
scleral  layers,  in  man  are  connected  with  special  expansions  or  "  reticulmn- 
plates."  Critical  examination  of  these  structures  with  high  powers  shows 
them  to  be  composed  of  fibrillae  of  the  greatest  tenuity. 

Additional  sensory  fibres,  constituting  the  foregoing  fourth  group,  bear 
an  intimate  relation  to  the  ciliary  muscle.  The  twigs  given  off  from  the 
annular  plexus  consist  principally  of  medullated  fibres ;  they  soon  divide 
and  intertwine,  and  contain  sparingly  interspersed  ganglion-cells,  chiefly 
along  the  course  of  the  thinner  nerve-bundles.  On  following  the  medul- 
lated fibres  in  their  further  ramifications  within  the  mass  of  the  ciliary 
muscle,  they  are  seen  to  give  off  small  offshoots,  which  retain  their  medul- 
lary sheath  until  they  have  reached  a  different  plane,  when  they  soon  divide 
into  two  non-medu Hated  fibres.  The  latter  almost  immediately  break  up  into 
a  number  of  secondary  fibrillse,  which  in  turn  are  resolved  into  terminal 
threads,  the  entire  group  of  fibrillae  forming  a  special  end-arborization,  of 
which  two  or  more  are  connected  with  the  filaments  derived  from  a  single 
primary  lateral  twig. 

The  terminal  arborizations  connected  with  the  nerves  in  question  are 
distinguished  from  other  nervous  endings  within  the  ciliary  muscle  by  the 
relatively  great  thickness  of  the  varicose  fibrillse,  which  end  in  free  knob- 
like  expansions.  These  end-arborizations  are  situated  at  various  levels,  but 
occupy  particularly  the  posterior  and  inner  segment  of  the  ciliary  body. 
They  lie  within  the  intermuscular  connective  tissue  between  the  bundles 
composing  the  ciliary  muscle.  According  to  Arnstein,1  these  terminal  rami- 
fications represent  a  special  nervous  apparatus  for  the  perception  of  muscle- 
sensibility  excited  mechanically  by  the  contraction  of  the  surrounding 
muscular  bundles. 

THE   IRIS. 

The  iris  constitutes  the  anterior  and  inner  segment  of  the  vascular  tunic 
of  the  eyeball,  forming  the  perforated  membrane  or  diaphragm  which 
stretches  across  in  front  of  the  crystalline  lens.  Its  inner  or  pupillary 
margin  rests  upon  the  anterior  surface  of  the  lens ;  its  periphery  or  root  is 
connected  with  the  choroidal  tract  just  anterior  to  the  ciliary  processes. 

Viewed  in  section,  the  iris  presents,  in  addition  to  its  variously  tinted 
stroma,  covered  in  front  by  the  endothelium  of  the  anterior  chamber,  a 
deeply  pigmented  posterior  stratum  directly  continuous  with  the  pigment 
layers  clothing  the  ciliary  processes ;  this  dark  lamella  represents  the  rudi- 
mentary anterior  segment  of  the  nervous  tunic,  and  constitutes  the  pars 
iridica  retinae.  The  iris,  therefore,  is  composed  of  two  genetically  distinct 
parts, — that  contributed  by  the  mesoderm,  and  that  secondarily  derived 
from  the  ectoderm  through  the  optic  vesicle. 

The  various  components  of  the  iris  and  their  morphological  relations 
may  be  grouped  as  follows  : 

1  Arnstein :  loc.  cit. ,  p.  560. 


THE    MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL.  273 

1.  Anterior  endotheliiim. 

2.  Anterior  boundary  layer.          C«ntmuatl^  f  the  mesodermic  tissues 

3.  Vascular  stroma  layer.         °f  the  ch°r°ldal  tract'  ^^tituting  the 
D    ,./..,.      7      „       drama  zone. 

4.  Posterior  limiting  lamella.  } 

5.  Pigment  layer. 


a.  Anterior  layer  of  pig- 
mented  spindle-cells,  representing 

OUTER  LAYER 

6.  Posterior  layer  of  pig- 
mented  polygonal  cells,  represent- 
ing INNER  LAYER 


of  optic  vesicle. 


The  anterior  endothelium  consists  of  a  single  layer  of  irregular  poly- 
hedral plates,  composed  of  finely  granular  protoplasm  and  containing  ovoid 
or  reniform  nuclei,  which  are  uninterruptedly  continued  over  the  front 
surface  of  the  iris  as  far  as  the  pupillary  margin.  This  endothelium,  the 


FIG.  40. 


Radial  section  of  iris.— a,  endothelium;  6,  anterior  boundary  layer;  c,  vascular  stroma;  e,  posterior 
limiting  lamella ;  /,  pigmented  retinal  zone ;  v,  blood-vessels.    Magnified  170  diameters. 

presence  of  which  is  demonstrable  after  silver  staining,  is  a  part  of  the 
general  lining  of  the  anterior  chamber ;  it  is,  therefore,  a  direct  continuation 
of  the  endothelial  cells  which  cover  the  membrane  of  Descemet  and  invest 
the  trabeculje  of  the  ligamentum  pectinatum.  Even  when  the  stroma  layer 
is  loaded  with  pigment,  as  in  irides  of  very  deep  color,  the  cells  of  the 
anterior  endothelium  remain  clear  and  uninvaded  by  pigment. 

The  anterior  boundary  layer  has  no  existence  as  an  independent  layer, 

since  it  consists  of  the  modified  and  condensed  foremost  stratum  of  the 

stroma,  in  which  the  connective-tissue  cells  of  the  general  iris-stroma  are 

unusually  closely  arranged  on  account  of  the  relatively  small  amount  of  the 

VOL.  I.— 18 


274  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

intercellular  fibrous  tissue  and  small  size  of  the  interfascicular  lymph-spaces. 
As  a  result  of  these  peculiarities  the  cells  lie  nearer  together,  producing  an 
appearance  which  Michel 1  interprets  as  indicating  a  layer  of  reticulated 
tissue  somewhat  resembling  a  dense  lymphoid  structure.  The  layer  under 
consideration,  however,  differs  from  ordinary  iridial  stroma,  as  pointed  out 
by  Koganei,2  only  in  being  more  condensed,  the  existence  of  a  reticulum 
seemingly  composed  almost  exclusively  of  irregularly  spindle  and  stellate 
plate-like  elements,  as  assumed  by  Michel,  being  suggested  by  the  incon- 
spicuous representation  of  the  fibrous  bundles  in  this  part  of  the  iris.  The 
fibrous  tissue  is,  however,  distinctly  present,  and  exists,  as  emphasized  by 
Eetzius,3  in  the  form  of  irregularly  interlacing,  extremely  delicate  bundles, 
which  pass  between  the  cells  as  far  as  the  anterior  endothelium.  The 
irregular  connective-tissue  cells  occupying  the  anterior  layer,  which  are 
sometimes  round,  oval,  or  stellate  elements,  in  the  deepest  part  of  the 
boundary  layer  assume  the  disposition  of  the  elements  of  the  general  iris- 
stroma,  into  which  they  insensibly  pass. 

The  minute  clefts  which  occupy  the  interspaces  between  the  elements 
constituting  the  anterior  boundary  layer  represent  lymph-spaces,  and  some- 
times contain  lymph-corpuscles  or  migratory  cells  in  varying  number,  as  in 
other  parts  of  the  iris-stroma.  At  the  posterior  margin  of  the  boundary 
layer  the  intercellular  clefts  become  of  larger  size,  and  gradually  pass  into 
the  lymph-spaces  of  the  stroma  layer.  In  deeply  colored  irides  the  proto- 
plasm of  the  cells  composing  the  anterior  boundary  layer,  as  well  as  their 
processes,  becomes  invaded  by  pigment  particles.  Blood-vessels  are  wanting 
within  this  part  of  the  iris. 

The  vascular  stroma  layer,  which  constitutes  the  bulk  of  the  iris,  consists 
of  a  loosely  disposed  mesh-work  of  connective-tissue  fibres  and  cells  sup- 
porting a  rich  supply  of  blood-vessels  and  nerves  and  enclosing  irregular 
lymph-spaces.  In  addition  to  these  elements,  this  stratum,  in  the  vicinity 
of  the  pupillary  margin,  contains  the  muscular  tissue  constituting  the 
sphincter  pupillce,  as  well  as  an  irregular  layer  of  radially  disposed  muscle- 
bundles  which  extend  from  the  pupillary  towards  the  marginal  zone,  and 
represent  an  imperfect  dilator  pupillce. 

The  strength  of  the  supporting  framework  depends  less  on  the  con- 
nective-tissue elements  than  upon  the  radially  coursing  blood-vessels  and 
nerve-trunks  which  they  invest  (Schwalbe),  and  around  which  they  form 
sheaths  of  considerable  size.  The  structural  elements  composing  the  layer 
of  vascular  stroma  include  delicate  bundles  of  fibrous  tissue,  on  and  between 
which  lie  the  irregular  connective-tissue  cells.  While  the  fibrous  tissue  is 
aggregated  principally  as  the  sheaths  investing  the  vessels  and  the  nerves, 

1  Michel :  Die  histologische  Structur  des  Irisstroma,  Erlangen,  1875. 

2  Koganei :  Untersuchungen  tiber  den  Bau  der  Iris  des  Menschen  und  der  "VVirbel- 
thiere,  Archiv  f.  mik.  Anat.,  Bd.  xxv.,  1885. 

'Ketzius:  Zur  Kenntniss  vom  Bau  der  Iris,  Biologische  Untersuchungen,  Neue 
Folge,  v.,  1893. 


THE    MICROSCOPICAL   ANATOMY  OP  THE   EYEBALL. 


275 


the  intervening  territory  is  occupied  by  delicate  bands  irregularly  inter- 
woven to  form  a  loose  or  spongy  tissue,  the  interfibrillar  interstices  of  which 
may  be  regarded  as  lymph-spaces.  The  connective-tissue  cells  are  particu- 
larly numerous  in  the  vicinity-of  the  larger  perivascular  sheaths,  accom- 
panying the  blood-vessels  in  conspicuous  groups ;  within  the  general  loose 
mesh-work  characterizing  other  parts  of  the  stroma  layer  the  cells  are  much 
less  numerous.  The  fibrous  investments  of  the  blood-vessels,  arteries  as 
well  as  veins,  are  particularly  conspicuous  in  sections  passing  parallel  to 
the  pupillary  margin,  in  which  the  radially  disposed  vessels  are  cut  generally 
at  right  angles.  These  perivascular  sheaths,  which  in  thickness  not  infre- 
quently equal  the  external  diameter  of  the  enclosed  vessel,  are  composed 
principally  of  circularly  disposed  fibres ;  the  latter  often  deviate  sufficiently 

FIG.  41. 


Tangential  section  of  iris.— a,  endothelium  ;  6,  anterior  boundary  layer ;  c,  vascular  stroma ;  e,  pos- 
terior limiting  lamella  ;  /,  pigmeuted  retinal  zone;  g,  portion  of  sphincter  pupillse  muscle  cut  parallel 
to  course  of  fibres ;  h,  transversely  cut  dilator  fibres.  Magnified  170  diameters. 

in  their  transverse  arrangement  to  produce  obliquity,  which  results  in  an 
interweaving  of  the  fibrillse.  While  the  circular  fibres  constitute  the  most 
important  part  of  the  sheath,  Retzius l  has  shown  that  longitudinal  fibrillse 
exist  both  without  and  within  the  chief  fibrous  layer,  the  perivascular  invest- 
ment thereby  materially  gaining  in  strength.  A  perivascular  lymph-space 
usually  separates  the  sheath  from  the  blood-vessel,  the  exterior  of  the  latter 
being  invested  by  a  more  or  less  perfect  endothelial  covering. 

The  stroma  in  dark  irides  contains  a  variable  amount  of  pigment,  dis- 
tributed principally  as  irregular  aggregations  of  pigmented  cells  in  which 

1  Eetzius  :  loc.  cit. 


276 


THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 


the  nuclei  alone  remain  uuinvaded  ;  here  and  there  additional  smaller  non- 
nucleated  masses  of  colored  particles  are  encountered.  The  anterior  layer 
and  the  pupillary  zone  are  the  portions  of  the  iris-stroma  that  contain  the 
pigment-cells  in  greatest  number;  in  irides  of  only  moderate  darkness 
these  situations  include  almost  the  entire  pigment.  The  stroma  of  very 
dark  irides  presents  a  much  more  general  distribution  of  the  pigment,  since 
in  these  cases  all  portions  of  the  stroma  layer  are  filled  with  the  colored 
particles ;  the  latter  are  contained  either  within  irregular  spherical  masses 
of  pigment-cells  or  within  more  isolated  stellate  elements.  The  vicinity  of 
the  circulus  arteriosus  minor — that  is,  the  outer  margin  of  the  pupillary  zone 
— is  especially  rich  in  accumulations  of  pigment. 

FIG.  42. 


Radial  section  of  pupillary  zone  of  iris.— a,  termination  of  pigmented  layers  at  pupillary  margin ; 
b,  stroma  layer ;  c,  transversely  cut  bundles  of  sphincter  pupillae ;  d,  pigmented  retinal  zone ;  e,  dilator 
fibres.  Magnified  70  diameters. 

The  muscular  tissue  which,  as  already  intimated,  lies  within  the  vascular 
stroma  layer  forms  one  of  the  most  important  constituents  of  the  iris. 

The  distribution  of  this  muscular  tissue,  while  a  matter  of  much  interest, 
has  been  and  still  is  a  subject  concerning  which  the  views  of  competent 
observers  widely  diverge.  While  the  details  of  the  arrangement  of  the 
iridial  musculature,  particularly  the  presence  or  absence  of  a  dilator  pupillse, 
are  still  matters  for  discussion,  it  is  agreed  that  anatomically  the  most 
important  muscular  tract  surrounds  the  pupil  as  an  annular  band, — the 
sphincter  pupillce. 

The  latter  consists  of  a  zone  of  involuntary  muscle,  varying  between 
.040  and  .080  millimetre  in  width,  according  to  the  state  of  contraction,  and 
measuring  about  one-tenth  as  much  in  thickness ;  it  is  situated  nearer  the 
posterior  than  the  anterior  surface  of  the  iris-stroma.  The  individual 
muscle-bundles  composing  the  pupillary  sphincter  somewhat  interlace  during 
their  circular  course,  being  contained  within  and  separated  from  one  another 
by  delicate  investments  of  connective  tissue. 

The  muscle  extends  almost,  but  not  quite,  to  the  pupillary  margin,  the 
immediate  free  border  of  the  latter  being  composed  of  the  attenuated  con- 
tinuation of  the  retinal  layers,  which  thus  shut  out  the  iris-stroma  from  the 
pupil.  During  contraction,  however,  the  pigmented  retinal  layers  become 
compressed  to  such  a  degree  that  the  sphincter  muscle  apparently  forms  the 


—  6 


THE   MICROSCOPICAL   AXATOMY  OF  THE   EYEBALL.  277 

direct  boundary  of  the  pupillary  opening.  The  observation  of  Steinach,1 
that  the  muscular  elements  of  the  sphincter  are  sometimes  pigmented,  is 
of  interest  in  connection  with  the  presence  of  colored  particles  within  the 
spindle-cells  of  the  posterior  boundary  layer  of  the  iris. 

In  marked  contrast  to  our  definite  knowledge  regarding  the  universally 
admitted  presence  of  a  sphincter  muscle  stand  the  data  concerning  the 
existence  of  an  antagonizing 

dilator     pupil  lae.       Not  with-  FIG.  43. 

standing,  however,  the  dis- 
agreement as  to  their  interpre- 
tation, the  presence  of  certain 
anatomical  details  has  been 
established  by  many  obser- 
vations. 

It  is  admitted  by  all  that 
the  iris-stroma  is  separated 
from  the  deeply  pigmented 
posterior  zone  by  a  delicate 
stratum,  the  posterior  boun- 
dary layer,  or  membrane  of 
Bruch,  within  which,  or  at 
least  closely  associated  with  it, 
exist  numerous  radially  dis- 
posed delicate  spindle  fibre- 
cells.  These  elements,  often 
more  or  less  pigmented,  are 
the  particular  objects  concern- 
ing which  investigators  disagree,  since  some  regard  the  spindle-cells  as 
muscular  in  nature,  while  others  consider  them  as  modified  connective 
tissue. 

The  character  of  these  cells  has  been  repeatedly  the  subject  of  critical 
investigations  by  competent  observers  in  the  iris  of  the  albino  rabbit,  which 
tissue,  owing  to  the  absence  of  the  obscuring  pigment,  offers  an  unusually 
favorable  opportunity  of  study. 

Sections  of  such  tissue,  either  when  cut  meridionally  or  parallel  to  the 
surface  of  the  iris,  demonstrate  the  presence  of  long,  delicate  spindle-cells, 
the  general  form  and  nuclei  of  which  strongly  resemble  elongated  muscular 
elements.  These  cells  are  especially  well  displayed  in  tissues  fixed  in  a  four 
per  cent,  solution  of  formaldehyde  and  stained  with  hsematoxylin.  The 
spindle-cells  in  the  rabbit,  however,  do  not  constitute  a  continuous  layer,  but 
are  disposed  as  delicate  bundles  radiating  from  the  pupillary  margin  well 
towards  the  ciliary  zone.  The  presence  of  similarly  arranged  spindle-cells 


Radial  section  of  iris  of  rabbit  (Retzlus.)— b,  anterior 
boundary  layer:  g,  vascular  stroma  layer;  d,  dilator 
fibres ;  r,  retinal  layer.  Magnified  about  300  diameters. 


1  Steinach  :  tfber  den  Sphincter  Pupillse  des  Frosches,  Archiv  f.  d.  ges.  Physiologic, 
Bd.  LIII.,  1893. 


278  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

has  been  established  by  Heese l  in  a  number  of  other  animals,  including  the 

cat,  dog,  sheep,  ox,  and  pig. 

Direct  observations  confirming  the  existence  of  corresponding  elements 

within  the  human  iris  have 

FIG.  44.  been  made  by  a  number  of 

investigators,  among  which 
the  testimony  of  Merkel,2 
Retzius,3  and  Schafer 4  is  the 
most  positive.  The  last- 
named  observer  states,  "  I 
have  myself  obtained  unmis- 

Surface  view  of  dilator  fibres  of  human  iris.     (Retzius.)  * 

Magnified  about  400  diameters.  takable  evidence  of  the  pres- 

ence of  a  thin  layer  of  fibres 

at  the  back  of  the  iris,  under  the  pigment-cells,  having  all  the  appear- 
ance of  flat,  plain  muscle-cells."  The  study  of  numerous  sections  of 
human  irides  cut  parallel  to  the  pupillary  margin  leads  the  writer  to  agree 
as  to  the  existence  of  an  almost  unbroken  layer  of  elongated  elements 
which  strongly  resemble  involuntary  muscle-cells. 

The  observations  of  Ewing6  suggest  the  close  relations  between  the 
dilator  fibres  and  the  ciliary  region  :  he  describes  the  presence  of  numerous 
delicate  bundles  of  radiating  connective-tissue  fibres,  which  pass  from  the 
valleys  between  the  ciliary  processes,  through  the  root  of  the  iris,  to  join 
with  the  radially  disposed  muscular  bundles  representing  the  dilator. 

While  the  existence  of  a  more  or  less  complete  stratum  of  delicate 
spindle-cells  within  the  posterior  border  of  the  stroma  layer  of  the  iris  of 
man  and  many  other  animals  may  thus  be  assumed  as  positively  demon- 
strated, the  interpretations  as  to  the  exact  nature  of  these  elements  are  far 
from  identical.  The  difficulty  of  determining  the  nature  of  the  cells  in 
question  solely  upon  their  morphological  characteristics  is  admitted  by  those 
most  skilled  in  microscopical  investigations,  and  even  so  experienced  an 
observer  as  Retzius  confesses  his  inability  to  decide  positively  the  muscular 
character  of  the  spindle-cells  from  their  histological  details. 

The  authorities,  however,  who  have  regarded  the  evidence  as  sufficiently 
conclusive  to  warrant  the  belief  in  the  existence  of  a  definite  dilator  pu- 
pillae  muscle  include  a  number  of  the  most  trustworthy  observers,  among 
whom  are  Henle,  Kolliker,  Luschka,  Merkel,  Dogiel,  Eversbusch,  Schafer, 
and  others.  Conspicuous  among  those  who,  on  the  other  hand,  consider 

1  Heese :  Ueber  den  Einfluss  des  Sympathicus  auf  das  Auge,  insbesondere  auf  die 
Irisbewegung,  Archiv  f.  d.  ges.  Physiolog.,  Bd.  LIT.,  1893. 

1  Merkel  :  Ergebnisse  der  Anatomic  u.  Entwickelungsges.,  Bd.  in.,  1893  (Anmer- 
kung,  S.  288). 

3  Ketzius  :  Zur  Kenntniss  vom  Bau  der  Iris,  Biologiscbe  Untersuch.,  Neue  Folge,  v., 
1893. 

4  Schafer:  in  Quain's  Anatomy,  10th  ed.,  vol.  in.,  Pt.  3,  1894. 

6  Ewing :  Ueber  ein  Bauverhaltniss  des  Iris-Umfanges  beim  Menschen,  Archiv  f. 
Ophthalmol.,  Bd.  xxxiv.,  1888. 


THE   MICROSCOPICAL   ANATOMY  OF  THE  EYEBALL.  279 

the  muscular  nature  of  the  spindle-cells  as  insufficiently  established,  and 
therefore  look  upon  these  elements  as  elastic  in  character,  are  Schwalbe  and 
Grunhagen.  The  latter,  who  has  long  regarded  the  expansive  movements 
of  the  iris  as  associated  with  the  contraction  of  the  blood-vessels,  has  ad- 
mitted in  a  more  recent  paper l  that  the  finely  fibrillar  ground-substance  of 
Bruch's  membrane  possesses  muscular  contractility:  Griinhagen's  views, 
therefore,  are  in  closer  acco.rd  with  those  of  the  majority  of  observers  than 
formerly.  As  a  result  of  his  comparative  and  embryological  investigations, 
Gabrielides  2  also  accepts  and  figures  a  dilator  in  the  human  iris. 

In  connection  with  the  question  of  the  presence  of  a  dilator  muscle  in 
man,  the  fact  is  suggestive  that  a  distinct  dilator  pupillse  exists  not  only  in 
birds,  where  a  robust  dilator  is  found,  but  also  in  many  mammals,  as  estab- 
lished by  the  comparative  investigations  of  Koganei' 3  and  of  Dostoiewsky.4 
Within  the  iris  of  the  seal  and  the  common  otter  the  dilator  is  very  well 
developed  and  constitutes  a  conspicuous  structure ;  that  of  the  otter,  ac- 
cording to  Eversbusch,5  in  correspondence  with  the  triangular  form  of  the 
pupil,  consists  of  three  bands  of  radially  disposed  fibres. 

The  admitted  uncertainty  of  definitely  establishing  the  muscular  char- 
acter of  the  spindle-cells  upon  purely  histological  data  has  given  great  value 
to  physiological  investigations  concerning  the  movements  of  the  iris,  as 
capable  of  supplying  additional  corroborative  evidence  in  connection  with 
the  existence  of  a  pupillary  dilator  muscle.  The  various  views  held  by 
observers  who  from  time  to  time  have  offered  explanations  concerning  the 
phenomena  attending  the  movements  of  the  iris  may  be  arranged,  as 
suggested  by  Heese,6  under  three  general  groups. 

1.  A  sphincter  and  a  dilator  muscle  both,  exist ;  the  former  is  controlled 
by  the  oculo-motor  nerve,  the  latter  is  innervated  by  the  sympathetic.     The 
movements  of  the  iris  and  its  conditions  of  dilation  or  contraction  are, 
therefore,  the  direct   result  of  the  antagonizing  influence  of  these  two 
muscles. 

2.  A  dilator  muscle  does  not  exist,  the  size  of  the  pupil  being  regulated 
by  the  balance  between  the  contraction  of  the  sphincter  and  the  elasticity 
of  the  iris-stroina.     The  influence  of  the  sympathetic  upon  the  pupil  is 
secondary,  being  directly  exerted  upon  the  blood-vessels,  which  by  their 

1  Grunhagen :   Ueber  die  Mechanik  der  Irisbewegung,  Nachtrag,  Archiv  f.  d.  ges. 
Physiologic,  Bd.  LIII.,  1893. 

2  Gabrielides  :  Rechercb.es  sur  I'embryogenie  et  1'anatomie  comparee  de  Tangle  de  la 
chambre  anterieure  chez  le  poulet  et  chez  1'homme,  Archiv.  d'Ophthal.,  t.  xv.,  1895. 

3  Koganei :  Untersuch.  iiber  den  Bau  der  Iris  des  Menschen  und  der  Wirbelthiere, 
Archiv  f.  mik.  Anat.,  Bd.  xxv.,  1885. 

*  Dostoiewsky  :  Ueber  den  Bau  des  Corpus  ciliare  und  der  Iris  von  Saugethieren, 
Archiv  f.  mik.  Anat ,  Bd.  xxvin.,  1886. 

5  Eversbusch:  Vergleichende  Studien  iiber  den  fein.  Bau  der  Iris  der  Saugethiert-... 
Zeitsch.  f.  vergl.  Augenheilkunde,  Bd.  in.,  1^85. 

•  Heese :  Ueber  den  Einfluss  des  Sympathetic^  auf  das  Auge,  insbesondere  t 
Irisbewegung,  Archiv  f.  d.  ges.  Physiolog.,  Bd.  LII.,  1893. 


280  THE    MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

thereby  induced  contraction  effect  a  mechanical  dilation  of  the  pupillary 
opening. 

3.  A  dilator  muscle  does  not  exist,  but  the  dilation  of  the  pupil  follow- 
ing stimulation  of  the  sympathetic  nerve  is  due,  not  to  the  contraction  of 
the  blood-vessels,  but  to  a  restraining  or  inhibitory  action  exerted  by  the 
sympathetic  upon  the  sphincter.  In  consequence  of  such  influence  brought 
about  by  the  sympathetic  nerve,  the  dilation  follows  as  the  result  of  the 
temporary  inaction  and  partial  passiveness  of  the  sphincter  muscle. 

The  physiological  aspects  of  the  pupillary  movements  have  been  ex- 
haustively considered  in  an  elaborate  paper  by  Langley  and  Anderson,1  to 
which  the  reader  is  referred  for  a  critical  discussion  of  the  various  views 
pertaining  to  the  changes  within  the  iris,  as  well  as  for  the  details  of  the 
interesting  experimental  investigations  there  recorded.  The  present  pur- 
pose will  be  served  by  a  brief  account  of  the  conclusions  arrived  at  by 
these  authors. 

The  experimental  proofs  of  the  existence  of  a  radially  arranged  con- 
tractile substance  within  the  iris  adduced  by  Langley  and  Anderson  include 
the  demonstration  of  the  following  facts  : 

1.  When  local  dilation  of  the  pupil  passes  a  certain  limit,  the  opposite 
side  of  the  iris  is  dragged  towards  the  stimulated  side ;  that  this  displace- 
ment is  not  due  to  inhibition  of  the  sphincter  muscle  is  shown  by  the  fact 
that  the  sphincter  can  be  made  to  contract  at  the  same  time. 

2.  If  a  radial  strip  of  iris  be  isolated  from  the  adjacent  parts  of  the 
iris,  stimulation  of  the  sympathetic  causes  shortening  of  the  thus  isolated 
strip.     This  shortening  may  be  produced  prior  to  and  without  any  con- 
traction of  the  blood-vessels. 

3.  Direct  examination  of  the  iris  during  stimulation  of  the  cervical 
sympathetic  shows  that  dilation  of  the  pupil  precedes  the  contraction  of  the 
vessels.     The  dilating  action  of  the  sympathetic  is  thus  demonstrated  to  be 
independent  of  contraction  of  the  blood-vessels. 

4.  There  vis  no  proof  of  elastic  tissue  in  the  iris,  since  a  radial  strip  does 
not  always  retract  on  being  stretched  ;  if  the  iris  be  left  until  its  muscular 
tissue  is  dead,  a  radial  strip  does  not  shorten. 

5.  There  is  no  evidence  that  the  sympathetic  causes  inhibition  of  the 
sphincter  ;  it  causes,  on  the  contrary,  radial  shortening  of  a  portion  of  the 
iris  without  the  least  trace  of  relaxation  in  the  tone  of  the  sphincter  border. 

These  facts,  when  taken  in  conjunction  with  the  histological  details  to 
which  attention  has  already  been  directed,  warrant  the  assumption  that  the 
presence  of  a  definite  dilator  pupillse  muscle  has  been  established. 

The  posterior  limiting  lamella  has  been  the  subject  of  considerable  con- 
fusion in  the  description  of  the  posterior  border  of  the  iris-stroma  as  given 
by  various  authors  :  this  has  been  due  largely  to  the  uncertainty  as  to  the 

1  Langley  and  Anderson  :  On  the  Mechanism  of  the  Movements  of  the  Iris,  Journal 
of  Physiology,  vol.  xin.,  1892. 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL.  281 

relations  and  significance  of  the  stratum  of  spindle-cells  which  have  been 
already  described.  These  elements,  in  the  light  of  our  present  knowledge, 
must  be  regarded,  for  the  reasons  already  presented,  as  representing  a  con- 
tractile sheet,  which  is  accepted  by  many,  including  the  writer  as  the  ex- 
pression of  a  distinct  dilator  pupillse  muscle.  While  the  more  or  less  perfect 
layer  of  spindle-cells  is  intimately  united  with  the  structure  forming  the 
posterior  boundary  of  the  iris-stroma,  critical  examination  of  suitably  pre- 
pared sections  shows  that  the  spindle-cells  are  separated  from  the  posterior 
pigmented  zone  by  a  stratum  of  great  delicacy,— the  posterior  limiting 
lamella  proper. 

This  last-named  structure,  also  called  the  posterior  boundary  lamella,  the 
vitreous  lamella,  the  basal  membrane,  or  the  membrane  of  Bruch,  appears 
as  a  clear  lamella  of  great  tenuity,  its  maximum  thickness  not  exceeding 
.002  millimetre,  which  closely  adheres  to  the  outer  stratum  of  the  pig- 
mented retinal  zone  behind  and  is  intimately  related  to  the  sheet  of  spindle- 
cells  in  front.  The  almost  homogeneous  appearance  of  the  limiting  lamella 
under  moderate  amplification  gives  place  to  a  distinct,  though  delicate,  radial 
striation  when  examined  with  high  powers.  After  prolonged  maceration, 
according  to  Schwalbe,1  the  striated  lamella  breaks  up  into  fine  stiff  fibrillae, 
entirely  distinct  from  muscle-fibres.  The  intimate  union  of  the  lamella 
with  the  subjacent  pigmented  tissue  not  infrequently  results  in  the  adherence 
of  portions  of  the  outer  retinal  layer  to  the  lamella,  in  consequence  of  which 
oval  nuclei  and  particles  of  the  pigmented  cells  remain  attached  to  and 
seemingly  form  constituents  of  the  limiting  membrane. 

The  posterior  limiting  lamella  of  the  iris  may  probably  be  regarded  as 
the  anterior  continuation  of  the  membrane  of  Bruch,  as  found  in  the  ciliary 
zone  and  posterior  parts  of  the  choroidal  tract ;  in  constitution  it  is  the  result 
of  a  local  condensation  of  the  connective-tissue  stroma,  and  corresponds  to 
a  membrana  propria  in  other  locations.  The  recognition  by  Merkel  *  of  the 
posterior  limiting  lamella  as  distinct  from  the  superimposed  stratum  of 
spindle-cells  has  been  recently 3  reaffirmed  by  him ;  the  existence  of  the 
limiting  lamella  is  also  admitted  bySchafer;4  Retzius,5  on  the  contrary, 
seems  to  consider  the  dilator  stratum  and  the  membrane  of  Bruch  as 
inseparable. 

The  pigmented  layer  covering  the  posterior  surface  of  the  iris  as  far  as 
the  anterior  margin  of  the  pupillary  opening,  while  constituting  anatomically 
an  integral  part  of  the  diaphragm,  possesses  a  distinct  morphological  value, 
since  it  represents  the  remains  of  the  anterior  limits  of  the  ectodermic 
secondary  optic  vesicle. 

1  Schwalbe :  Anatomie  der  Sinnesorgane,  S.  206. 

2  Merkel :  Handbuch  der  topograph.  Anatomie,  Bd.  I.,  1887,  S.  256. 

3  Merkel:  Ergebnisse  der  Anatomie  u.  Entwickelungsges.,   Bd.  ill.,  1893  (Anmer- 
kung,  S.  287). 

*  Schafer:   Quain's  Anatomy,  10th  ed.,  vol.  in.,  Pt.  3,  1894,  p.  3 

5  Ketzius  :  Zur  Kenntniss  vom  Bau  der  Iris,  Biolog.  Untersuch.,  Neue  Folge,  v.,  1898. 


282  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

The  deeply  colored  posterior  zone  in  suitable  preparations  is  shown 
to  consist  of  two  distinct  layers, — an  outer,  composed  of  low,  irregular 
spindle-cells,  and  an  inner,  made  up  of  short,  cylindrical,  polygonal  ele- 
ments. These  layers  represent  the  outer  and  inner  lamella  of  the  embry- 
onal optic  cup,  and  at  their  extreme  anterior  limits,  corresponding  to  the 
front  margin  of  the  pupil,  they  become  continuous,  as  in  their  earlier  stage 
of  development. 

The  outer  layer  consists  of  a  single  row  of  deeply  piginented  fusiform 
elements,  about  .070  to  .080  millimetre  in  length  and  .009  to  .010  milli- 
metre in  thickness,  the  general  disposition  of  which  is  radial,  and  closely 

follows  the  minute  inequalities  of 

FIG-  45.  the  posterior  boundary  layer  with 

which  these  cells  are  almost  in- 
separably united.  The  unbroken 
investment  thus  formed  is  con- 
tinuous at  the  ciliary  border  of  the 
iris  with  the  low  columnar  or  poly- 
Radial  section  of  poster^'portion  of  human  hedral  pigmented  elements  cousti- 

iiif.    (Retzius.)— d,  dilator  fibres ;  dz,  r,  anterior  and     tilting  the  Ollter  lamella  of  the  pars 
posterior  layers  of  retinal  zone.    Magnified  about       . , . 

400 diameters.  ciliaris  retina?;  at  this  point  the 

radial  arrangement  of  the  spindle- 
cells  gives  place  to  a  more  circular  disposition.  At  the  pupillary  margin 
the  low,  irregular  cells  of  the  outer  layer  pass  directly  into  the  densely 
pigmented  elements  of  the  posterior  layer,  the  two  «trata  being  directly 
continuous. 

The  inner  layer  consists  of  cells  in  which  the  pigment  particles  are  so 
crowded  that  ordinarily  all  demarcation  between  the  individual  elements  is 
obscured,  the  layer  appearing  as  one  continuous  and  unbroken  pigmeuted 
zone.  Favorable  preparations,  as  afforded  by  albino  eyes,  or  after  the  re- 
moval of  the  color  by  means  of  bleaching  solutions,  conclusively  show  that 
this  layer  consists  of  short  columnar  or  polygonal  elements,  .030  to  .035 
millimetre  in  thickness,  the  boundaries  of  which  are  sharply  defined  when 
not  obscured  by  the  usual  dense  accumulation  of  pigment  granules.  The 
cells  likewise  possess  spherical  nuclei.  The  dimensions  of  these  elements, 
as  well  as  of  those  of  the  outer  layer,  are  evidently  largely  influenced  by 
the  contraction  and  dilation  of  the  pupil,  in  which  variations  the  cells  of 
the  pigment  layer  are  entirely  passive. 

At  the  ciliary  border  of  the  iris  the  cells  of  the  posterior  layer  are 
directly  continuous  with  the  elements  which  constitute  the  inner  stratum  of 
the  pars  ciliaris  retinae ;  the  pigment  particles  become  less  closely  aggregated 
and  gradually  abandon  the  innermost  portion  of  the  cells. 

The  inner  surface  of  the  pigment  layer  is  covered  by  an  extremely  deli- 
cate membrane  of  a  homogeneous  cuticular  character,  the  limitans  iridis. 
which,  as  suggested  by  Schwalbe,  is  probably  the  continuation  of  the  similar 
cuticular  investment  of  the  pars  ciliaris  retina?.  Its  great  delicacy,  and  the 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL.  283 

readiness  with  which  it  splits  and  becomes  separated  from  the  iris,  account 
for  the  quite  common  absence  of  the  membrane  in  usual  preparations. 

^  The  Blood-  Vessels  of  the  Irw.— The  arteries  supplying  the  iris  are  given 
off  from  the  anterior  border  of  the  circulus  arteriosus  iridis  major,  which,  as 
already  stated,  is  situated  within  the  ciliary  region  immediately  around  the 
outer  margin  of  the  iris.  The  iridial  branches  thus  originating  spring  from 
points  which  closely  correspond  to  the  attachments  of  the  ciliary  processes 
to  the  iris,  several  vessels,  however,  not  infrequently  arising  within  the  area 
belonging  to  a  single  process.  (Leber.) 

The  radially  disposed  arterial  stems  proceed  through  the  stroma  layer 
as  far  as  the  outer  border  of  the  pupillary  zone,  giving  off  branches  at  acute 
angles  during  their  course ;  on  reaching  the  sphincter  zone  they  freely  inos- 

Fio.  46. 


Arterial  supply  of  the  iris.  (Sappey.)— 1, 1,  long  ciliary  arteries  giving  off  their  superior  (2,  2)  and 
inferior  (3,  3)  branches  ;  4,  4,  recurrent  branches  to  choroid ;  5,  5, 6,  6,  anterior  ciliary  arteries ;  7,  net- 
work surrounding  pupil. 

culate  with  one  another  and  with  their  branches  to  form  a  second  annular 
anastomotic  circuit,  the  circulus  arteriosus  iridis  minor,  which  lies  near  the 
anterior  surface  of  the  iris  and,  during  foetal  life,  communicates  with  the 
vascular  pupillary  membrane. 

The  lesser  arterial  circle  distributes  branches  in  three  directions  :  an 
inner  set  continues  towards  the  pupillary  margin,  to  break  up  into  a  capillary 
net-work  within  the  sphincter  muscle,  a  posterior  group  contributes  a  capil- 
lary net-work  occupying  the  posterior  surface  of  the  stroma  layer,  and  a 
third  anterior  set  provides  a  capillary  reticulum  which  ramifies  within  the 
anterior  boundary  layer. 

These  capillaries  become  tributary  to  venous  radicles  which  pursue  a 
general  radial  course  directed  towards  the  ciliary  border ;  the  veins  thus 
arising  unite  at  acute  angles  to  form  vessels  which  are  continued  as  venous 
trunks  along  the  inner  surface  of  the  ciliary  muscle,  in  company  with  the 


284 


THE    MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


veins  from  the  ciliary  processes,  and  finally  empty  into  the  large  venae 
vorticosae. 

In  structure,  the  arteries  of  the  iris-stroma  are  characterized  by  a  rela- 
tive preponderance  of  elastic  tissue  and  meagreness  of  muscular  elements  j 
they  possess,  as  do  also  the  veins,  a  robust  adventitious  sheath,  contributed 
by  the  condensed  surrounding  iris-stroma,  in  which  stellate  cells  are  often 
conspicuous.  According  to  Michel,1  the  vessels  of  the  iris  are  provided 
with  an  additional  endothelial  sheath,  which  thus  partially  lines  the  peri- 
vascular  lymph-space  situated  within  the  thickened  adventitious  coat. 

PIG.  47. 


General  surface  view  of  the  distribution  of  the  motor  nerves  of  the  iris  of  rabbit  after  injection  of 
the  living  animal  with  methylene-blue.  (Hosch.)— As  a  matter  of  convenience,  many  of  the  minute 
twigs  within  the  sphincter  border  have  not  been  represented.  Magnified  70  diameters. 

The  Lymphatics  of  the  Iris. — Distinct  lymphatic  vessels  do  not  exist 
within  the  iris,  the  absorbent  system  being  represented  by  the  lymph-spaces 
within  the  stroma  layer. 

1  Michel :  Die  histologische  Structur  des  Irisstroma,  1875. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL. 


285 


These  interfascicular  lymphatic  clefts,  which  occur  in  such  profusion  in 
all  parts  of  the  iris  that  the  entire  stroma  layer  possesses  the  character  of  a 
sponge-like  tissue,  constitute  a  freely  intercommunicating  system  of  spaces. 
The  tissue-juices  contained  within  the  lymphatic  channels  are  carried 
towards  the  periphery  of  the  iris,  at  which  position  the  iridial  spaces,  in 
addition  to  communicating  with  the  interstitial  clefts  within  the  ciliary 
body,  probably  communicate  with  the  intra-pectinate  spaces  of  Fontana,  as 
maintained  by  Schwalbe. 

The  question  as  to  the  occurrence  of  direct  absorption  of  the  aqueous 
humor  through  the  anterior  surface  of  the  iris  has  been  both  affirmatively 
and  negatively  answered,  but  the  recent  careful  investigations  of  Tiicker- 
mann1  render  the  probability  of  such  absorption  very  questionable,  and,  at 
best,  only  to  the  very  limited  extent  to  which  it  takes  place  through  the 
posterior  surface  of  the  cornea.  In  both  situations  particles  within  the 
aqueous  humor  may  be  taken  up  by  the  protoplasm  of  the  endothelial  cells. 
The  loose  character  of  the  stroma  and  the  particularly  rich  vascular  supply 
fully  suffice  to  account  for  the  contents  of  the  lymph-spaces  of  the  iris 
without  assuming  a  par- 
ticipation of  the  fluids  Fjo-  48. 
within  the  anterior 
chamber ;  the  only  com- 
munication between  the 
latter  cavity  and  the 
iridial  channels  is  the 
indirect  path  by  way  of 
the  spaces  of  Fontana. 

The  Nerves  of  the 
Iris. — The  classic  paper 
of  Arnold,  together 
with  the  subsequent 
contributions  of  Faber, 
Pause,  Formad,  Meyer, 

Furst,  and  Eversbusch,  forms  the  basis  of  our  knowledge  concerning  the 
distribution  of  the  nerves  within  the  iris. 

The  nerve-trunks  supplying  the  iris  proceed  inward  from  the  annular 
intra-muscular  plexus,  the  orbiculus  gangliosus,  formed  by  the  ciliary  nerves 
within  the  muscle  of  accommodation.  The  nervous  stems,  which  are  at 
first  composed  principally  of  medullated  fibres  and  pursue  a  spiral  course, 
pass  into  the  ciliary  border  of  the  iris,  and  upon  entering  the  stroma  layer 
divide  into  branches  which  are  united  and  rearranged  to  constitute  plexuses 
of  various  character.  In  the  disposition  of  the  principal  nerve-trunks 
there  seems  to  be  no  close  correspondence  between  their  course  and  that  of 
the  blood-vessels.  , 

i  Tiickermann  :  Ueber  die  Vorgange  bei  der  Resorption  in  die  vordere  Kamu.er 
injizierter  kOrniger  Farbstoffe,  Archiv  f.  Ophthalmol.,  Bd.  xxxvm.,  1* 


6  b 

Surface  view  of  sphincter  muscle  of  iris  of  rabbit  after  gold  stain- 
ing. (Meyer.) — o,  plexus  of  pale  nerve-fibres  which  become  con- 
tinuous at  6  with  intermuscular  flbrillae.  High  amplification. 


286 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


According  to  Meyer,1  the  iridial  nerves  provide  nerve-endings  of  three 
kinds :  1,  motor  endings  within  the  muscular  tissue ;  2,  sensory  endings 
within  the  superficial  layers  of  the  stroma ;  3,  vaso-motor  endings  in  the 
walls  of  the  arteries  and  the  capillaries. 

The  fibres  destined  for  the  supply  of  the  muscular  tissue,  after  forming 


FIG.  49. 


—  d 


d 


Horizontal  section  of  rabbit's  iris  after  staining  with  gold,  showing  motor  nerve-endings  in  dilator 
layer.  (Retzius.)— d,  d,  muscle-fibres ;  n,  nerve  breaking  up  into  intermuscular  fibrillse.  Magnified 
about  950  diameters. 

a  coarse  mesh-work  as  they  journey  towards  the  pupillary  zone,  break  up 
into  smaller  non-medullated  twigs,  which  unite  at  the  outer  border  of  the 
sphincter  muscle  to  form  an  annular  pupillary  plexus.  Numerous  delicate 
non-medullated  fibres  pass  from  the  latter  into  the  muscular  tissue,  within 

FIG.  50. 


Surface  view  of  sensory  nerves  of  iris  of  rabbit  after  gold  staining.  (Meyer.) — ch,  choroid;  s, 
sphincter  pupillse ;  art,  circulus  arteriosus  iridis  major;  n,  spirally  coursing  nerves.  All  nerves  repre- 
sented are  medullated  ;  others  have  been  omitted.  Magnified  7  diameters. 

which  they  sooner  or  later  break  up  into  the  attenuated  ultimate  fibrillse 
for  the  supply  of  the  individual  muscle-cells  between  which  the  nerve- 

1  Meyer:  Die  Nervendigungen  in  der  Iris,  Archiv  f.  mik.  Anat.,  Bd.  xvn.,  1880. 


THE    MICROSCOPICAL    ANATOMY   OP   THE   EYEBALL.  287 

filaments  end.  The  ultimate  fibrillae  correspond  in  their  course  with  the 
general  disposition  of  the  muscular  tissue,  present  varicosities,  and  terminate 
in  free  endings  between  the  contractile  elements.  It  is  a  significant  fact  in 
connection  with  the  determination  of  the  character  of  the  spindle-cells  of 
the  posterior  lamella  of  the  iris-stroma  that  Retzius l  has  discovered  definite 
motor  nerve-endings  within  this  layer. 

The  sensory  nerves  of  the  iris  form  an  extensive  superficial  plexus  of 
largely  medullated  fibres  within  the  anterior  plane  of  the  stroma  layer. 
The  terminal  part  of  this  sensory  plexus  is  composed  of  non-medu Hated 
fibres  of  especial  delicacy,  which  lie  close  beneath  the  anterior  endothelium. 

The  vaso-motor  nerves  are  represented  by  the  delicate  bundles  of  non- 
medullated  fibres  which  accompany  the  blood-vessels  and  terminate  in  free 
endings  within  the  muscular  tunic  of  the  arteries,  or  in  pale  fibres  entwining 
the  capillaries. 

The  existence  of  ganglion-cells  within  the  iris  has  been  a  subject  of 
conflicting  opinion.  Among  those  who  have  described  nerve-cells  as  present 
in  this  situation  are  Arnold  and  Faber,  while  Pause,  Iwanoff,  Formad, 
Fiirst,  and  Schwalbe  have  denied  their  existence.  Meyer 2  records  having 
found  in  the  human  iris  small  multipolar  cells  which,  while  possessing 
apparently  no  connection  with  the  nerve-fibres,  strongly  resembled  in  their 
general  appearance  ganglion-cells.  Hosch,3  likewise,  describes  small  (.012 
to  .015  millimetre  in  diameter)  multipolar  elements  within  the  sphincter 
zone  of  the  human  iris,  which  not  only  are  identical  in  appearance  with 
ganglion-cells,  but  are  directly  connected  with  the  nerve-fibres.  These 
instances  of  the  direct  observation  of  the  nerve-cells,  it  will  be  noted,  per- 
tain to  human  tissue ;  both  Meyer  and  Hosch  agree  with  the  majority  of 
investigators  in  denying  the  presence  of  ganglion-cells  within  the  rabbit's 
iris,  which  has  been  the  usual  subject  of  observation.  While  admitting 
the  possible  presence  of  small  nerve-cells  along  the  course  of  the  fibres 
taking  part  in  the  formation  of  the  pupillary  plexus,  it  is  certain  that  their 
number  and  size  are  so  insignificant  that  the  usually  accepted  doctrine,  that 
the  iris  is  without  ganglion-cells,  may  be  still  regarded  as  true. 

THE   NERVOUS   TUNIC. 

The  nervous  tract  of  the  eyeball  consists  essentially  of  the  highly  dif- 
ferentiated expansion  of  the  optic  vesicle  and  its  cerebral  extension,  the 
direct  derivatives  of  the  neural  ectoderm,  which  form  the  retina  and  parts 
of  the  optic  nerve.  Regarded  in  the  light  of  the  newer  views  concerning 
the  retina,  as  based  upon  the  investigations  of  Tartuferi,  Golgi,  Cajal, 
Dogiel,  Retzius,  van  Gehuchten,  His,  and  others,  this  tunic  of  the  eyeball 
can  no  longer  be  placed  in  the  same  category  with  the  remaining  coats,  but 

1  Retzius  :  Zur  Kenntniss  zum  Bau  der  Iris,  Biolog.  Untersuch.,  Neue  Folge,  v.,  1893. 

2  Meyer  :  loc.  cit. 

•HoBch:    Ehrliclvs    Methylenblaumethode    und   ihre  Anwendung  auf 
Archiv  f.  Ophthal.,  Bd.  xxxvii.,  1891. 


288  THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 

must  be  conceded  a  morphological  position  of  far  greater  dignity  and  im- 
portance. The  retina  must  be  considered  as  a  true  nervous  centre  and  as  a 
peripherally  situated  portion  of  the  central  nervous  system  rather  than  as 
merely  the  complex  percipient  apparatus  of  light-stimulus. 

The  retinal  tract  in  its  entirety,  as  representing  the  structures  evolved 
from  the  optic  vesicle,  extends  from  the  entrance  of  the  optic  nerve  pos- 
teriorly as  far  forward  as  the  anterior  margin  of  the  pupillary  margin. 
This  extensive  sheet,  from  the  profound  variations  which  certain  of  its 
parts  undergo  both  in  structure  and  in  function,  becomes  differentiated 
into  two  sharply  contrasting  segments.  The  posterior  of  these,  extending 
from  the  optic  entrance  to  the  ora  serrata,  and  embracing  about  two-thirds 
of  the  eyeball,  constitutes  the  actively  functionating  nervous  centre,  the 
retina  proper  ;  the  anterior  segment  continues  from  the  ora  serrata  over  the 
posterior  surface  of  the  ciliary  body  and  the  iris  as  far  forward  as  the 
anterior  pupillary  margin,  and  is  known  under  the  names  of  pars  cilia  ris 
and  pars  iridica  retinae,  the  representatives  of  the  rudimentary  anterior 
portions  of  the  two  layers  of  the  optic  cup. 

The  retina  proper,  or  pars  optica  retinse,  consists  of  an  inner  and  an 
outer  lamina,  which  correspond  to  the  very  unequally  developed  inner  and 
outer  layers  of  the  optic  vesicle.  The  outer  lamina  includes  the  pigment 
layer  alone,  while  to  the  inner  lamina  belong  all  the  remaining  layers  of 
the  fully  formed  retina.  The  inner  lamina  permits  of  further  subdivision 
of  its  structures,  as  suggested  by  Schwalbe,  into  the  neuro-epithelial  and  the 
cerebral  layer. 

The  relations  of  these  divisions  to  the  individual  retinal  layers  may  be 
expressed  as  follows : 

I.  OUTER  LAYER  OF  (  pi  ,  j  A    pi  } 

OPTIC  VESICLE.   I  / 


Layer  of  rods  and  cones. 

Layer  of  bodies  of  visual  cells  (outer 

nuclear  layer). 
External  plexiform  layer  (outer  re-  -, 

ticular  layer). 


•  B.  Neuro-epithelial  layer. 


II.  INNER  LAYER  OF 

OPTIC  VESICLE.  "    Layer  of  bipolar  cells  (inner  nuclear 

Internal  plexiform  layer  (inner  re-  f  C  Cerebral  layer- 

ticular  layer). 
Layer  of  ganglion-cells. 
Layer  of  optic  nerve-fibres. 

-  The  retina,  in  common  with  other  parts  of  the  central  nervous  system, 
consists  of  two  varieties  of  elements,  the  nervous  and  the  sustentacular. 
The  latter  constitute  the  supporting  neuroglia  which  appears  as  a  reticular 
framework  composed  of  columnar  segments,  the  long  fibres  of  Miiller, 
which  extend  the  entire  thickness  of  the  retina,  and  by  apposition  of  their 
expanded  outer  and  inner  extremities  produce  the  seemingly  continuous 
structures  known  as  the  external  and  internal  limiting  membranes. 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL.  289 

While  the  advances  in  the  comprehension  of  the  structural  details  of  the 
retina  will  ever  be  deeply  indebted  to  the  classic  investigations  of  Heinrich 
Miiller,  Max  Sclmltze,  Kolliker,  Krause,  W.  Miiller,  Schwalbe,  and  others, 
the  close  of  the  last  decade  left  still  much  uncertainty  concerning  the  exact 
mode  of  transmission  of  the  light-stimuli  from  the  percipient  elements  to 
the  nervous  centres. 

The  almost  simultaneous  introduction  of  the  chrome-silver  impregnation 
method  of  Golgi,1  as  presented  in  his  later  and  more  important  communi- 
cation, and  the  methylene-blue  staining  of  Ehrlich,2  supplied  a  marked 
impetus  to  renewed  investigations  of  the  nervous  system  which  have  borne 
fruit  in  the  many  important  advances  made  in  our  knowledge  of  the  form 
and  relations  of  the  nervous  elements.  As  was  to  be  expected,  the  capa- 
bilities of  these  recent  methods  were  soon  taken  advantage  of  in  renewed 
study  of  the  retina,  the  intricate  structure  of  which  has  always  remained 
an  inviting  field  for  the  foremost  histologists. 

A  new  epoch  in  the  anatomy  of  the  retina  was  inaugurated  by  the  ap- 
pearance of  Tartuferi's3  paper,  in  1887,  recording  the  results  of  his  appli- 
cation of  the  Golgi  silver  staining  to  that  structure.  These  investigations 
were  immediately  followed  by  those  of  Dogiel,4  in  which  the  discoveries 
yielded  by  the  use  of  the  methylene-blue  stainings  were  communicated. 
The  first  of  the  brilliant  researches  of  Ramon  y  Cajal 5  was  announced 
almost  at  the  same  time.  The  years  intervening  since  the  publication  of 
these  important  papers  have  witnessed  great  activity  in  the  investigation 
of  the  nervous  elements  of  the  retina,  as  evidenced  by  the  appearance  of 
an  extended  series  of  communications  by  the  authors  just  named,  as  well 
as  of  the  contributions  by  Baquis,6  Fromaget,7  Retzius,8  and  others. 

The  interest  and  importance  of  these  observations,  particularly  those  of 
Golgi  and  of  Cajal,  lay  not  only  in  the  decided  advance  in  the  more  accu- 
rate knowledge  of  the  relations  of  the  nervous  elements  of  the  retina,  but 
in  the  establishment  of  the  broader  theorem  of  the  independence  of  nerve- 
cells  and  their  extensions  as  axis-cylinders  in  general.  Upon  the  evi- 
dence advanced  by  the  labors  of  these  investigators,  as  well  as  by  their 

I  Golgi :  Sulla*  fina  anatomia  degli  organ!  centrali  del  sistema  nervosa,  1886,  which  was 
preceded  ten  years  by  his  first  paper,  Sulla  fina  structure  dei  bulbi  olfactorii,  1875. 

II  Ehrlich  :  Ueber  die  Methylenblaureaktion  der  lebenden  Nervensubstanz,  Deutsch. 
med.  Wochenschr.,  No.  4,  1886. 

3  Tartuferi :  Sulla  anatomia  della  retina,  Archivio  per  le  scienze  mediche,  vol.  XI., 
1887;  and  Internat.  Monatsschrift  fur  Anat.  u.  Physiolog.,  Bd.  IV.,  1887. 

4  Dogiel :  Ueber  das  Verhalten  der  nervosen  Elemente  in  der  Retina  der  Ganoiden, 
Keptilien,  Vogel  und  Saugethiere,  Anatom.  Anzeiger,  Bd.  in.,  1888. 

5  Kamon  y  Cajal :  Estructura  de  la  retina  de  las  aves,  Kevista  trim,  de  Histologia 
normal,  1888. 

6  Baquis  :  Sulla  retina  della  faina,  Anatom.  Anzeiger,  Bd.  v.,  1890. 

7  Fromaget :  Contribution  a  1'etude  de  1'histologie  dela  retina,  Archiv.d'ophthalmol., 
t.  xii.,  1892. 

8  Retzius :  Ueber  die  neueren  Prinzipien  in  der  Lehre  von  den  Einrichtungeu 
sensiblen  Nervensystems,  Biolog.  Untersuch.,  Neue  Folge,  IV.,  1892. 

VOL.  I.— 19 


290 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


own  supplementary  confirmatory  studies,  many  of  the  foremost  authorities, 
including  Kolliker,  His,  v.  Lenbossek,  Retzius,  and  others,  accept  the 
view  that  every  nerve-cell  exists  as  an  independent  individual  element,  and 
neither  ends  in  net-works  formed  by  its  own  ramifications  joining  with  those 
of  other  cells,  nor  communicates  by  actual  union  with  other  cells.  Direct 
anatomical  continuity  between  nervous  elements,  therefore,  is  no  longer 
accepted,  contiguity,  as  represented  by  approximation  and  contact,  being 
regarded  as  the  closest  relation  into  which  such  elements  enter.  Although 
accepted  by  the  majority  of  anatomists  at  present,  it  should  be  mentioned 
that  direct  contiguity  between  certain  elements  is  still  maintained  by  some 
authorities,  among  whom  Dogiel,  Waldeyer,  and  Merkel  are  conspicuous. 

FIG.  51. 


Diagram  illustrating  the  relation  of  the  retinal  elements.  (Kallius.)— A,  layer  of  rods  and  cones; 
B,  limitans  externa ;  C,  outer  nuclear  layer  (bodies  of  visual  cells) ;  D,  Henle's  external  fibrous  layer 
(composed  principally  of  rod-fibres) ;  E,  outer  plexiform  layer ;  F,  inner  nuclear  layer  (rod  and  cone 
bipolars) ;  G,  inner  plexiform  layer ;  H,  layer  of  large  ganglion-cells ;  J,  fibre-layer ;  K,  limitans  in- 
terna ;  a,  supporting  fibre  of  Miiller ;  6,  c,  rod  and.  cone  visual  cells ;  d,  bipolars  belonging  to  rod-cells ; 
e-i,  bipolars  belonging  to  cone-cells;  k-m,  horizontal  nerve-cells;  n,  centrifugal  nerve  fibre;  o-t,  gan- 
glion-cells connected  with  optic  fibres;  a-e,  amacrines  arranged,  in  layers;  £-#,  diffuse  amacrines; 
ij,  nervous  amacrine. 

The   fanciful   conceptions  of   the   structure  of   the   retina   advanced    by 
Johnson x  can  hardly  be  considered  seriously. 

The  application  of  the  Golgi  silver  staining,  particularly  in  the  modified 
methods  introduced  by  Cajal,  has  been  of  especial  importance  in  revealing 
the  nature  and  relations  of  many  retinal  elements  which  before  were 
obscure.  Before  proceeding  to  the  detailed  consideration  of  the  individual 
elements  composing  the  retina,  a  general  sketch  of  the  anatomy  of  this 
complicated  nervous  sheet,  based  upon  the  newer  accepted  views,  will  be  of 
advantage.  This  purpose  will  be  served  by  a  study  of  the  accompanying 
diagram  illustrating  the  present  views  concerning  the  structure  of  the  retina 

1  Johnson :  Observations  on  the  Macula  Lutea,  Archives  of  Ophthalmology,  vol. 
xxiv.  3,  1895;  ibid.,  vol.  xxv.  1,  1896. 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL.  291 

which  has  been  prepared  by  Kallius l  in  accordance  with  the  observations 
of  Ramon  y  Cajal. 

Passing  from  the  outer  or  choroidal  surface  towards  the  inner  boundary 
of  the  retina,  ignoring  the  pigment  layer,  the  first  two  strata,  which  consti- 
tute the  neuro-epithelial  layer,  are  occupied  by  the  external  and  internal 
parts  of  the  visual  cells.  The  former  comprises  the  layer  of  rods  and 
cones,  the  latter  the  external  nuclear  layer,  of  the  older  nomenclature.  It 
will  be  noticed  that  the  central  or  inner  terminations  of  the  visual  cells  are 
not  limited  to  the  neuro-epithelial  layer,  but  pass  for  a  short  distance 
within  the  subjacent  outer  plexiform  or  reticular  stratum. 

Within  the  latter  zone  the  two  forms  of  visual  cells  end  in  a  somewhat 
different  manner ;  the  rod-cells  terminate  in  knob-like  expansions  which  are 
embraced  by  the  arborescent  processes  of  the  bipolar  nerve-cells  of  the 
outer  ganglionic  (inner  nuclear)  layer.  The  centrally  directed  axis-cylin- 
der processes  of  these  bipolar  cells  pass  as  far  as  the  inner  ganglionic 
layer,  where  they  end  in  arborizations  surrounding  the  large  ganglion-cells. 

The  inner  terminations  of  the  cone-cells  within  the  outer  plexiform 
layer  are  more  expanded  than  those  of  the  rods,  and  also  give  off  a  few  short 
lateral  processes.  The  bases  of  the  cone-cells  come  into  intimate  relation 
with  the  ramifications  which  proceed  from  the  bipolar  cells  of  the  outer 
ganglionic  layer. 

It  will  be  noted  that  the  centrally  coursing  axis-cylinder  processes  of  the 
last-mentioned  cells  are  confined  to  the  inner  plexiform  zone,  but  that  all 
do  not  reach  the  same  level,  some  breaking  up  into  the  terminal  ramifica- 
tions immediately  after  entering  the  zone,  while  others  pass  to  various  levels 
within  the  same  layer.  The  cone  ganglion-cells,  therefore,  map  out  a  series 
of  secondary  zones  of  progressively  deeper  level.  The  central  expansions 
of  the  bipolar  ganglion-cells  of  the  cones  come  into  close  relation  with  the 
arborizations  of  the  large  nerve-cells  of  the  inner  ganglion  layer.  The 
cone-  and  the  rod-cells  are,  therefore,  in  relation  with  bipolar  ganglion- 
cells  which  are  of  two  kinds,  each  form  of  visual  cell  having  relations  with 
a  distinct  nervous  element. 

The  axis-cylinders  of  the  large  cells  of  the  inner  ganglion  layer  are 
continued  brainward  as  the  fibres  of  the  optic  nerve.  The  presence,  how- 
ever, of  "  centrifugal"  fibres  which  extend  outward  as  far  as  the  outer 
boundary  of  the  inner  plexiform  layer  has  been  demonstrated. 

In  addition  to  the  foregoing  relations  of  the  more  important  retinal  ele- 
ments which  thus  constitute  a  direct  pathway  for  the  conveyance  of  im- 
pressions received  by  the  visual  cells,  the  recent  studies  of  the  retina  have 
discovered  the  existence  of  numerous  additional  elements  which  probably 
establish  paths  for  the  indirect  transmission  of  impressions. 

The  elements  in  question  include  the  horizontal  or  basal  cells  found  in  the 
external  plexiform  layer  the  processes  of  which  ramify  about  the  extremi- 

1  Kallius  :  Ergebnisse  der  Anatomic  und  Entwickelungsgesch.,  Bd.  n.,  1892,  S.  251. 


292 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


ties  of  the  visual  cells,  sometimes,  however,  extending  centrally  as  far  as 
the  internal  plexiform  zone.  Additional  elements  are  encountered  within 
the  inner  plexiform  layer,  as  the  various  spongioblasts  or  "  amacrine  cells" 
of  Cajal.  Three  varieties  of  these  elements  have  been  described  :  a,  those 
which,  as  the  "  layered"  spongioblasts,  extend  to  various  depths  within  the 
plexiform  layer,  and  there  end  by  dividing  into  ramifications  which  possess 
a  generally  horizontal  disposition  ;  6,  those  which,  as  the  "  diffuse"  spongio- 
blasts, possess  large  bodies  and  send  off  more  or  less  elaborate  arborizations 
into  the  plexiform,  and  partly  also  the  inner  ganglion,  layer ;  and,  c,  those 
which,  as  the  "  nervous"  spongioblasts,  possess  a  recognizable  axis-cylinder 
extending  into  the  fibre-layer. 

The  sustentacular  elements  of  the  retina  are  contributed  by  the  compli- 
cated and  irregular  fibres  of  Miiller,  which  pass  from  the  layer  of  rods  and 
cones  through  the  retinal  strata  to  form  the  innermost  boundary  of  the 
nervous  sheet ;  in  their  course  they  enter  the  several  strata  and  give  off 
secondary  ramifications  which  supply  the  tissue  supporting  the  cells  of  the 
individual  retinal  layers. 

The  general  arrangement  and  relations  of  the  retinal  elements  having 
been  broadly  sketched,  a  more  detailed  consideration  of  the  individual 
layers  and  their  component  elements  claims  further  attention. 

The  Pigment  Layer. — Reference  to  the  section  on  the  Development 
of  the  Eye  will  show  that  the  pigment  layer  is  the  direct  and  sole  repre- 
sentative of  the  outer  lamella  of  the  optic  cup,  and  therefore  corresponds 

to  that  segment  of  the  primary  optic  vesicle 
which  has  not  suffered  invagination.  The  outer 
layer  very  early  evinces  the  disposition  to  remain 
thin  and  to  become  pigmented,  the  accumulation 
of  colored  particles  first  taking  place  near  the 
anterior  lip  of  the  optic  cup  and  extending 
towards  the  posterior  pole. 

The  layer  of  retinal  pigment  in  the  fully  de- 
veloped eye  consists  of  a  single  stratum  of  nucle- 
ated polygonal  cells  the  protoplasm  of  which, 
when  seen  in  surface  views,  is  almost  completely 
filled  with  colored  particles,  with  the  exception 
of  the  nuclear  areas.  The  cells  are  usually  hex- 
agonal in  outline,  but  numerous  exceptions  are 

observed  in  which  the  number  of  sides  is  reduced  to  four  or  five ;  on  the 
other  hand,  as  shown  by  Boden  and  Sprawson,1  they  may  be  increased  to 
seven  or  even  to  ten.  The  ordinary  diameter  of  the  pigment-cells  varies 
between  .012  and  .018  millimetre,  here  and  there  groups  of  smaller  cells 
being  interspersed,  or,  on  the  contrary,  isolated  cells  of  unusual  size  being 


FIG.  52. 


Surface  view  of  pigmented 
retinal  epithelium.  Magnified 
500  diameters. 


1  Boden  and  Sprawson  :  The  Pigment-Cells  of  the  Retina,  Quart.  Jour,  of  Mic.  Science, 
vol.  xxxiu.,  1892. 


THE   MICROSCOPICAL   ANATOMY   OP  THE   EYEBALL. 

surrounded  by  smaller  ones.  In  the  immediate  vicinity  of  the  ora  serrata 
the  pigmented  elements  are  of  exceptionally  large  size  throughout  a  zone 
1  to  1.5  millimetres  in  breadth,  in  which,  as  pointed  out  by  Kuhnt1  the 
cells  are  so  much  more  deeply  colored  than  those  of  the  centrally  situated 
areas  that  a  difference  in  the  general  tint  of  the  peripheral  zone  is  noted 
with  the  unaided  eye.  The  large  cells  of  this  deeply  colored  zone  pos- 
sess nearly  always  more  than  six  sides,  and  very  frequently  have  more  than 
a  single  nucleus. 

Surface  views  give  the  impression  that  the  entire  cell,  with  the  exception 
of  the  nucleus,  contains  pigment ;  that  such,  however,  is  not  the  case  is 
shown  by  vertical  sections,  in  which  it  is  seen  that  each  cell  consists  of  three 
parts, — an  outer  zone,  containing  the  nucleus,  wanting  in  pigment  and  pre- 
senting a  smooth  surface  towards  the  choroid  ;  a  middle  zone,  deeply  pig- 
mented and  sometimes  known  as  the  "  base"  of  the  cell ;  and  an  irregular 
inner  zone,  consisting  of  indefinite  protoplasmic  processes  which  extend 
betweeu  the  outer  segments  of  the  visual  cells,  whose  ends  are  thus  received 
within  the  pigment  layer.  According  to  Kuhnt2  and  others,  the  outer  sur- 
face of  the  pigment-cells  possesses  a  delicate  keratose  cuticle  which  also 
invests  the  sides  of  the  closely  approximated  cells,  and  appears  in  surface 
views  as  the  clear  boundary  lines  defining  the  deeply  colored  hexagonal  areas. 

In  many  of  the  lower  animals,  as  certain  fishes,  amphibians,  and  birds, 
the  outer  zone  of  the  pigment-cells  contains  foreign,  and  sometimes  brightly 
colored,  substances,  of  which  aleuronoid 
particles  and  fat  droplets  are  the  most 
frequent.  Angelucci,3  who  has  carefully 
studied  these  inclusions,  states  that  the 
aleuronoid  particles  are  absent  in  the  ret- 
inal cells  of  mammals,  but  that  excep- 
tionally, as  in  the  rabbit  and  the  ox,  oil 
droplets  may  exist.  None  of  these  foreign 
particles,  however,  have  been  observed 
within  the  human  retina.  The  investigations 
of  Kerschbaumer 4  show  that  in  many  cases 
decrease  of  the  retinal  pigmentation,  par- 
ticularly within  the  extreme  anterior  zone 
in  the  vicinity  of  the  ora  serrata,  accompa- 
nies other  senile  changes  within  the  eye. 
The  source  of  the  color  within  the  retinal 
epithelium  is  the  blood,  from  which  the  substances,  in  a  condition  of  solu- 

1  Kuhnt :    Grosszellenzone  im  Pigmentepithel  des   Menschen,  Bericht  uber  d.  12. 
Versamml.  d.  ophthalmol.  Gesellsch.  in  Heidelberg,  1879. 

2  Kuhnt :  loc.  cit. 

»  Angelucci :    Histologische  Untersuchungen  uber  das  retinale  Pigmentepitl 
Wirbelthiere,  Archiv  f.  Anat.  u.  Physiologic,  1878. 

« Kerschbaumer:  tfber  Altersveranderungen  der  Uvea,  Archiv  f   Opht 
xxxviii.,  1892. 


Fio.  53. 


Surface  view  of  pigmented  retinal 
epithelium  from  an  aged  subject.  The 
cells  exhibit  loss  of  pigmentation  and 
multiple  nuclei.  Magnified  500  diam- 
eters. 


294  THE   MICROSCOPICAL    ANATOMY   OF   THE    EYEBALL. 

tion,  are  deposited  within  the  protoplasm  of  the  cells ;  in  this  connection, 
as  pointed  out  by  Scherl,1  the  fact  is  suggestive  that  the  first  deposit  of 
retinal  pigment  occurs  in  close  relation  to  the  earliest  vascular  supply  of  the 
interior  of  the  primitive  organ.  While  Scherl  failed  to  find  iron  in  the 
coloring-matter,  Mays,2  by  the  use  of  a  mixture  of  a  ten  per  cent,  solution 
of  hydrochloric  acid  and  a  five  per  cent,  solution  of  potassium  sulpho- 
cyanide,  succeeded  in  obtaining  a  characteristic  iron  reaction. 

The  pigment  granules  occur  in  the  form  of  minute  crystals,  their  long 
axes  being  placed  generally  at  right  angles  to  the  retinal  free  surface.  The 
distribution  of  the  pigment  within  the  protoplasm  during  life,  moreover, 
is  by  no  means  constant,  but  varies  greatly  according  to  the  intensity  of 
light-stimulus  to  which  the  tissue  is  subjected. 

The  researches  of  Kiiline  and  Sewall,3  Angelucci,4  Englemann,5  Grade- 
nigo,6  v.  Genderen-Stort,7  Fick,8  and  others  leave  little  doubt  that  the  ele- 
ments of  the  retinal  epithelium  undergo  marked  change  during  exposure 
to  light.  Under  such  stimulus  the  pigment  particles  advance  along  the 
protoplasmic  extensions  of  the  epithelial  cells  between  the  rods  and  cones 
until  the  outer  segments  of  the  visual  cells  are  buried  within  the  pigment. 
After  prolonged  exclusion  of  light,  on  the  contrary,  the  pigment  particles 
are  withdrawn  from  the  processes  and  become  once  more  aggregated  within 
the  basal  portion  of  the  cells. 

It  has  been  shown  that  the  migration  of  the  pigment  particles  is  not 
effected  by  the  protrusion  or  retraction  of  the  protoplasmic  processes  them- 
selves, but  is  due  rather  to  the  displacement  of  the  particles  by  currents 
streaming  within  the  cell  protoplasm,  somewhat  similar  to  the  transporta- 
tion of  granules  within  the  pseudopodia  of  the  amoeba. 

In  addition  to  the  migration  of  the  pigment  particles  along  the  proto- 
plasmic extensions  of  the  epithelium,  Englemann  and  v.  Genderen-Stort 
observed  that  conspicuous  changes  in  the  elements  of  the  percipient  layer 
also  marked  the  effect  of  light-stimulus.  These  alterations  consist  particu- 
larly in  the  shortening  of  the  inner  cone-segment  so  that  the  entire  cone 

1  Scherl :  Einige  Untersuchungen   liber  das  Pigment  des  Auges,  Archiv  f.  Ophthal- 
mol.,  Bd.  xxxix.,  1893. 

2  Mays:  Ueber  den  Eisengehalt  des  Fuscins,  Archiv  f.  Ophthalmol.,  Bd.  xxxix., 
1893. 

8  Ktihne  and  Sewall :  On  the  Physiology  of  the  Retinal  Epithelium,  Journal  of 
Physiology,  vol.  m.,  1879. 

4  Angelucci :  De  Faction  de  la  lumiere  et  des  couleurs  sur  1 'epithelium  retinum,  Soc. 
de  Med.  de  Gand,  t.  LX.,  1882. 

5  Englemann  :  Ueber  Bewegungen  der  Zapfen  und  Pigmentzellen  unter  dem  Einfluss 
des  Lichtes  und  des  Nervensystems,  Pfluger's  Archiv,  Bd.  xxxvi. ,  1885. 

8  Gradenigo :  Ueber  den  Einfluss  des  Lichtes  und  der  Warme  auf  die  Retina  des 
Frosches,  AHgemein.  Wiener  med.  Zeitg.,  No.  28,  1885. 

7  v.  Genderen-Stort :    Ueber   Form-   und   Ortsveranderungen  der   Netzhautelemente 
unter  Einfluss  vom  Licht  und  Dunkel,  Archiv  f.  Ophthalmol.,  Bd.  xxxin.,  1888. 

8  Fick :  Untersuchungen  iiber  die  Pigmentwanderung  in  der  Netzhaut  des  Frosches, 
Archiv  f.  Ophthalmol.,  Bd.  xxxvn.,  1891. 


THE    MICROSCOPICAL   ANATOMY    OF   THE   EYEBALL.  295 

lies  nearer  to  the  external  limiting  membrane,  the  shortening  depending 
upon  the  contraction  of  the  protoplasmic  part  of  the  cone  which  connects 
the  cone-granule  with  the  ellipsoid. 

Associated  with  the  foregoing  alterations  in  the  position  of  the  pigment 
and  the  relations  between  the  percipient  elements  and  the  retinal  epithe- 
lium, a  variation  is  to  be  noted  in  the  intimacy  of  attachment  between 
the  pigment  layer  and  the  remaining  portions  of  the  retina.  With  the 
withdrawal  of  the  pigment  from  between  the  visual  cells,  the  attachment 
between  the  percipient  and  the  pigmented  layer  of  the  retina  seemingly 
becomes  weakened  and  the  tendency  for  the  two  lamellse  to  separate  more 
marked.  In  eyes  subjected  to  the  action  of  light  just  before  death,  and 

FIG.  54. 


Sections  of  frog's  retina,  showing  the  effect  of  the  action  of  light  and  of  darkness  upon  the  pig- 
ment-cells and  upon  the  rods  and  cones,  (v.  Genderen-Stort.)  Nitric  acid  preparations.  Magnified  600 
diameters.— A.  Pigment  and  neuro-epithelium  after  remaining  forty-eight  hours  in  absolute  darkness. 
The  pigment  is  collected  within  the  outer  nucleated  part  of  the  cells ;  the  cones  are  greatly  elongated. 
B.  Pigment  and  neuro-epithelium  after  live  hours'  exposure  to  daylight.  The  pigment  has  migrated 
nearly  to  the  bases  of  the  rods,  and  the  cones  have  retracted  almost  to  the  outer  limiting  membrane. 

rapidly  fixed,  the  pigment  and  remaining  retinal  strata  are  intimately 
attached,  while  in  eyes  treated  in  an  identical  manner  after  death,  but 
protected  from  light  before,  the  connection  between  the  pigment  and  other 
parts  of  the  nervous  coat  is  but  slight ;  the  retina,  when  removed  from 
eyes  so  treated,  is  but  imperfectly  covered  by  pigmented  cells,  since  the 
majority  of  these  remain  adherent  to  the  choroid. 

The  primary  cleft  between  the  outer  and  inner  lamellae  of  the  embryonic 
optic  vesicle  does  not  undergo  complete  obliteration,  but  is  represented  by 
the  interspaces  between  the  rods  and  the  cones  and  the  processes  extended 
from  the  pigmented  cells.  These  intervals  are  occupied  by  a  clear,  prol>a- 
bly  fluid,  substance  (Schwalbe)  which  may  be  regarded  as  a  modified  lymph, 


296 


THE    MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 


since  substances  injected  beneath  the  pial  sheath  of  the  optic  nerve  some- 
times find  their  passage  into  the  space  between  the  pigment  layer  and  the 
rods  and  cones.  This  fact  is  regarded  by  Schwalbe  as  positive  evidence  of 
the  communication  between  the  subpigmentary  space  in  question  and  the 
central  lymph-tracts,  a  relation  partially  anticipated  by  the  early  continuity 


Sections  of  retinae,  showing  the  effect  of  exposure  to  light  and  darkness  on  the  pigment  and  cones, 
(v.  Geuderen-Stort.)  Nitric  acid  preparations.  Magnified  870  diameters.—  A,  Cones  from  the  retina  of 
pig  kept  in  absolute  darkness  for  twenty-four  hours.  B,  Cones  from  the  retina  of  pig  exposed  to  light 
for  two  hours.  C,  Cones  from  human  retina  kept  in  darkness  for  fourteen  hours  before  death. 

between  the  optic  vesicle  and  the  brain-cavities.  GifFord,1  likewise,  finds  a 
well-developed  lymph-space  between  the  retinal  pigment  and  the  rods  and 
cones. 

The  Layer  of  Neuro- Epithelium. — The  neuro-epithelial  layer  comprises 
the  visual  cells  in  their*entirety,  and  hence  includes  two  zones  frequently 
described  as  independent  strata  of  the  retina,  the  layer  of  rods  and  cones 
and  the  outer  nuclear  layer.  Since  the  former  of  these  represents  the 
highly  specialized  outer  portion  and  the  latter  the  bodies  of  the  visual 
cells,  convenience  alone  justifies  the  retention  of  their  recognition  as  dis- 
tinct strata,  both  belonging  to  the  layer  constituted  by  the  light-stimulus- 
perceiving  elements. 

The  layer  of  rods  and  cones,  about  .060  millimetre  in  thickness,  there- 
fore embraces  the  outer  parts  of  the  two  kinds  of  visual  cells.  Although 
differing  in  their  details,  the  rods  and  cones  possess  many  points  in 
common.  Each  consists  of  an  inner  and  an  outer  segment,  of  which  the 
former  is  the  greater  in  diameter,  especially  in  the  cones.  The  position  of 
juncture  between  the  segments  in  general  corresponds  about  to  the  middle 
of  the  entire  stratum,  although  the  outer  portions  of  the  rods,  which  con- 
stitute about  half  of  the  entire  length  of  these  structures,  embrace  a  greater 


'Gifford:    Weitere  Versuche  uber  die  Lymphstrome  und  Lymphwege  des  Auges, 
Archiv  f.  Augenheilkunde,  Bd.  xxvi.,  1893. 


THE    MICROSCOPICAL    ANATOMY    OF   THE   EYEBALL. 


297 


proportion  of  the  depth  of  the  layer  than  do  the  corresponding  parts  of  the 
cones,  which  are  usually  much  shorter  and  do  not  reach  the  outer  limits  of 
the  zone.  It  has  been  maintained  by  v.  Genderen-Stort,1  however,  that 
during  life  the  junction  between  the  outer  and  inner  segments  of  the  cones 


Outer  layers  of  frog's  retina,  showing  the  effect  of  exposure  to  light  for  varying  periods  on  the  form 
and  position  of  the  elements,  (v.  Genderen-Stort.)— .4,  after  six  minutes'  exposure  to  diffuse  light;  B, 
after  thirty  minutes'  exposure  to  weak  diffuse  light ;  C,  after  thirty  minutes'  exposure  to  strong  diffuse 
light. 

lies  well  outward  between  the  outer  segments  of  the  rods,  and,  therefore, 
farther  from  the  membrana  limitans  externa  than  is  generally  pictured. 

The  outer  segments  of  the  rods  are  nearly  of  the  same  size  as  the  inner, 
while  the  outer  tapering  portions  of  the  cones  are  much  smaller  than  the 


Outer  layers  of  frog's  retina,  showing  the  effect  of  darkness  on  the  position  and  form  of  retinal 
elements,  (v.  Genderen-Stort.) — A,  after  one  hour  in  darkness ;  B,  after  two  hours  in  darkness ;  C,  after 
four  hours  in  darkness.  Maximum  retraction  of  pigment  into  bases  of  pigmented  cells  and  about  the 
apices  of  the  cone-pyramids. 

expanded  conical  inner  divisions.  The  outer  segments  of  both  rods  and 
cones  exhibit  well-marked  differences  from  the  inner  segments  in  their 
chemical  and  optical  properties,  as  shown  by  their  behavior  towards  stains 
and  refracting  powers.  The  inner  divisions  stain  readily  with  carmine, 
hsematoxylin,  iodine,  and  other  coloring  solutions,  while  the  outer  segments 
remain  unaffected.  The  latter  are  doubly  refracting,  the  inner  singly  re- 
fracting, in  their  action  upon  light. 

Additional  structural  peculiarities,  presently  to  be  described,  emphasize 
the  distinction  between  the  outer  and  the  inner  segments.  The  early  in- 
vestigations of  Heinrich  Muller,  Kolliker,  Max  Schultze,  and  others  during 
the  two  decades  following  the  middle  of  the  present  century  laid  the  founda- 
tion of  our  present  knowledge  of  the  structure  of  the  retina;  and,  while 
the  results  of  these  achievements  have  become  now  so  much  a  part  of 
the  common  possession  of  anatomy  that  a  detailed  reference  to  the  many 


1  v.  Genderen  Stort :  loc.  cit. 


298 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


FIG.  58. 


individual  papers  contributed  by  these  writers  seems  no  longer  necessary, 
every  one  engaged  in  a  study  of  the  retina  will  be  glad  to  acknowledge 
the  deep  obligations  of  science  to  the  indefatigable  industry  of  these 
histologists. 

The  rods  of  the  human  retina  appear  of  an  elongated,  cylindrical  form, 
about  .060  millimetre  in  length  and  .002  millimetre  in  diameter,  and  con- 
sist of  an  outer  and  an  inner  di- 
vision which  share  almost  equally 
the  entire  length  of  the  cylinder. 

The  outer  rod-segment,  as  usu- 
ally observed,  possesses  a  uniform 
diameter  and  appears  as  a  homo- 
geneous, highly  refracting  cylin- 
der. Its  affinity  for  the  ordinary 
stains  is  so  slight  that  it  remains 
colorless  after  the  application  of 
carmine  or  hsematoxylin.  In 
many  respects  this  part  of  the  rod 
must  be  regarded  as  corresponding 
in  its  nature  to  a  cuticular  forma- 
tion which  rests  upon  the  proto- 
plasmic portion  of  the  visual  cell 
represented  by  the  inner  segment. 
High  amplification  in  favorable 
preparations  demonstrates  a  fine 
longitudinal  striation  which  has 
been  attributed  to  the  presence  of 
delicate  canals.  Max  Schultze 
describes  additional  transverse 
markings  ;  these  he  regards  as  in- 
dications of  the  presence  of  minute 
disks,  approximately  .0006  milli- 
metre in  thickness,  of  which  the 
outer  segment  is  made  up.  Pro- 
longed treatment  with  salt  and 
alkaline  solutions  emphasizes  the 

transverse  markings  probably  by  inducing  swelling  of  the  cement-substance 
uniting  the  disks;  in  some  instances  solution  of  the  cement-substance  is 
attended  by  cleavage  resulting  in  separation  and  displacement  of  the  disks. 
The  elaborate  investigations  of  Kiihne1  indicate  the  presence  of  a 
structureless  envelope  of  great  delicacy  resembling  neuro-keratin  in  nature  ; 
the  substances — both  the  disks  and  the  intervening  cement — included  within 


Section  of  human  retina,  showing  the  usual  ap- 
pearance of  the  component  layers  in  ordinary  prepa- 
rations.—a,  pigment  layer;  b,  rods  and  cones;  c,  ex- 
ternal limiting  membrane ;  d,  outer  nuclear  layer  ;  e, 
outer  plexiform  layer ;/,  innernuclear  layer ;  g,  inner 
plexiform  layer ;  h,  layer  of  ganglion-cells ;  i,  fibre 
layer;  k,  internal  limiting  membrane;  i,  base  of 
Miiller's  fibre ;  v,  blood-vessel.  Magnified  475  diame- 
ters. 


1  Kiihne :  various  papers  in  Untersuchungen  aus  d.  physiolog.  Institut  der  Univer- 
sitat  Heidelbergs,  Bd.  II.,  1878. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL. 


899 


Fio.  69. 


this  investment  present  in  their  reactions  great  similarity,  although  not 
identity,  to  certain  parts  the  coverings  of  nerve-fibres;  they  have  been, 
therefore,  named  "  myeloid"  by  Kiihne,  as  expressing  their  close  relation  to 
the  myelin  of  nerves. 

The  external  segments  of  the  rods  possess  additional  interest  on  account 
of  being  the  chief,  if  not  the  exclusive,  seat  of  the  visual 
purple  or  rhodopsin,  as  shown  by  Boll l  and  Kiihne.2  The 
visual  purple  is  uniformly  distributed  through  the  entire 
outer  segment  of  the  rods,  which  consequently  possess  a 
uniform  purplish-red  tint.  The  fovea  centralis,  when  the 
rods  are  absent,  therefore,  is  marked  as  an  area  devoid  of 
the  color  presented  by  other  portions  of  the  living  retina. 

The  inner  rod-segment  presents  a  slightly  increased 
diameter  as  well  as  a  feebly  emphasized  elliptical  profile, 
the  thickness  at  the  centre  being  somewhat  greater  than  at 
either  end.  Each  segment  exhibits  a  differentiation  into 
two  parts, — an  outer,  which  appears  faintly  striated  longi- 
tudinally and  reaches  as  far  as  the  external  division  of 
the  rod,  and  an  inner,  which  retains  the  finely  granular  or 
almost  homogeneous  character  of  the  cell-protoplasm  and 
seemiugly  rests  upon  the  external  limiting  membrane.  The 
outer  of  these  portions  of  the  inner  rod-segment,  on  account 
of  its  fibrillar  texture,  contrasts  strongly  with  the  granular 
structure  of  the  inner  division,  and  is  frequently  described 
as  the  rod-ellipsoid  (Schwalbe)  or  the  lenticular  body  (M. 
Schultze).  The  constancy  of  this  diiferentiation  of  the 
inner  segment  of  the  rod  is  remarkable,  since,  in  a  more 
or  less  developed  condition,  it  is  found  in  the  majority  of 
vertebrate  retinae  ;  in  the  human  and  the  mammalian  eye  the 
rod-ellipsoid  is  less  well  defined  than  in  some  lower  types. 

The  body  of  the  rod-visual  cell  includes  that  part  of  the 
ueuro-epithelial  element  which  lies  within  the  outer  nuclear 
layer,  through  the  entire  depth  of  which  it  extends.  It 
consists  of  the  greatly  attenuated  protoplasmic  column, 
the  rod-fibre,  and  the  conspicuous  nucleus,  the  rod-granule. 

The  rod-fibre,  at  its  outer  end,  is  directly  continuous  with  the  granular 
protoplasmic  inner  division  of  the  internal  segment  of  the  rod,  while  its 
inner  end  extends  through  the  entire  thickness  of  the  outer  nuclear  layer 
a  short  distance  into  the  outer  reticular  or  plexiform  layer.  Within  this 
latter  stratum  the  rod -fibre  ends  in  a  minute  expansion,  or  end-knob,  in 
close  relation  with  the  surrounding  and  enveloping  terminal  arborizations 

1  Boll :   Zur  Anatomic  und  Physiologie  der  Retina,  Archiv.  f.  Anat.  u.  Physiol., 
Physiol.  Abth.,  1877. 

2  Kiihne  :  Zur  Photochemie  der  Netzhaut  und  iiber  den  Sehpurpur,  Untei 
aus  d.  physiolog.  Institut  d.  Univ.  Heidelbergs,  Bd.  i.,  1877. 


Semi  -  diagram- 
matic view  of  a  rod 
and  a  cone  from  the 
human  retina.  (Max 
Schultze.)—?,  I,  po- 
sition of  the  exter- 
nal limiting  mem- 
brane, below  which 
the  nucleated  body 
of  the  visual  cell 
lies. 


300  THE   MICROSCOPICAL   ANATOMY   OP   THE    EYEBALL. 

of  the  bipolar  nerve-cells  which  are  particularly  devoted  to  the  rod-cells. 
The  course  of  the  fibre  within  the  outer  nuclear  layer  is  marked  by  numer- 
ous varicosities  which  give  the  cell-body  an  irregular  beaded  profile ;  the 
tendency  to  the  formation  of  these  enlargements  appears  to  be  unusually 
pronounced  after  the  action  of  diluted  solutions  of  chromic  and  osmic  acid. 
(Schwalbe.) 

The  rod-granule,  as  the  nucleus  of  the  rod-visual  cells  is  termed,  being 
of  much  greater  diameter  than  the  fibre,  marks  the  position  of  a  conspicu- 
ous spindle-form  swelling  in  which  the  thin  sheet  of  protoplasm  contributed 
by  the  cell  envelops  the  elliptical  nuclear  body.  The  nuclei  of  the  rod- 
cells  occur  at  all  levels  of  the  outer  nuclear  zone,  a  single  nucleus  being 
connected  with  each  fibre.  Owing  to  the  numerical  preponderance  of  the 
rods  over  the  cones,  they  constitute  the  chief  constituents  of  the  deeply 
staining  granules  characterizing  the  stratum. 

The  elliptical  nuclei,  from  .006  to  .007  millimetre  in  length,  when  ex- 
amined under  high  amplification  in  favorable  preparations,  exhibit  alter- 
nating dark  and  light  cross-stripes.  The  darker,  more  deeply  staining  sub- 
stance always  occupies  the  poles  of  the  nucleus,  the  intermediate  zone  being 
appropriated  by  the  lighter  and  faintly  coloring  material ;  not  infrequently 
the  single  light  central  stripe  is  subdivided  by  an  additional  dark  zone ;  in 
such  cases  five  instead  of  three  bands  are  present.  Flemming l  has  described 
the  appearance  within  the  nuclei,  after  treatment  with  osmic  acid,  of  minute 
additional  bodies  which  he  regards  as  nucleoli.  According  to  the  same 
authority,  the  boundaries  between  the  dark  and  the  light  bands  are  not 
smooth,  but  uneven  and  sometimes  serrated. 

The  cone-visual  cells  present  the  same  divisions  as  do  the  rod- cells,  each 
being  made  up  of  the  specialized  external  part,  the  cone,  and  the  internal 
attenuated  cell-body  occupying  the  external  nuclear  layer. 

Each  cone  consists  of  an  outer  and  an  inner  segment,  which,  however, 
differ  from  the  corresponding  parts  of  the  rods  in  the  marked  inequality  of 
their  length,  as  well  as  of  their  diameters.  While  the  outer  segment  of  the 
rod  is  almost  as  long  and  wide  as  the  inner  segment,  that  of  the  cone,  as 
usually  observed,  forms  only  about  one-third  of  the  .031  to  .036  millimetre 
representing  the  entire  length  of  the  cone.  The  outer  piece  rapidly  tapers 
from  its  base,  about  .002  millimetre  in  diameter,  where  it  joins  the  inner 
segment  to  a  blunted  point.  The  outer  ends  of  the  cones  lie  farther  removed 
from  the  pigment  layer,  scarcely  reaching  so  far  as  the  middle  of  the  outer 
rod-segment.  The  inner  cone-segment  is  likewise  shorter  than  the  corre- 
sponding division  of  the  rod.  The  line  of  juncture  ordinarily  presented 
between  the  outer  and  inner  cone- segments  falls  within  that  of  the  rods. 

The  outer  cone-segment  is  further  distinguished  from  the  similar  part 
of  the  rod  by  the  absence  of  the  visual  purple,  although  in  many  of  its 
characteristics — the  possession  of  high  refractive  properties,  pale  color, 

1  Flemming:  Zellsubstanz,  Kern  und  Zelltheilung,  1882. 


FIG.  GO. 


Diagrammatic  representation  of  the  visual  cells.  (Modified  from  Schultze  and  Schwalbe.)— A,  outer 
portion  of  visual  cells,  corresponding  to  the  layer  of  rods  and  cones;  B,  inner  portion,  constituting  the 
outer  nuclear  layer;  C,  outer  plexiform  layer  in  which  the  visual  cells  end;  a, a',  outer  segments  of 
rods  and  cones;  b,  V,  inner  segments  of  rods  and  cones;  c,  c',  the  rod-  and  cone-ellipsoids;  D,  external 
limiting  membrane,  internal  to  which  lie  the  attenuated  bodies  of  the  visual  cells  represented  by  the 
nucleated  rod-  and  cone-fibres  (d,  d'),  which  end  in  the  outer  plexiform  layer  in  relation  with  the 
arborizations  of  the  bipolar  nerve-cells. 


FIG.  61 

v  ur  ' 


Section  of  human  retina.    (Bohm  and  v.  Davidoff.)-«,  a',  outer  and  ,nn 
outer  and  inner  segments  of  cones ;  c,  membrana  limitans  extenm ;  d   outer  nuclear 
plexiform  layer;  /inner  nuclear  layer;  g,  inner  plexiform  layer;  k,  layer  of  ganglio 
layer.    Magnified  700  diameters. 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL.  301 

keratine  envelope,  and  tendency  to  split  up  into  disks — it  closely  resembles 
the  outer  segment  of  the  rods. 

The  inner  segment,  or  body  of  the  cone,  presents  a  notable  increase  in 
diameter  over  all  other  parts  of  both  cones  and  rods,  being  ellipsoidal  rather 
than  conical  in  profile,  with  a  diameter  of  about  .0065  millimetre  at  the 
point  of  greatest  swelling.  The  length  of  the  inner  cone-segment  Is  be- 
tween .022  and  .024  millimetre.  The  outer  two-thirds  of  this  segment  are 
appropriated  by  an  ellipsoidal  structure  which  presents  a  delicate  longitu- 
dinal striation  similar  to  that  found  in  the  inner  segment  of  the  rods. 

The  body  of  the  cone-visual  cell  lies  within  the  outer  nuclear  layer,  and, 
like  that  of  the  rod-cell,  consists  of  the  slender  cone-fibre  and  the  cone- 
granule. 

The  cone-fibre,  the  attenuated  protoplasmic  column  of  the  neuro-epithe- 
lial  cell,  extends  the  entire  thickness  of  the  outer  nuclear  stratum  and  ends 
within  the  external  zone  of  the  adjoining  reticular  layer  in  an  expanded 
base  or  foot.  The  fibre  is  by  no  means  of  uniform  diameter,  but  begins 
next  the  limitans  externa  with  a  width  about  equal  to  that  of  the  adjoining 
portion  of  the  cone  of  which  it  is  the  direct  continuation.  The  most  ex- 
panded part  of  the  fibre,  about  .005  millimetre  in  diameter,  is  always  next 
the  cone,  and  contains  the  conspicuous  nucleus  or  cone-granule ;  the  wider 
portion  of  the  fibre,  including  approximately  the  outer  third  of  the  entire 
length,  beyond  the  nucleus  rapidly  tapers  to  a  slender  stalk,  which  main- 
tains a  uniform  diameter  of  about  .0012  millimetre  throughout  the  re- 
maining depth  of  the  zone  until  it  reaches  the  subjacent  reticular  layer,  in 
the  outer  part  of  which  it  terminates  in  an  expanded  base,  from  which 
numerous  minute  processes  are  given  oif  which  stand  in  close  relation  with 
the  arborizations  of  the  nerve-cells.  Hosch  l  expresses  the  belief  that  the 
relation  between  the  fibrils  proceeding  from  the  base  of  the  cone-cells  and 
the  processes  of  the  nerve-cells  is  more  intimate  than  mere  apposition,  since 
he  finds  in  the  Golgi  preparations  evidences  of  direct  anatomical  continuity 
between  these  elements. 

The  cone-granules,  or  nuclei  of  the  cone-visual  cells,  contribute  to  the 
characteristic  appearance  of  the  outer  nuclear  layer,  but  to  a  less  degree 
than  the  rod-cells,  owing  to  the  numerical  preponderance  of  the  latter.  The 
nuclei  of  the  cone-cells  occupy  the  enlarged  outer  end  of  the  fibre,  and 
consequently  are  limited  to  a  zone  immediately  subjacent  to  the  external 
limiting  membrane.  This  strongly  contrasts  with  the  disposition  of  the  rod- 
nuclei,  which  are  distributed  throughout  various  levels  of  the  outer  nuclear 
stratum.  The  cone-granules  are  larger  than  the  nuclei  of  the  rod-cells, 
possess  well-marked  nucleoli,  but  do  not  exhibit  the  cross-stripes  seen  in 
the  corresponding  part  of  the  rod-elements. 

Peculiar  modifications  of  the  cone-cells  are  sometimes  observed  in  whi.-l. 

1  Hosch  :   Bau  der  Saugethiernetzhaut  nach  Silberpraparaten,  Archiv  f.  Ophthalmol., 

Bd.  XLI.    1895. 


302 


THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 


FIG.  62. 


two  of  these  elements  are  connected,  producing  the  double  or  "  twin  cones" 

first  described  by  Hannover.1  The  more 
usual  form  of  union  is  fusion  of  the  bases 
or  inner  segments,  but  Hannover,2  Pacini,3 
Borysiekiewicz,4  and  Norris  and  Wallace5 
have  described  looped  connections  between 
the  outer  segments,  the  observations  of  the 
last-named  authors  being  made  upon  the 
human  retina.  Norris,6  in  a  recent  commu- 
nication, accepts  the  view  that  "  the  external 
extremities  of  the  cones  and  rods  are  loops, 
the  outer  member  of  a  cone  bending  over  to 
become  continuous  with  the  outer  member  of 
an  adjacent  rod,  or  less  frequently  with  the 
outer  member  of  another  cone  (twin  cones). 
Adjacent  rods  unite  also  by  their  curved  outer 
segments,  ending  thus  also  in  peripheral  loops. 
.  .  .  The  outer  member  of  a  cone,  having 
thus  curved  on  itself,  ruus  down  along  the 
side  of  the  inner  segment  as  a  cylinder  having  about  the  same  calibre  as 
at  the  turn,  and,  after  perforating  the  external  limiting  membrane,  passes 
alongside  of  the  nucleus  at  the  base  of  the  cone,  and 
may  be  followed  for  some  distance  in  a  tortuous  course 
between  the  nuclei  of  the  so-called  outer  nuclear  layer, 
anastomosing,  at  times,  with  some  of  the  other  nerve- 
fibrils  of  this  layer." 

The  numerical  ratio  between  the  rod-  and  cone- 
visual  cells  varies  in  different  portions  of  the  retina. 
On  examining  the  surface  of  the  retina  about  midway 
between  its  anterior  margin  and  the  posterior  pole, 
the  field  is  seen  to  be  studded  with  closely  placed 
smaller  and  larger  circles  which  represent  the  rods 
and  cones  seen  from  their  outer  extremity.  The 
larger  circles  contain  smaller  figures,  which  are  the 
foreshortenings  of  the  outer  cone-segments,  the  sur- 
rounding outline  being  caused  by  the  greater  diam- 
eter of  the  inner  part  of  the  cone. 


Diagrammatic  view  of  twin  cones. 
(Hannover.) 


FlO.  63. 


Surface  view  of  retina, 
showing  disposition  and 
relative  number  of  the 
rods  and  cones.  (Kolliker.) 
— 1,  from  the  fovea— only 
cones ;  2,  from  the  margin 
of  the  macula  lutea;  3, 
from  midway  between  the 
fovea  and  the  ora  serrata ; 
a,  profile  of  larger  inner 
segment;  6,  of  smaller 
outer  segment ;  c,  rod. 


1  Hannover:  Recherches  microscopiques  sur  le  systeme  nerveux,  1844. 

2  Hannover :  La  retine  de  1'homme  et  des  vertebres,  Copenhague,  1876. 
s  Pacini :  Nuove  ricerche  sulla  tessitura  intima  della  retina,  1845. 

*  Borysiekiewicz :  Untersuchungen  iiber  den  feineren  Bau  der  Netzhaut,  1887. 

5  Norris  and  Wallace :   A.  Contribution  to  the  Anatomy  of  the  Human  Retina,  with  a 
Special  Consideration  of  the  Terminal  Loops  of  the  Rods  and  the  Cones,  University  Medi- 
cal Magazine,  vol.  vi.,  March,  1894. 

6  Norris  :  The  Terminal  Loops  of  the  Cones  and  Rods  of  the  Human  Retina,  Trans- 
actions of  the  American  Ophthalmological  Society,  1895. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL.  303 

The  cones  are  separated  from  one  another  by  a  space  including  three  or 
four  rods,  this  relation,  however,  becoming  markedly  changed  in  the  vicinity 
of  the  macula.  In  the  latter  region  the  number  of  cones  is  so  materially 
increased  that  immediately  outside  the  macula  the  cones  are  separated  by 
only  a  single  row  of  rods ;  within  the  macula  the  preponderance  of  the 
cones  is  marked  to  such  an  extent  that  the  rods  are  practically  wanting, 
the  cone-cells  alone  being  present  at  the  point  of  greatest  visual  acuity. 
While  this  anatomical  detail  warrants  regarding  the  cone-cells  as  the  more 
important  factor  in  the  percipient  layer  in  man  and  other  mammals  possess- 
ing a  similar  arrangement  in  the  area  of  sharpest  vision,  the  fact  that  in 
some  of  the  lower  types,  as  in  the  frog,  the  rods  greatly  preponderate 
cautions  against  assuming  that  the  cone-cells  are  invariably  the  more  essen- 
tial structures  in  the  perceptive  apparatus.  Additional  evidence  as  to  the 
importance  of  the  cones  is  supplied  by  the  fact  that  the  number  of  the 
cones  rapidly  diminishes  towards  the  ora  serrata,  at  which  point  they  are 
almost  entirely  wanting,  the  well-established  low  acuity  of  vision  of  this 
portion  of  the  retina  agreeing  with  the  numerical  paucity  of  the  cones. 

The  total  number  of  cone-cells  contained  within  the  human  retina  at 
birth  has  been  placed  by  Salzer l  at  3,360,000.  The  cones  in  the  adult 
retina  have  been  estimated  by  Krause  2  at  7,000,000,  of  which  13,000  are 
included  within  the  non-vascular  area  of  the  macula.  (Becker.3)  The  esti- 
mates of  both  Salzer  and  Krause  place  the  number  of  cone-cells  at  about 
seven  times  the  number  of  the  fibres  of  the  optic  nerve.  The  entire  number 
of  rod-cells  present  in  the  adult  human  retina  is  undoubtedly  many  times 
that  of  the  rod-elements.  Krause  has  placed  such  estimate  as  high  as 
130,000,000  rods. 

The  modifications  in  the  form  and  arrangement  of  the  visual  cells 
within  the  macula  lutea  and  at  the  ora  serrata  will  be  considered  later  with 
the  descriptions  of  these  specialized  portions  of  the  retina. 

The  External  Plexiform  or  Outer  Reticular  Layer. — This  zone  repre- 
sents the  first  of  the  strata  which  collectively  constitute  the  cerebral  portion 
of  the  retina.  As  seen  in  usual  preparations,  this  stratum  appears  as  a 
narrow,  finely  granular  zone  about  .010  millimetre  in  width ;  when  ex- 
amined with  high  powers,  the  granular  matrix  is  resolvable  into  a  deli- 
cate reticulum.  The  true  nature  of  the  reticulation  of  this  layer,  however, 
is  apparent  only  after  the<  successful  application  of  the  improved  methods 
of  staining  the  processes  of  nerve-cells. 

Eecent  investigations  based  upon  the  results  of  the  Golgi  silver  and  the 
methylene-blue  stainings  have  shown  that  the  outer  reticular  layer,  in  addi- 
tion to  the  extremely  delicate  framework  of  sustentacular  tissue,  is  made  up 

1  Salzer :    Ueber  die  Anzahl  der  Sehnervenfasern   und  der  Retinazapfen  im  Auge 
des  Menschen,  Sitzungsberich.  der  Wiener  Akademie,  Bd.  LXXXI.,  Abth.  iii.,  1880. 

2  Krause:   Handbuch  der  menschlichen  Anatomie,  Bd.  II.,  1879. 

3  Becker:  Die  Gefasse  der  menschlichen  Macula  lutea,  Archiv   f.  Ophthalmol.,  Bd. 
xxvii.,  1881. 


304  THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 

of  the  terminations  of  the  visual  cells,  the  processes  and  arborizations  pro- 
ceeding from  the  bipolar  nerve-cells  situated  within  the  inner  nuclear  layer, 
and  from  the  "  horizontal  cells"  belonging  to  the  outer  reticular  stratum 
itself. 

The  exact  relation  between  the  central  ends  of  the  visual  cells  and  the 
terminations  of  the  nerve-fibres  has  long  been  a  subject  of  investigation 
and  speculation.  The  observations  of  Tartuferi,  Dogiel,  Cajal,  and  others 
have  demonstrated  beyond  question  the  mode  of  ending  of  the  visual  cells 
within  the  external  plexiform  layer.  As  already  noted,  the  cone-elements 
terminate  within  this  layer  by  an  expanded  coniform  base  or  "  foot,"  while 
the  slender  rod-elements  end  in  smaller  knob-like  thickenings  which  usu- 
ally occupy  the  outermost  zone  of  the  reticular  stratum.  These  free  end- 
ings of  the  rod-fibres  were  undoubtedly  seen  by  Max  Schultze,1  as  evi- 
denced by  his  classic  delineations  of  the  retina,  and  also  recognized  by 
Hannover,2  although  their  full  significance,  as  now  appreciated,  was  not 
recognized. 

Within  the  inner  nuclear  layer,  composed  principally  of  the  bipolar 
nerve-cells,  lie  certain  elements  which  are  destined  particularly  for  relation 
with  the  rod-elements,  others  especially  for  the  cone-cells.  These  "  rod" 
and  "  cone"  bipolar  cells  send  off  processes  into  the  outer  plexiform  layer 
which  break  up  into  rich  arborizations  of  terminal  filaments  immediately 
beneath  the  corresponding  visual  elements  and  surround  the  latter  with  a 
close  ramification  of  fibrils. 

Cajal,  Retzius,  and  those  accepting  their  teaching  regard  the  undoubt- 
edly close  relations  between  these  terminal  filaments  and  the  bases  of  the 
visual  cells  as  limited  to  intimate  juxtaposition  and  contact,  and  deny  the 
existence  of  any  direct  anatomical  continuity  between  the  percipient  and 
the  nervous  elements.  Dogiel,  Waldeyer,  and  Merkel  are  less  ready  to 
accept  the  doctrine  of  contact  alone,  and  admit  a  possible  continuity  be- 
tween the  delicate  threads  proceeding  from  the  bases  of  the  visual  cells 
and  the  processes  of  the  nerve-cells.  The  weight  of  evidence,  not  only 
from  retinal  preparations,  but  also  from  the  conditions  obtaining  in  other 
neuro-epithelia,  leads  the  author  to  accept  the  independent  termination  of 
the  visual  cells,  without  anatomical  continuity,  as  the  true  relation  between 
the  percipient  elements  and  the  nervous  processes. 

In  addition  to  the  intricate  reticulation  produced  by  the  ascending 
processes  and  terminal  ramifications  of  the  bipolar  nerve-cells,  the  pres- 
ence of  the  "  horizontal  cells"  and  their  extensions  still  further  conduces 
to  the  complexity  of  the  arrangement  of  fibrils. 

The  horizontal,  basal,  or  stellate  cells  have  a  wide  distribution  within 
mammalian  retinae,  their  presence  having  been  demonstrated  by  Merkel 3 

1  Schultze:  Zur  Anatomic  und  Physiologie  der  Retina,  Archiv  f.  mik.  Anat.,  Bd.  n., 
1866. 

2  Hannover:  La  retina  de  1'homme  et  des  vertebres,  1876. 

8  Merkel :  Ueber  die  menschlichen  Retina,  Archiv  f.  Ophthalmol.,  Bd.  xx. 


THE    MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL.  305 

and  Kolliker 1  in  the  calf,  Golgi  and  Manfred!2  and  Rivolta8  in  the  horse, 
Schwalbe4  and  Dogiel5  in  man,  and  others,  including  Krause,6  Tartuferi' 
and  Cajal.8 

The  horizontal  cells  exist  within  the  outer  plexiform  layer  as  two 
kinds,  the  smaller  outer  and  the  large  inner  cells.  The  outer  horizontal  cells 
appear  as  stellate,  flattened  elements,  which  occupy  the  outermost  zone  of 

FIG.  64. 


Surface  view  of  horizontal  cells  from  retina  of  ox  stained  with  methy lene-blue.  (Cajal.) — a,  intensely 
stained  cell-body;  b,  richly  branched  dendrits:  c,  axis-cylinder  process;  d,  d,  scattered  axis-cylinder 
processes  undergoing  repeated  branching. 

the  plexiform  layer  and  possess  a  variable  diameter,  some  being  as  small  as 
.012  millimetre,  others  as  large  as  .040  millimetre.  These  elements  must  be 
regarded  as  nerve-cells  which  take  part  in  the  "  indirect  conduction,"  since 
they  are  provided  with  both  branched  protoplasmic  and  long  axis-cylinder 
processes  which  extend  for  considerable  distances  within  the  plexiform 

1  Kolliker:  Handbuch  der  Gewebelehre,  6te  Aufl.,  1867. 

8  Golgi  and  Manfredi :  Annotazioni  istologiche  sulla  retina  del  cavallo,  Accad.  di  med. 
di  Torino,  1872. 

3  Eivolta :  Delle  cellule  multipolari  che  formano  lo  strato  intergranuloso  o  intermedio 
nella  retina  del  cavallo,  Giorn.  di  Anat.,  Fisiol.  e  Patholog.  degli  Animali,  anno  in.,  1871. 

4  Schwalbe:  Handbuch  d.  ges.  Augenheilk.,  Graefe  und  Saemisch,  Bd.  I.,  1874. 

5  Dogiel :  Ueber  die  Ketina  des  Menschen,  Internal.  Monatsschr.  f.  Anat.  u.  Histolog., 
Bd.  i.,  1884. 

6  Krause:  Handbuch  der  menschlichen  Anatomic,  Bd.  I.,  1876. 

7  Tartuferi:  Sull'  anatomia  della  retina,  Internal.  Monatsschr.  f.  Anat.  u.  Physiol., 
1887. 

8  Cajal :  La  retine  des  vertebres,  La  Cellule,  t.  IX.,  1893. 

VOL.  I.— 20 


306  THE   MICROSCOPICAL   ANATOMY    OF   THE   EYEBALL. 

layer  and  end  in  arborizations  surrounding  the  terminations  of  the  visual 
cells. 

The  inner  horizontal  cells,  of  much  larger  size  than  the  corresponding 
outer  elements,  have  been  especially  studied  by  Tartuferi,  Baquis,  Dogiel, 
and  Cajal.  Following  the  description  of  Cajal,1  the  inner  horizontal  cells 
include  two  varieties  of  elements,  those  provided  with  descending  processes 
and  those  without  descending  processes. 

The  inner  horizontal  cells  with  descending  processes  are  large,  pyriform 
or  conical  in  form,  with  the  base  directed  outward,  from  which  a  number 
of  stout,  horizontally  extending  dendrits  are  given  off.  These  rather  short 
protoplasmic  processes  rapidly  become  thinner,  irrespective  of  their  division, 
and,  after  a  limited  dichotomous  branching,  break  up  into  an  arborization 
composed  of  short  varicose  threads  which  end  in  minute  terminal  knobs. 

The  descending  protoplasmic  process  arises  from  the  centrally  directed 
apex  of  the  cell-body.  On  reaching  the  outer  half  of  the  internal  plexi- 
form  layer,  it  divides  usually  into  two  branches,  which  extend  horizontally 
for  some  distance  and  end  either  by  forming  a  rich  horizontal  plexus  within 
the  inner  reticular  stratum  or  by  gradually  fading  away. 

The  neurit,  or  axis-cylinder  process,  of  these  cells  is  remarkably  robust. 
Beginning  usually  as  a  conical  enlargement  on  the  cell-body,  it  runs  in  a 
generally  horizontal  direction,  at  some  distance  from  the  outer  reticular 
layer,  throughout  a  remarkably  extended  course,  the  fibre  being  traceable 
sometimes  for  nearly  one  millimetre  without  materially  changing  its  course. 
The  statement  of  Dogiel  that  these  processes  bend  centrally  eventually  to 
become  fibres  of  the  fibre  layer  is  not  sustained  by  Cajal,  who  denies  that 
they  become  thus  deflected  from  their  horizontal  course,  and  maintains,  on 
the  contrary,  that  they  probably  end  within  the  outer  reticular  layer. 

The  inner  horizontal  cells  without  descending  processes  have  been  most 
accurately  described  by  Cajal,2  who  regards  them  as  the  most  common  of 
the  inner  horizontal  elements,  being  much  more  frequent  than  those  pos- 

FIG.  65. 


Horizontal  cell  from  retina  of  ox.   (Cajal.)— a,  axis-cylinder  process  giving  off  collateral  branches. 

sessing  the  centrally  directed  process.  This  authority  recognizes  two  va- 
rieties of  these  cells, — (a)  those  presenting  a  spindle- form  or  crescentic 
body,  but  slightly  protruding,  and  having  only  few  horizontal  proto- 
plasmic processes,  and  (6)  those  exhibiting  a  large  volume,  with  conspic- 


1  Cajal:  Die  Retina  der  Wirbelthiere,  uebersetzt  von  R.  Greeff,  Wiesbaden,  1894. 

2  Cajal:  loc.  cit.,  p.  121. 


THE   MICROSCOPICAL   ANATOMY  OF  THE   EYEBALL.  307 

uous  bulging  on  their  lower  surface,  and  giving  off  a  large  number  of 
divergent  processes. 

The  axis-cylinder  process  of  these  cells  is  very  robust,  and  stretches 
horizontally,  at  some  distance  from  the  outer  reticular  layer,  throughout  an 
extended  course,  the  first  part  of  which,  however,  is  not  infrequently  some- 
what curved. 

The  foregoing  description  of  the  elements  composing  the  external  plexi- 
form  layer  has  emphasized  the  fact  that  by  far  the  greater  portion  of  the 
delicate  reticulum  constituting  this  stratum  depends  for  its  formation  upon 
the  interlacement  of  the  processes  of  nervous  elements.  The  vertically 
coursing  fibrils  are  principally  contributed  by  the  bipolar  cells  of  the  sub- 
jacent zone,  together  with  the  similarly  coursing  processes  of  the  horizontal 
cells ;  the  horizontally  coursing  filaments  are  derived  from  the  extended 
ramifications  of  the  horizontal  cells  of  the  reticular  layer  itself.  While 
undoubtedly  the  contributions  from  the  several  cells  compose  the  great 
bulk  of  this  layer,  yet  the  presence  of  a  delicate  framework  of  sustentacu- 
lar  tissue  must  not  be  ignored.  The  latter  is  represented  by  the  apparently 
granular  substance  which  occupies  the  interstices  of  the  fibrillar  reticulation, 
and,  while  small  in  amount,  corresponds  closely  with  the  supporting  sub- 
stance unmistakably  present  in  the  better  developed  inner  reticular  layer, 
as  shown  by  Schiefferdecker *  and  Dogiel 2  and  emphasized  by  Merkel.5 

The  Layer  of  Bipolar  Nerve-Cells,  or  the  ^nner  Nuclear  Layer. — In 
ordinary  preparations  of  the  retina  this  stratum  appears  similar  to  the 
outer  nuclear  zone,  being  conspicuous  by  reason  of  the  large  number  of 
deeply  staining  cells  which  seemingly  form  the  major  part  of  the  entire 
zone.  The  layer  varies  in  thickness  in  different  parts  of  the  retina,  in  the 
vicinity  of  the  optic  entrance  measuring  about  .035  millimetre,  near  the 
ora  serrata  diminishing  to  about  .018  millimetre. 

The  complex  constitution  of  this  layer  and  the  varying  character  of  its 
cellular  elements  were  early  recognized,  although  the  subdivision  into  an 
outer  zone  of  nerve-cells,  the  ganglion  retinse,  and  an  inner  zone  of  spongio- 
blasts,  as  made  by  W.  Miiller  and  accepted  by  many  subsequent  authors, 
as  Schwalbe,  Krause,  and  others,  must  be  somewhat  modified  and  supple- 
mented in  view  of  the  results  of  the  more  recent  investigations. 

The  researches  of  Tartuferi,  Dogiel,  and  Cajal,  already  cited,  have  con- 
clusively shown  that  the  bipolar  nerve-cells,  the  principal  elements  of  the 
layer,  consist  of  two  distinct  varieties  :  1,  bipolars  especially  related  to  the 
terminations  of  the  rod  visual  cells;  2,  bipolars  particularly  destined  for 
the  cone  elements. 

These  may  be  designated,  as  suggested  by  Schiifer,4  respectively  the 

1  Schieiferdecker :  Studien  zur  vergleichenden  Histologie  der  Retina,  Archiv  f.  mik. 
Anat.,  Bd.  xxvui.,  1886. 

2  Dogiel :  Neuroglia  der  Retina  beim  Menschen,  Archiv  f.  mik.  Anat.,  Bd.  XLI.,  1898. 

3  Merkel :  Ergebnisse  der  Anatomic  und  Entwickelungsgesch.,  Bd.  II.,  1893,  S.  257. 

4  Schafer:  Quain's  Anatomy,  vol.  in.,  Pt.  8,  1894,  p.  43. 


308  THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 

rod-  and  the  cone-bipolars.  Both  fulfil  the  purpose  of  supplying  the  con- 
necting link  between  the  percipient  elements  and  the  large  nerve-cells  of 
the  ganglion  layer  in  the  transmission  of  the  light-impulses,  their  outer  ends 
being  closely  related  to  the  rod  and  the  cone  visual  elements,  their  central 
expansions  with  the  large  ganglion-cells.  The  cell-body  of  each  gives  off 
two  main  processes,  an  ascending  protoplasmic  or  dendrit  and  a  descending 
axis-cylinder  or  neurit.  The  two  varieties  of  bipolars  present  in  their  mode 
of  ramification  distinguishing  peculiarities  which  call  for  brief  consideration. 

The  rod-bipolars  are  robust  elements  of  crescentic  or  oval  form,  and 
send  off  a  variable  number  of  ascending  protoplasmic  processes  which  freely 
branch  and  collectively  present  a  rich  arborization  within  the  outer  zone  of 
the  external  plexiform  layer.  Here  the  ascending  processes  end  in  minute 
terminal  twigs  which  closely  surround  the  knobbed  enlargements  of  the  rod- 
fibres  which  descend  from  the  superimposed  outer  nuclear  layer. 

The  arborizations  of  the  larger  rod-bipolars  are  of  such  extent  that  they 
embrace  the  free  terminations  of  from  fifteen  to  twenty  rod-fibres ;  those 
of  the  smaller  cells  are  much  less  expanded,  and  include  the  terminations 
of  only  three  or  four  rods.  As  already  stated,  there  seems  no  sufficient 
evidence  for  assuming  a  direct  anatomical  continuity,  close  contact  between 
the  expanded  rod-fibre  and  the  embracing  fibrils  being  the  extent  of  the 
intimacy  between  the  neuro-epithelial  and  the  nervous  filaments. 

The  descending  axis-cylinder  processes  of  the  rod-bipolars  are  very  long, 
passing  entirely  through  the  subjacent  inner  plexiform  layer  to  reach  the 
large  elements  of  the  ganglion-cell  layer.  Immediately  above  these  latter 
the  processes  split  up  into  short  twigs,  which,  after  closely  ramifying  over 
the  ganglion-cells,  end  in  ellipsoidal  or  spherical  varicosities. 

The  cone-bipolars,  while  occurring  at  all  levels,  are  more  numerous 
within  the  deeper  zone  of  the  inner  nuclear  layer.  Their  ascending  proto- 
plasmic processes  do  not  pass  as  deeply  into  the  outer  reticular  stratum  as 
do  those  of  the  rod-bipolars,  but  end  at  the  level  occupied  by  the  expanded 
conical  bases  of  the  cone-fibres.  The  mode  of  termination  of  these  processes 
contrasts  strongly  with  the  vertical  arborizations  of  the  rod-bipolars,  since 
the  terminal  fibres  in  question  assume  a  horizontal  direction,  expanding 
into  a  rich  and  extensive  arborization  composed  of  delicate  fibrils  which 
never  are  deflected  vertically.  These  ramifications  lie  beneath  the  expanded 
bases  of  the  cone-fibres,  from  which  latter  twigs  pass  that  bear  close  relations 
to  the  terminal  processes  of  the  bipolar  cell. 

As  pointed  out  by  Cajal,1  the  extent  of  the  area  included  within  the 
expansions  of  the  arborizations  of  the  cone-bipolars  renders  it  highly 
probable  that  the  more  deeply  situated  rod-fibres  also  at  times  may  enjoy 
intimate  contact  with  the  processes  derived  from  the  second  (cone)  variety 
of  bipolar  nerve-cells,  thus  establishing  an  additional  path  for  the  convey- 
ance of  the  particular  light-impulses  taken  up  by  the  rods. 

1  Cajal :  loc.  cit. 


PLATE  I. 

The  elements  of  the  mammalian  retina  based  on  the  investigations  of  Ramon  y  Cajal 
by  means  of  the  Golgi  method  of  silver  staining.  (Cajal.) 

FIG.  2. — Section  of  dog's  retina. — a,  a,  cone-fibres  ;  6,  6,  rod-granules  and  fibres  ;  c,  d, 
bipolar  nerve-cells  with  erect  arborizations  destined  for  the  rod-elements ;  e,  e,  bipolar 
nerve-cells  with  horizontal  arborizations  destined  for  the  cone-elements ;  /,  giant  bipolar 
with  horizontal  ramifications ;  h,  diffuse  amacrine  cell  the  processes  of  which  ramify 
directly  upon  the  large  ganglion-cells;  i,  i,  ascending  axis-cylinder  processes ;  j,  j,  cen- 
trifugal nerve-fibres ;  </,  g,  special  elements  whose  relations  are  still  uncertain ;  n,  one 
of  the  large  ganglion-cells  receiving  the  ramifications  of  the  rod-bipolars ;  m,  m,  nerve- 
fibres  penetrating  the  inner  plexiform  layer. 

FIG.  3. — Horizontal  cells  from  dog's  retina. — A,  outer  horizontal  cell ;  B,  inner  hori- 
zontal cell  of  moderate  size  without  descending  protoplasmic  process ;  C,  inner  horizontal 
cell  of  small  dimension  ;  a,  a,  horizontally  coursing  axis-cylinder  processes. 

FIG.  4. — Nerve-cells  from  retina  of  ox. — a,  a,  bipolars  with  erect  arborization  ;  b, 
bipolar  with  horizontal  arborization  for  cone-cells ;  c,  d,  e,  similar  bipolars  the  arbori- 
zations of  which  are  distributed  more  superficially  ;  /,  bipolar  with  extensive  arborization 
and  irregularly  coursing  descending  process  ;  g,g,  bipolars  with  very  extensive  horizontal 
arborizations  ;  h,  h,  ovoid  cells  situated  outside  the  outer  plexiform  layer ;  i,  j,  m,  amacrine 
cells  of  the  inner  plexiform  layer  lying  at  and  distributed  to  various  levels. 

FIG.  5. — Horizontally  coursing  axis-cylinder  process  from  the  outer  plexiform  layer. 
— a,  profile  view  of  the  terminal  arborization ;  b,  axis-cylinder. 

FIG.  6. — Another  form  of  terminal  arborization  of  a  similar  process. 

FIG.  7. — Nervous  elements  of  retina  of  ox,  especially  various  forms  of  amacrine  cells 
distributed  to  different  planes. — A,  crescentic  amacrine  with  greatly  extended  process; 
E,  large  amacrine  with  thick,  widely  extended  branches ;  C,  peculiar  amacrine  with 
very  slender  processes ;  D,  amacrine  with  radiating  processes ;  E,  large  amacrine  dis- 
tributed to  deepest  stratum  ;  F,  small  amacrine  ;  G,  H,  amacrines  destined  for  deeper  part 
of  layer ;  a,  small  ganglion-cell ;  &,  ganglion-cell  the  processes  of  which  form  arborizations 
at  three  different  levels ;  c,  ganglion-cell  of  limited  expansion ;  d,  ganglion-cell  of  moder- 
ate size ;  e,  large-sized  ganglion-cell ;  /,  ganglion-cell  the  richly  branched  processes  of 
which  form  arborizations  within  the  deeper  as  well  as  the  superficial  levels  of  the  layer. 

FIG.  8. — Amacrine  and  ganglion  cells  from  dog's  retina. — A,  B,  C,  D,  E,  F,  G, 
various  forms  of  amacrine  cells  distributed  to  different  levels  of  the  inner  plexiform  layer; 
a,  b,  d,  e,f,  g,  i,  ganglion-cells  whose  arborizations,  of  varying  form  and  extent,  terminate 
at  different  planes. 

FIG.  9. — Ganglion-cells  from  dog's  retina. — a,  b,  c,  d,  /,  g,  h,  i,  ganglion-cells  of 
various  size  and  form  terminating  in  arborizations  which  occupy  different  levels  within 
the  inner  plexiform  layer;  A,  B,  C,  nervous  elements  constituting  the  so-called  "  spongio- 
blasts." 


THE    MICROSCOPICAL   ANATOMY   OP  THE   EYEBALL. 


309 


This  same  observer  has  described,  cone-bipolars  of  especially  large  size, 
—giant  bipolars, — which  usually  lie  immediately  beneath  the  external  plexi- 
form  layer.  From  the  large  pyramidal  or  conical  bodies  of  these  giant 
cells  processes  ascend  to  form  extensive  horizontal  ramifications  within  the 
reticular  stratum. 

The  descending  axis-cylinder  processes  of  the  cone-bipolars  differ  mark- 
edly in  their  distribution  from  the  corresponding  extensions  of  the  rod- 
cells.  The  processes  in  question,  instead  of  penetrating  into  the  layer  of 
ganglion- cells,  are  limited  to  the  inner  reticular  stratum,  within  which  they 
terminate  at  definite  but  various  levels.  On  reaching  the  particular  depth 
for  which  it  is  destined,  the  process  breaks  up  into  fine  terminal  filaments 
possessing  numerous  varicosities  :  these  expansions  come  into  close  relation 
with  the  arborizations  of  the  ascending  processes  of  some  of  the  large  cells 
situated  within  the  ganglion  layer. 

The  terminations  of  the  cone-bipolars  within  the  inner  plexiform  layer 
occur  within  five  definite  planes,  which  arrangement  results  in  the  appear- 
ances long  recognized  as  the  secondary  zones  of  this  layer. 

FIG.  66. 


Nerve-cells  from  retina  of  ox  stained  with  methylene-blue.  (Cajal.) — a,  bipolar  for  cones;  6,  giant 
bipolar  with  horizontally  expanding  processes;  c,  cone-bipolar  with  deeply  situated  nucleus;  d,  cres- 
centic  cell  with  delicate,  long  fibres  distributed  to  the  deepest  part  of  the  inner  plexifora  layer;  /,/./, 
diffuse  amacrines  forming  plexus  at  g;  e,  h,  i,j,  amacrines ;  *.  giant  ganglion-eell ;  m,  ganglion-cell. 

The  Amacrine  Cells,  or  Spongioblasts. — W.  Miiller  years  ago  recognized 
the  fact  that  the  inner  nuclear  layer  contained  cells  not  included  with  the 
nervous  elements  constituting  the  outer  aggregation  termed  the  ganglion 
retince.  These  elements,  situated  in  the  deeper  zone  of  the  inner  reticular 
layer,  Miiller  regarded  as  closely  concerned  in  the  production  of  the  sup- 
porting framework  of  the  stratum,  and  hence  suggested  the  name  of 
"  spongioblasts"  as  appropriate.  While  the  precise  nature  and  relation  of 
these  elements  are  still  somewhat  uncertain,  much  has  been  added  to  our 
knowledge  of  their  form  and  ramifications  by  the  more  recent  methods  of 
staining.  Their  peculiarity  of  apparently  being  without  an  axis-cylinder 
process  has  led  Cajal,  by  whom  they  are  regarded  as  nervous  elements,  to 
name  them  the  amacrine  cells,  although  Dogiel  has  shown  that  the  homolo- 
gous structures  in  the  retina  of  amphibia,  reptiles,  and  birds,  and  possibly 
also  some  of  the  cells  in  man,  undoubtedly  possess  such  processes. 

The  amacrine  cells  or  spongioblasts  of  the  mammalian  retina  have  been 


310  THE   MICROSCOPICAL    ANATOMY   OF   THE    EYEBALL. 

studied  with  great  care  by  Tartuferi,  Dogiel,  and  Cajal.  The  last-named 
authority  concludes1  from  his  investigations  that  there  are  present  two 
types  of  amacrines,  the  diffuse  and  the  stratiform. 

The  diffuse  amacrine  cells  occur  as  large  and  as  small  elements.  The 
former  possess  a  triangular,  crescentic,  or  mitral  body,  from  which  two  or 
three  robust  processes  obliquely  descend,  freely  branch,  and  finally  form  a 
rich  arborization  composed  of  varicose  fibrils.  These  expansions  are  dis- 
tributed principally  within  the  lowermost  stratum  of  the  inner  reticular 
layer,  immediately  above  the  ganglion-cells. 

The  smaller  diffuse  amacriues  exhibit  the  same  general  character  as  the 
larger  elements ;  they  diifer,  however,  in  their  oval  or  "  udder-form"  body, 
the  smaller  number  of  primary  branches,  and  the  position  of  the  terminal 
twigs,  which,  while  distributed  to  the  inner  two-thirds  of  the  plexiform 
layer,  lie  within  a  somewhat  higher  plane  than  the  endings  of  the  large 
cells. 

The  stratiform  amacrine  cells,  so  designated  on  account  of  the  manner  in 
which  their  processes  are  disposed,  are  divided  by  Cajal  into  three  chief 
groups,  although  additional  subvarieties  have  been  described  based  upon 
their  relations  to  the  five  individual  strata  of  the  inner  plexiform  layer  in 
which  the  cell-processes  end. 

While  possessing  in  common  descending  processes  and  horizontal  ramifi- 
cations, these  cells  differ  in  size,  in  the  robustness  of  their  primary  branches, 
and  in  the  expansion  and  delicacy  of  their  terminal  twigs. 

Type  I.  includes  amacrines  which  possess  very  large  cell-bodies  and 
thick  primary  stalks  ;  the  latter  extend  into  the  inner  plexiform  layer  and 
ramify  within  one  of  its  sublayers  throughout  a  considerable  area.  The 
terminal  arborization,  however,  consists  of  comparatively  few  and  relatively 
coarse  fibrils. 

Type  II.  comprises  smaller  amacrines  from  the  medium-sized  pyriform 
body,  of  which  a  straight  process  passes  into  the  inner  plexiform  layer, 
where  it  ends,  within  one  of  the  substrata,  in  an  arborescence  of  moderate 
expansion  and  close  interlacement  of  the  component  fibrils. 

Type  III.  is  represented  by  the  amacrines  of  small  or  medium  size 
giving  off  a  slender  process  which  enters  the  inner  reticular  layer  and 
breaks  up  into  a  tuft  of  delicate,  horizontally  coursing  fibrils ;  these  ex- 
tend radially  within  one  of  the  substrata  and  form  a  terminal  arborescence 
of  often  very  considerable  size.  In  those  cases  in  which  the  eud-fibrillfe 
are  distributed  to  the  outermost  zone  of  the  inner  plexiform  layer,  and  con- 
sequently lie  immediately  beneath  the  cell-body  of  their  governing  element, 
the  chief  process  is  replaced  by  a  number  of  small  twigs  which  at  once  take 
part  in  the  production  of  the  arborescence. 

In  addition  to  the  nervous  elements  contained  within  the  inner  nuclear 
layer,  the  sustaining  neurogliar  framework  derived  from  the  fibres  of 

1  Cajal:  loc.  cit.,  p.  134. 


THE   MICROSCOPICAL   ANATOMY  OF  THE   EYEBALL.  31 1 

Miiller,  presently  to  be  described,  also  contributes  a  certain  number  of  the 
nuclei,  which  become  apparent  after  staining  with  carmine  or  hjematoxylin. 
These  nuclei  correspond  to  the  local  thickenings  of  the  sustentacular  fibres 
within  the  inner  nuclear  layer,  and  occur  at  various  levels.  The  supporting 
framework  within  this  layer  consists  for  the  most  part  of  irregular,  flattened 
bands  and  trabeculae,  which,  while  imperfectly  separating  the  nerve-cells 
suffice  to  sustain  and  hold  in  place  the  more  important  elements. 

The  Internal  Plexiform  or  Inner  Reticular  Layer. — This  stratum  in  or- 
dinary preparations  of  the  retina  is  very  conspicuous,  appearing  as  a  broad, 
lightly  stained  zone  about  .040  millimetre  in  breadth,  which  contrasts 
sharply  with  the  more  deeply  colored  elements  of  the  adjoining  inner  nu- 
clear and  ganglion  layers.  Seen  in  such  specimens,  the  stratum  apparently 
is  composed  of  a  granular  substance,  or  of  a  close  reticulum  of  fine  fibrilhe, 
traversed  by  the  long  fibres  of  Miiller. 

FIG.  67. 


Vertical  section  of  retina  of  ox.  (Cajal.) — o,  b,  horizontal  cells  of  inner  nuclear  layer;  c,  d,  ama- 
crines  distributed  within  inner  plexiform  layer ;  e,  small  ganglion-cell ;  f-j,  various  types  of  neuroglia 
cells  of  the  fibre-layer ;  k,  interstitial  amacrine  cell. 

While  undoubtedly  these  latter  contribute  largely  to  the  entire  bulk  of 
the  layer  by  means  of  the  delicate  reticulum  of  sustentacular  tissue  derived 
from  them  as  lateral  processes,  it  will  be  seen  from  the  foregoing  descrip- 
tions of  the  retinal  elements  that  the  terminal  ramifications  of  the  processes 
of  the  cells  contained  within  the  inner  nuclear  and  the  ganglion  layer  form 
a  very  considerable  proportion  of  the  structures  formerly  included  within 
the  "  spongiosum." 

As  already  mentioned,  the  internal  plexiform  layer  presents  a  differen- 
tiation into  five  subzones;  this  specialization  depends  upon  the  peculiar 
manner  in  which  the  descending  processes  of  the  cone-bipolars  and  the  asso- 
ciated ascending  processes  of  the  large  ganglion-cells  expand  at  various  levels 
into  the  horizontal  arborescences.  The  general  reticular  appearance  is  still 
further  promoted  by  the  interlacement  of  the  multitude  of  fibrils  derived 
from  the  richly  branching  amacrine  cells,  one  variety  of  which — the  strati- 
form amacrines — terminates  in  horizontal  ramifications. 


312  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

Various  authors — H.  Miiller,1  Nagel,2  Borysiekiewicz,3  and  Cajal 4 — have 
described  the  presence  of  small,  irregular  cells  within  the  inner  reticular 
layer.  These  elements,  the  interstitial  amacrine  cells  of  Cajal,  seem  to  be 
displaced  cells,  whose  fusiform  or  triangular  bodies,  instead  of  occupying 
the  usual  position  within  the  inner  nuclear  stratum,  have  suffered  displace- 
ment, although  their  processes  terminate  in  ramifications  having  the  usual 
destination  within  the  horizontal  substrata  of  the  layer.  In  exceptional 
cases,  however,  the  distribution  of  the  processes  of  the  displaced  cell  is 
thoroughly  erratic.  According  to  Cajal,  the  second  substratum  is  the  most 
frequent  location  of  these  aberrant  elements.  Parallel  examples  of  trans- 
posed cells  are  presented  in  the  unusual  positions  in  which  the  bipolars  and 
ganglion-cells  are  found  in  the  retinae  of  some  of  the  lower  types.  It  is 
probable,  according  to  Cajal,  that  the  terminal  ramifications  of  a  certain 
number  of  "  centrifugal"  nerve-fibres  may  still  further  add  to  the  complex 
constitution  of  the  internal  plexiform  layer. 

The  Layer  of  Ganglion- Cells.— The  especial  characteristic  of  this 
stratum  is,  as  indicated  by  its  name,  the  presence  of  the  large,  conspicuous 
nervous  elements  which  are  closely  related  with  the  fibres  composing  the 
optic  nerve. 

Throughout  the  greater  part  of  the  retina  these  cells  are  disposed  as  a 
single  closely  placed  row,  but  towards  the  visual  pole  they  become  more 
numerous,  and  in  the  vicinity  of  the  macula  lutea  are  arranged  as  a  double 
layer.  Within  the  yellow  spot  they  become  greatly  increased  in  number, 
being  superimposed  to  such  an  extent  that  they  lie  from  eight  to  ten  deep. 
Towards  the  periphery  of  the  retina,  on  the  contrary,  the  number  of  the 
ganglion-cells  decreases,  and  at  the  ora  serrata  they  are  no  longer  plentiful 
enough  to  constitute  a  complete  row,  but  lie  isolated  and  widely  apart. 

The  ganglion-cells  possess  the  common  characteristics  of  well-defined 
multipolar  nerve-cells,  the  neurits  or  the  axis-cylinder  processes  passing  cen- 
trally into  the  nerve-fibres  of  the  fibre  layer  and  optic  nerve,  aud  the  deudrits 
or  branched  protoplasmic  processes  entering  the  internal  plexiform  layer.  In 
their  ultimate  structure,  likewise,  the  ganglion-cells  of  the  retina  correspond 
with  the  nervous  elements  of  other  parts  of  the  central  nervous  system. 
Lenhoss6k 5  and  Dogiel,6  in  their  descriptions  of  the  retinal  ganglion-cells, 
recognize  the  presence  of  two  distinct  substances, — a  chromophilous  readily 
staining  and  an  achromatic  non-staining  material.  The  disposition  of  the 

1  H.  Miiller :  Anatomisch-phys.  Untersuchungen  iiber  die  Retina  des  Menschen  und 
der  Wirbelthiere,  Zeitschrift  f  wiss.  Zoologie,  Bd.  Tin.,  1857. 

2  Nagel:    Die  fettige  Degeneration  der  Netzhaut,  Archiv  f.  Ophthalmol.,  Bd.  vi., 
1860. 

8  Borysiekiewicz :  Untersuchungen  uber  den  feineren  Bau  der  Netzhaut,  1887. 

4  Cajal :  Die  Retina  der  Wirbelthiere,  1894,  p.  137. 

8  Lenhossek :  Der  feinere  Bau  des  Nervensystems  im  Lichte  neuester  Forschungen, 
1895. 

6  Dogiel :  Die  Structur  der  Nervenzellen  der  Retina,  Archiv  f.  mik.  Anat.,  Bd.  XLVI., 
1895. 


THE   MICROSCOPICAL  ANATOMY   OF   THE   EYEBALL.  313 

readily  tinged  constituent  of  the  cell,  as  demonstrated  after  staining  by 
Nissl's  or  DogiePs  modified  methylene-blue  method,  varies  greatly,  the 
component  granules  being  arranged  singly,  grouped,  in  rows,  or  as  fibrill®. 
In  opposition  to  the  views  of  Nissl,  Bach,1  and  others,  Dogiel  supports  the 
older  teaching  of  Max  Schultze  as  to  the  existence  of  fibrillation  of  the 
nerve-cells,  and,  further,  regards  the  dendrits  and  neurits  as  practically 
identical  in  structure.  According  to  the  recent  investigations  of  the  last- 
named  observer,  both  protoplasmic  and  axis-cylinder  processes  are  composed 
of  the  same  constituents, — namely,  chromophilous  substance,  ground-sub- 
stance, and  fibrillse, — only  in  varying  quantities.  The  axis-cylinder  process 
contains  only  an  insignificant  amount  of  the  chromophilous  and  ground- 
substance,  and  consists  principally  of  fibrillse ;  in  the  protoplasmic  processes, 
on  the  contrary,  the  fibrillse  are  inconspicuous,  while  the  staining  material 
and  the  interstitial  matrix  are  present  in  much  larger  quantities. 

The  more  accurate  methods  of  staining  now  employed  reveal  differences 
in  the  details  of  the  mode  of  the  termination  of  the  dendrits  which  have 
led  to  the  recognition  of  two  principal  types  of  the  ganglion-cells : 

I.  Ganglion-cells   the  protoplasmic  processes  of  which  terminate  in 
horizontal   ramifications  within  definite   substrata  of  the  inner  reticular 
layer. 

II.  Ganglion-cells  the  protoplasmic  processes  of  which  end  diffusely  by 
ramifications  distributed  to  the  entire  layer. 

The  first  class  further  includes  two  subgroups, — (1)  the  monostratified 
cells,  or  those  which  are  distributed  to  a  single  substratum  of  the  inner 
plexiform  layer,  and  (2)  the  bistratified  and  multistratified  cells,  or  those 
whose  processes  ramify  within  two  or  more  substrata. 

Each  of  these  subgroups  is  generally  represented  by  three  varieties  of 
cells, — (a)  large,  (6)  medium,  and  (c)  small, — the  variation  in  size  being 
usually  included  between  .03  and  .01  millimetre. 

Without  entering  upon  a  detailed  description  of  the  individual  elements 
ramifying  within  the  several  substrata,  the  general  character  of  the  three 
sizes  of  elements  may  be  noted. 

The  large  stratified  ganglion- cells  give  off  one,  two,  or  more  robust  pro- 
toplasmic processes  which  enter  the  reticular  layer  and  pursue  a  generally 
vertical  course  which  varies  in  length  according  to  the  level  for  which  the 
ramifications  are  destined.  The  terminal  arborizations  extend  over  a  con- 
siderable area,  are  open  in  arrangement,  and  come  into  close  relations  with 
the  horizontal  arborescences  formed  by  the  cone-bipolars ;  the  ramifications 
of  a  number  of  the  latter  are  often  in  relation  with  the  more  extended 
endings  of  a  single  ganglion-cell.  The  neurits  or  axis-cylinder  processes 
of  the  ganglion-cells  are  usually  thick,  and  pass  centrally  to  become  the 
axis-cylinders  of  the  nerve-fibres. 


1  Bach :    Die  menschliche  Netzhaut  in  normalen   und   pathologischen   Zustanden, 
Archiv  f.  Ophthalmol.,  Bd.  XLI.,  1895. 


314  THE  MICROSCOPICAL  ANATOMY   OF  THE   EYEBALL. 

The  medium-sized  stratified  ganglion-cells  present  considerable  variation 
in  their  dimensions,  but  are  generally  somewhat  smaller  than  the  members 
of  the  preceding  group.  The  bodies  of  the  elements  under  consideration 
are  pyriform,  the  smaller  end  being  directed  outward  and  penetrating  for 
some  distance  into  the  inner  reticular  layer.  The  terminal  arborescences 
of  these  cells,  composed  of  closely  interwoven  varicose  filaments  of  moderate 
delicacy,  are  less  flattened  than  those  of  the  other  stratified  ganglion-cells 
and  include  a  relatively  thick  zone. 

The  small  stratified  ganglion-cells  usually  possess  small  pyriform  cell- 
bodies  from  which  a  thin,  straight,  outwardly  pointing  stalk  ascends  for  a 
variable  distance  and  ends  by  breaking  up  into  a  terminal  arborization  of 
moderate  fineness.  When  the  latter  is  destined  for  the  deepest  substratum 
of  the  inner  plexiform  layer,  the  main  stalk  may  be  replaced  by  numerous 
delicate  processes  which  undergo  but  limited  division  and  almost  at  once 
take  part  in  the  terminal  ramifications. 

The  manner  in  which  the  stratified  ganglion-cells  of  the  human  retina 
terminate  within  the  substrata  of  the  internal  plexiform  layer,  as  shown 
by  the  studies  of  Dogiel  by  the  methylene-blue  method,  corresponds  in 
essential  details  with  the  descriptions  of  Cajal  based  upon  the  use  of  the 
Golgi  silver  stainings. 

The  diffuse  ganglion- cells  are  constant  elements  of  the  mammalian 
retina,  although  their  smaller  size  and  more  delicate  processes  render  them 
inconspicuous  in  comparison  with  the  giant  cells  devoted  to  the  horizontal 
arborizations  within  the  substrata.  These  elements,  as  suggested  by  their 
name,  differ  from  the  foregoing  ganglion-cells  in  the  manner  in  which 
their  peripherally  directed  dendrits  are  distributed.  Instead  of  forming 
arborescences  limited  to  definite  substrata,  their  protoplasmic  processes 
divide  into  filaments  which  ramify  throughout  the  plexiform  layer,  coming, 
possibly,  into  close  association  with  the  amacrine  cells  and  their  rami- 
fications. 

The  foregoing  descriptions  have  repeatedly  emphasized  the  close  rela- 
tions which  exist  between  the  terminal  expansions  of  the  bipolars  and  the 
ganglion-cells.  In  the  case  of  the  rod-bipolars,  the  latter  alone  contribute 
an  arborescence  which  embraces  the  upper  surface  of  the  ganglion-cell. 
The  expansions  of  the  cone-bipolars,  on  the  contrary,  are  limited  to  the 
inner  reticular  layer  when  they  are  brought  into  intimate  relations  with  the 
correspondingly  situated  expansions  of  the  ganglion-cells. 

The  nature  of  the  undeniably  close  relations  between  these  nervous  ele- 
ments has  long  been  the  subject  of  investigation  and  speculation.  The 
existence  of  a  mutual  conjunction  within  the  reticulum  formed  by  the  con- 
tributions of  both  has  been  strictly  maintained  by  many  authorities,  among 
whom  at  the  present  time  Dogiel  stands  conspicuous.  Equally  trustworthy 
authorities,  on  the  other  hand,  are  convinced  that  net- works  of  directly 
continuous  filaments  do  not  exist,  and  that  the  close  contact  of  the  free- 
ending  fibrils  composing  the  terminal  arborizations  of  bipolars  and  ganglion- 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL.  315 

cells  constitutes  the  limit  of  the  connection  between  the  two  and  the  path 
for  the  transmission  of  light-impulses. 

Cajal,  in  whom  the  "  contact  theory"  finds  one  of  its  stanchest  sup- 
porters, refers  to  these  relations  as  follows  : l  "  One  may  well  assume  that 
the  most  limited  and  most  individualized  paths  of  conduction  for  the  light- 
impulses  within  the  retina  always  consist  of  an  entire  group  of  bipolars 
which  transfer  their  impressions  to  a  single  ganglion-cell.  The  terminal 
ramification  of  the  ganglion-cell  is  greatly  expanded  in  comparison  with 
the  lower  arborizations  of  the  bipolars ;  it  is  therefore  possible  that  the 
ramification  of  a  single  ganglion-cell  comes  in  contact  with  a  more  or  less 
extended  group  of  bipolar  cells  and  receives  from  them  the  transmitted 
light-impulse.  The  most  extensive  paths  of  conduction,  consequently,  will 
be  afforded  by  the  diffuse  or  the  multistratified  ganglion -cells  to  which  is 
probably  transmitted  the  activity  from  a  large  number  of  bipolars.  .  .  . 

"  Finally,  the  object  of  multiplication  of  the  surfaces  of  contact,  or  the 
horizontal  arborizations  within  the  inner  plexiform  layer,  appears  to  be  to 
render  possible  a  large  number  of  fairly  isolated  independent  paths  of  con- 
duction within  a  limited  part  of  the  retina.  It  is  evident  that  were  but  a 
single  contact  stratum  present  within  the  inner  plexiform  layer  to  receive 
all  the  voluminous  and  extended  ramifications  contributed  by  the  two 
factors  of  the  apparatus  for  nervous  conduction  (the  arborizations  of  the 
bipolars  and  the  compressed  ramifications  of  the  ganglion-cells),  the  fairly 
isolated  impulses  conveyed  from  the  visual  cells  would  be  confused  within 
this  stratum  to  a  general  impulse,  and  so  the  greater  part  of  the  distinct- 
ness of  a  perception  be  lost." 

The  Layer  of  Nerve-Fibres. — The  vast  majority  of  the  nerve-fibres 
composing  this  layer  of  the  retina  are  the  continuations  of  the  centrally 
coursing  axis-cylinders  of  the  ganglion-cells  above  discussed ;  it  is  evi- 
dent, therefore,  that  the  customary  manner  of  speaking  of  the  fibres  as 
passing  from  the  optic  nerve  to  the  various  retinal  areas  is  a  conventional- 
ism, the  nerve-fibres  really  issuing  from  the  ganglion-cells  and  converging 
towards  the  optic  entrance  in  their  course  to  the  brain-centres  by  way  of 
the  optic  nerve  and  tract. 

The  direct  continuity  between  the  ganglion-cells  and  the  filaments  com- 
posing the  fibre  layer  has  long  been  recognized,  Corti 2  having  pointed  out 
their  connection  almost  half  a  century  ago  ;  the  details  of  the  distribution 
and  the  relations  of  the  ascending  protoplasmic  processes  of  the  ganglionic 
elements,  as  already  described,  on  the  contrary,  are  among  the  most  recent 
additions  to  retinal  anatomy. 

The  nervous  filaments  of  the  fibre  layer  are  generally  of  small  or  mod- 
erate size,  but  a  limited  number  of  very  large  fibres  are  also  present  which 
have  as  their  presiding  elements  the  ganglion-cells  of  exceptional  size. 

1  Cajal:  op.  cit.,  p.  141. 

a  Corti :  Beitrag  zur  Anatomic  der  Retina,  Muller's  Archiv,  1850. 


316  THE    MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 

The  nerve-fibres  arise  from  the  cells  as  axis-cylinder  processes  which  almost 
at  once  assume  a  horizontal  course  and  become  associated  with  others  in 
coarser  or  finer  bundles.  The  fibres  which  possess  an  unusually  large  diam- 
eter, from  .003  to  .005  millimetre,  generally  form  groups  limited  to  from 
four  to  six  axis-cylinders.  According  to  Tartuferi,1  all  the  nerve-fibres  of 
the  retina  exhibit  round  or  oval  varicosities  distributed  at  more  or  less 
regular  intervals  along  their  course,  the  fibres  consisting  of  the  naked  axis- 
cylinders  alone ;  Dogiel,2  however,  regards  the  varicosities  as  dependent  in 
a  measure  both  upon  the  freshness  of  the  tissue  examined  and  upon  the  size 
of  the  fibres,  since  he  has  found  that  in  the  perfectly  fresh  retina  the  vari- 
cosities are  small  and  inconspicuous,  and,  further,  that  the  largest  irregu- 
larities occur  along  the  axis-cylinders  of  greatest  diameter. 

Occasionally  the  nerve-fibres  of  the  human  retina  present  a  variation 
in  the  character  of  their  coats,  since  in  some  cases  the  medullary  sub- 
stance, the  white  matter  of  Schwann,  forms  a  covering  of  the  axis-cylin- 
ders before  the  fibres  have  passed  through  the  lamina  cribrosa,  the  usual 
position  at  which  they  obtain  their  medullary  investment.  In  these  rare 
instances  of  the  premature  acquisition  of  this  coat,  the  so-called  "  retention 
of  the  marrow-sheath,"  the  bundles  of  medullated  fibres  become  very 
conspicuous  when  seen  with  the  ophthalmoscope,  appearing  as  marked 
white  tracts  radiating  from  the  optic  papilla,  in  strong  contrast  to  the  ad- 
jacent parts  of  the  fibre  layer  retaining  their  usual  transparency.  It  is 
of  interest  to  note  that  in  some  of  the  lower  animals,  as  in  the  rabbit, 
bundles  of  medullated  fibres  are  usually  present  at  the  lateral  margins  of 
the  optic  disk. 

In  addition  to  the  centrally  coursing  fibres,  the  presence  of  very  fine 
peripherally  directed  or  "  centrifugal"  nerve-fibres  has  been  established ; 
these  terminate  practically  within  the  inner  plexiform  layer  and  have  no 
discoverable  connection  with  the  cells  of  the  ganglion  layer.  Cajal 3  has 
succeeded  in  demonstrating  two  varieties  of  such  centrifugal  fibres, — those 
which  ascend  through  the  inner  plexiform  stratum  and  end  in  free  vari- 
cose ramifications  in  relation  with  the  bodies  and  descending  processes  of 
the  amacrine  cells,  and  those  which  penetrate  the  plexiform  stratum  to 
various  levels  and  then  end  in  horizontal  twigs.  Regarding  the  central 
connections  of  the  centrifugal  fibres  little  is  known  with  certainty  beyond 
their  issuing  from  the  fibre  layer  and  assuming  a  vertical  course. 

In  their  ultimate  structure  the  axis-cylinders  of  the  retinal  fibres 
resemble  the  general  composition  of  the  nerve-cells,  consisting  of  deeply 
staining  fibrillse  and  an  interfibrillar  substance  which  possesses  but  weak 
affinity  for  dyes.  The  interfibrillar  matrix — the  axoplasm  of  SchieiFer- 

1  Tartuferi:  Sull'  anatomia  della  retina,  Internat.  Monatsschr.  f.  Anat.  u.  Physiol., 
1887. 

J  Dogiel:  Ueber  die  nervosen  Elemente  in  der  Ketina  des  Menschen,  Archiv  f.  mik. 
Anat.,  Bd.  XL.,  1892. 

3  Cajal:  loc.  cit.,  p.  143. 


THE   MICROSCOPICAL   ANATOMY   OP  THE   EYEBALL.  317 

decker 1— exists  in  such  meagre  amount  along  the  ordinary  course  of  the 
axis-cylinder  that  the  fibrillar  structure  of  the  latter  is  seldom  evident  in 
methylene-blue  preparations  ;  at  the  point  of  emergence  of  the  axis-cylinder 
from  the  ganglion-cell,  however,  the  fibrillation  is  distinct,  owing  to  the 
separation  of  the  component  threads  by  the  greater  quantity  of  the  axoplasm 
which  there  exists. 

The  individual  nerve-fibres  soon  become  grouped  into  bundles,  which 
while  pursuing  generally  radiating  courses  having  the  optic  entrance  as  the 
common  objective  point,  freely  intermingle  and  form  a  reticulum. 

Assuming,  as  a  matter  of  convenience,  that  the  optic  fibres  proceed  from 
the  nerve  towards  the  ora  serrata  in  spreading  over  the  retinal  area,  the 
disposition  of  the  bundles,  as  seen  in  surface  views,  presents  some  variation 
in  the  two  halves  of  the  nervous  sheet. 

The  direction  of  the  nerve-fibre  bundles  contained  within  the  mesial  or 
nasal  half  of  the  retina  is  strictly  radial ;  within  the  lateral  or  temporal  seg- 
ment, on  the  other  hand,  the  presence  of  the  macula  lutea  produces  a  dis- 
turbance of  the  typical  course  of  the  fibres,  since  the  space  between  the 
macula  and  the  optic  entrance  is  traversed  by  from  twenty-five  to  thirty 
bundles  of  exceptional  delicacy  which  possess  an  almost  straight  path 
between  their  point  of  entrance  and  the  macula,  within  which  they  dis- 
appear. These  groups  of  fibres  which  pass  between  the  yellow  spot  and 
the  optic  entrance  collectively  constitute  the  macular  bundle  of  Michel,2  by 
whom  the  arrangement  of  the  fibres  has  been  carefully  studied. 

In  consequence  of  the  departure  from  the  typical  radial  arrangement 
which  the  delicate  macular  bundle  makes,  the  adjacent  fibres  suffer  deflec- 
tion, the  upper  and  lower  bundles,  after  a  limited  radial  course,  arching 
above  and  below  the  macular  area.  Those  immediately  bounding  the 
macula,  after  sweeping  around  the  latter  in  bold  curves,  unite,  while  those 
adjacent,  but  less  closely  related,  pass  beyond  the  yellow  spot  and  after  a 
time  resume  their  typical  radial  disposition.  Towards  the  end  of  their 
course  from  the  optic  entrance  to  their  destination  the  strands  composing  the 
macular  bundle  become  smaller,  owing  to  the  deviation  of  many  fibres  to 
take  part  in  the  formation  of  a  rich  plexus.  In  the  vicinity  of  the  macula 
the  bundles  break  up  into  a  number  of  smaller  fasciculi ;  of  the  latter,  some 
become  resolved  into  individual  nerve-fibres  which  disappear  within  the 
macular  area,  while  others,  according  to  Dogiel,3  take  part  in  the  formation 
of  a  ring  of  fibres  which  encirclas  the  fovea  centralis.  Delicate  twigs  pass 
from  this  annular  bundle  obliquely  along  the  walls  of  the  depression,  ami. 
in  conjunction  with  additional  fibres  from  other  bundles,  form  a  wide- 
meshed  plexus  of  nervous  fibrillae  which  occupies  the  fundus,  as  well  as  the 

1  Schiefferdecker  und  Kossel :  Gewebelehre  mit  besonderer  Berucksichtigung  des 
menschlichen  Korpers,  Bd.  I.,  p.  200,  1891. 

"  Michel :  Ueber  die  Ausstrahlungsweise  der  Opticusfasern  in  der  menschlichen  Retina, 
Beitrage  zur  Anatomie  und  Physiologic,  Festschrift  fur  Ludwig,  1874. 

3  Dogiel:  loc.  cit.,  p.  32. 


318  THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 

sides,  of  the  fovea.  Dogiel  denies  that  the  fibre  layer  disappears  about 
four-tenths  of  a  millimetre  from  the  foveal  centre,  as  stated  by  Schwalbe l 
and  others,  but  maintains  that  it  is  present  within  the  fundus  fovese  as  an 
attenuated  layer  composed  of  minute  bundles  and  individual  axis-cylinders. 

According  to  this  same  observer,  the  arching  nerve-bundles  which  en- 
close the  macula  at  some  little  distance  from  the  boundary  of  the  yellow 
spot,  on  approaching  one  another  from  the  opposite  sides,  break  up  into 
small  fasciculi  or  separate  fibres  which  become  interwoven  to  constitute  a 
characteristic  net-work.  The  latter  plexus  appears  as  a  narrow  band, 
from  one  to  one  and  a  half  millimetres  in  breadth,  which  begins  about  one 
millimetre  beyond  the  outer  side  of  the  macula  and  extends  from  three  to 
six  millimetres  to  a  point  where  the  nerve-bundles  resume  their  usual  radial 
disposition. 

The  reticulum  formed  by  the  bundles  of  nerve-fibres  varies  in  the  size 
of  its  meshes  in  the  different  parts  of  the  retina,  the  net-work  becoming 
more  open  and  coarse,  the  bundles  at  the  same  time  growing  thinner,  in  the 
vicinity  of  the  ora  serrata,  at  which  point  all  traces  of  a  fibre  layer  disap- 
pear. On  approaching  the  optic  entrance  the  mesh-work  is  very  dense,  and 
the  general  thickness  of  the  fibre  layer  undergoes  a  corresponding  increase. 

Since  the  components  of  the  fibre  layer  depend  upon  the  ganglion-cells 
for  their  origin,  a  marked  inherent  variation  in  the  thickness  of  this  stratum 
in  the  several  portions  of  the  retinal  area  is  to  be  anticipated  from  the  in- 
equality in  the  distribution  of  the  ganglion-cells.  Where  these  are  numer- 
ous and  constitute  a  compact  row,  as  in  the  vicinity  of  the  posterior  pole, 
the  nerve-fibres  are  likewise  present  in  greater  profusion ;  towards  the 
periphery,  on  the  other  hand,  where  the  cells  occur  with  relative  infre- 
quency,  the  fibres  are  few  and  the  entire  thickness  of  the  layer  is  reduced. 

In  addition  to  these  inherent  differences  depending  upon  the  relative 
number  of  the  neWe-fibres  originating  within  a  given  area,  the  progressive 
accumulation  of  the  fibres  as  they  course  towards  their  common  place  of 
exit,  the  optic  papilla,  results  in  a  conspicuous  increase  in  the  entire  thick- 
ness of  the  fibre  layer  at  this  place. 

At  the  margin  of  the  optic  entrance  the  fibre  layer  constitutes  more  than 
half  the  entire  thickness  of  the  retina  ;  on  leaving  this  position,  however, 
the  stratum  rapidly  diminishes,  so  that  while  at  a  distance  of  one-half  milli- 
metre it  still  measures  two-tenths  of  a  millimetre,  at  a  point  one  centimetre 
farther  advanced  the  layer  possesses  an  insignificant  depth.  The  excep- 
tional thickness  at  the  papillar  margin  depends  not  upon  superimposed 
layers  of  bundles, — an  arrangement  existing  only  throughout  a  limited 
area  above  the  macula, — but  upon  the  larger  size  of  the  individual  bundles 
as  they  approach  their  point  of  exit. 

The  relation  and  position  of  the  bundles  of  nerve-fibres  proceeding  from 
the  various  portions  of  the  retinal  field  within  the  optic  nerve  have  elicited 

1  Schwalbe:  Lehrbuch  der  Anatomic  der  Sinnesorgane,  1887. 


THE   MICROSCOPICAL   ANATOMY   OP   THE   EYEBALL.  319 

much  attention  and  study.  The  observations  of  Gudden,1  Michel,*  Ganser  s 
Samelsohn,4  Schwalbe,5  and  others  have  shown  that  the  arrangement  of  the 
decussating  and  non-decussating  bundles  greatly  varies  among  the  lower 
animals ;  in  the  latter,  as  a  rule,  the  crossed  fibres  predominate,  while  in 
man  the  uncrossed  are  the  more  numerous. 

The  non-decussating  bundles  of  the  human  retina  are  derived  from  the 
lateral  or  temporal  two-thirds  of  both  retinae ;  the  crossed  fibres  largely 
proceed  from  the  inner  or  nasal  third,  but  undoubtedly  many  fibres  also  are 
contributed  by  the  outer  zones,  the  temporal  two-thirds,  therefore,  being 
represented  by  both  crossed  and  non-decussating  bundles. 

The  macular  fibres  include  both  varieties,  and  occupy  a  position  within 
the  optic  nerve  near  the  eyeball,  corresponding  in  transverse  sections  to  a 
narrow  triangular  area  the  apex  of  which  reaches  the  central  vessels  and 
its  base  the  periphery  of  the  optic  nerve  within  the  outer  and  lower  quad- 
rant. During  their  further  course  towards  the  chiasm  the  macular  bundle 
changes  its  relations,  assuming  gradually  a  more  central  and  dorsal  position, 
until  within  the  chiasm  they  lie  collected  on  the  upper  surface,  close  beneath 
the  brain.  The  well-known  case  of  double  central  scotoma  of  the  macula, 
recorded  by  Vossius,6  furnished  an  interesting  observation,  since  the  exist- 
ence of  two  distinct  atrophic  areas  corresponded  to  the  paths  of  the  crossed 
and  uncrossed  bundles,  showing  that  the  assumption  as  to  the  composite 
structure  of  the  macular  bundle  is  well  founded. 

THE   RETINAL   SUSTENTACULAR   TISSUE. 

As  in  other  parts  of  the  wall  of  the  neural  tube,  so  also  in  the  retinal 
area,  as  represented  by  the  optic  vesicles,  the  elements  undergo  differentia- 
tion into  two  groups, — the  nerve-cells  and  the  closely  related  neuro-epithe- 
lium,  and  the  supporting  tissue  or  neuroglia. 

The  neuroglia  or  sustentaeular  tissue,  a  derivative  of  the  ectoderm,  is 
present  within  the  retina  in  two  forms, — (1)  as  the  conspicuous  radial  fibres 
of  Muller,  and  (2)  as  the  spider-cells. 

The  fibres  of  Muller  constitute  a  sustaining  framework  of  considerable 
complexity  which  supports  the  delicate  retinal  elements  and  enjoys  an  inti- 
mate relation  to  all  parts  of  the  highly  specialized  nervous  structures.  The 
Miillerian  fibres  are  modified  cells  which  extend  through  the  entire  thick- 


1  Gudden :  Ueber  die  Kreuzung  der  Nervenfasern  im  Chiasma  nervorum  opticorum, 
Archiv  f.  Ophthalmol.,  Bd.  xxv.,  1879. 

2  Michel:    Zur  Frage  der  Sehnervenkreuzung  im  Chiasma,  Archiv  f.  Ophthalmol., 
Bd.  xxin.,  1877. 

3 Ganser:  Ueber  die  periphere  und  centrale  Anordnung  der  Sehnervenfasern  u.  s.  w., 
Archiv  f.  Psychiatric,  Bd.  xni.,  1882. 

*  Samelsohn  :  Zur  Topographic  des  Faserverlaufes  im  menschlichen  Sehnerven,  M 

Centralblatt,  No.  23,  1880. 

5  Schwalbe  :  Lehrbuch  der  Anatomie  der  Sinnesorgane,  1887. 

6  Vossius :  Ein    Fall  von  beiderseitigem  centralem  Skotom    mit   pathologic 
tomischem  Befund,  Archiv  f.  Ophthalmol.,  Bd.  xxvni.,  1883. 


320 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


Flo. 


ness  of  the  retina,  beginning  on  the  inner  surface  in  expanded  pyramidal 
bases  and  terminating  externally  in  delicate  septa  which  pass  between  the 
rods  and  cones.  While  in  general  the  fibres  constitute  slender  nucleated 

columns,  their  contributions  to  the  support  of 
the  elements  of  the  individual  retinal  layers 
vary  in  detail,  so  that  the  isolated  fibres  present 
a  succession  of  striking  differences  in  various 
parts  of  their  course. 

The  Miillerian  fibres  become  swollen  and 
indistinct  after  treatment  with  acetic  acid  and 
dilute  alkalies.  They  are  not,  however,  de- 
stroyed by  boiling  in  water.  The  most  satis- 
factory exhibitions  of  the  sustentacular  fibres 
of  the  retina  are  had  in  preparations  made  by 
the  modified  Golgi  process  or  stained  by  the 
second  of  the  hsematoxylin  methods  suggested 
by  Wolters,1  those  prepared  by  the  last- 
mentioned  procedure  being  particularly  in- 
structive. 

The  fibres  of  Miiller  are  found  in  all  parts 
of  the  retina,  but  they  are  unusually  conspic- 
uous in  the  vicinity  of  the  ora  serrata,  where 
they  are  especially  distinct  and  numerous. 
Within  the  macular  region,  according  to  Do- 
giel,2  they  are  also  very  well  developed  and 
of  exceptional  length. 

At  a  level  corresponding  to  the  juncture  of 
the  rods  and  cones  with  the  bodies  of  the  visual 
cells  the  adjacent  sustentacular  cells  come  into 
apposition  and  form  a  seemingly  continuous, 
fenestrated  lamella  through  which  the  outer 
segments  of  the  visual  cells — the  rods  and 
cones — project.  This  grill-like  structure  when 

seen  in  profile  constitutes  the  apparently  continuous  membrana  limitans 
exierna. 

The  external  surface  of  this  horizontally  extending  perforated  lamina  is 
beset  with  minute  vertical  processes  which  lie  between  the  inner  segments 
of  the  rods  and  cones  and  thus  probably  effect  an  isolation  of  the  per- 
cipient elements. 

The  expansions  of  the  fibres  of  Miiller  within  the  outer  nuclear  layer 
are  quite  complex  in  their  arrangement.  The  numerous  lateral  lamellar 


Supporting  fibres  of  Miiller, 
from  the  peripheral  area  of  retina 
of  ox,  after  Golgi  staining.  (Cajal.)— 
a,  a,  descending  lateral  processes 
passing  from  the  nucleated  enlarge- 
ment (6)  within  the  inner  nuclear 
layer. 


1  Wolters :    Drei    neue    Methoden   zur    Mark-    und    Axencylinderfarbung   mittelst 
Hsematoxylin,  Zeitschrift  f.  wissensch.  Mikroskopie,  Bd.  vn.,  1891. 

2  Dogiel :  Neuroglia  der  Retina  des  Menschen,  Archiv  f.  mik.  Anat,  Bd.  XLI.,  1893. 


THE   MICROSCOPICAL  ANATOMY  OF  THE  EYEBALL.  321 

extensions  given  off  from  the  fibres  break  up  into  secondary  plate-like 
septa  and  pass  between  the  visual  cells,  around 
which  they  form  a  close  investment,  whereby  Fl°-  69. 

lateral  diffusion  of  the  impulses  received  by 
these  elements  is  in  a  great  measure  prevented. 
The  intricate  mesh-work  formed  by  the  collat- 
eral extensions  of  this  part  of  the  fibre  consti- 
tutes a  striking  picture  when  seen  after  Golgi 
staining. 

The  contributions  from  the  sustentacular 
fibres  to  the  outer  plexiform  layer  are  very 
inconspicuous,  since  they  consist  of  lateral 
projections  of  such  delicacy  that  they  soon 
become  lost  amidst  the  maze  of  ramifica- 
tions proceeding  from  the  nerve-cells  sending 
their  processes  within  this  stratum.  The 
amount  of  sustaining  tissue  within  the  outer 
plexiform  layer  is  so  inconsiderable  that  its 
presence  has  been  overlooked  by  many  authors ; 
there  is,  however,  no  doubt  that  a  delicate 
framework  derived  from  the  Miillerian  fibres 
aids  in  supporting  the  constituents  of  the 
stratum,  a  fact  emphasized  by  Schiefferdecker,1 
Merkel,2  and  Dogiel.3 

Within  the  inner  nuclear  layer  the  susten- 
tacular fibre  usually  presents  its  greatest  width, 
its  considerable  but  uncertain  thickness  being 
augmented  by  the  presence  of  a  marked  local 
expansion  which  corresponds  to  the  position 
of  the  nucleus  of  the  fibre.  The  spherical 
or  ellipsoidal  nucleus  is  usually  surrounded 
by  an  area  which  suggests  the  earlier  condi- 
tion of  the  protoplasm  of  the  fibre  before  it 
had  so  completely  lost  the  characteristics  of 
the  primitive  cell. 

The  lateral  expansions  of  the  fibre  within 
this  stratum  are  less  extensive  than  those  found  within  the  outer  nuclear 
layer,  and,  while  affording  an  important  means  of  support  for  the  nervous 
elements,  suffice  to  form  only  an  incomplete  insulation  for  the  bipolars  and 
the  amacrine  cells  which  are  included  within  the  mesh-work.  The  enlarge- 
ment of  the  fibre  corresponding  to  the  position  of  the  nucleus,  according 

1  Schiefferdecker :  Studien  zur  vergleichenden  Histologie  der  Retina,  Archiv  f.  mik. 
Anat.,  Bd.  xxvm.,  1886. 

a  Merkel:  Ergebnisse  der  Anat.  u.  Entwickelung,  Bd.  II.,  1893,  S 
»  Dogiel:  Neuroglia  der  Retina  des  Menschen,  Archiv  f.  mik.  Anat.,  Bd.  XLi.,  U 
VOL.  I.— 21 


Supporting  fibres  of  Mailer, 
from  retina  of  ox,  in  the  vicin- 
ity of  the  papilla,  after  Golgi 
staining.  (Cajal.)  —  a,  processes 
supporting  and  isolating  the  rods 
and  cones ;  b,  outer  nuclear  layer ; 
c,  outer  plexiform  layer ;  d,  inner 
nuclear  layer,  containing  the  ex- 
panded and  nucleated  portions 
of  the  fibres;  e,  inner  plexiform 
layer ;  /,  ganglion  layer ;  g,  nerve 
fibre  layer,  through  which  the 
long  branched  fibres  extend. 


322  THE   MICROSCOPICAL   ANATOMY   OP   THE    EYEBALL. 

to  Cajal,1  frequently  gives  off  a  descending  process  of  considerable  size 
which  passes  centrally  into  the  subjacent  plexiform  layer,  within  which  it 
terminates  after  breaking  up  into  a  number  of  endings. 

The  granular  or  finely  reticular  appearance  of  the  inner  plexiform  layer 
is  largely  dependent  upon  the  intricate  ramifications  of  the  numberless 
lateral  processes  given  off  from  the  Miillerian  fibres  during  their  course 
through  the  stratum.  The  lateral  extensions  at  once  subdivide  into  delicate 
fibrils,  which  pass  in  a  generally  horizontal  direction  and  terminate  among 
the  arborizations  of  the  nervous  elements,  to  which  they  contribute  material 
support.  In  their  course  they  are  so  disposed  that  horizontal  spaces  are 
continually  being  left  between  the  bundles  of  fibrillse,  within  which  clefts 
the  expansions  of  the  nerve-cells  find  place.  As  already  noted,  the  endings 
of  the  descending  processes  given  off  from  the  fibres  at  the  nuclear  enlarge- 
ment also  contribute  to  the  maze  of  sustaining  tissue  within  the  plexiform 
stratum. 

The  ganglion  layer  receives  relatively  short,  thick,  and  irregular  pro- 
cesses from  the  compressed  Miillerian  fibres;  these  plate-like  processes 
extend  between  the  large  nerve-cells  of  the  stratum,  which  they  imperfectly 
surround  and  isolate,  the  ganglion-cells  being  lodged  within  niche-like 
recesses  which  correspond  in  size  with  the  dimensions  of  the  nervous 
element.  Within  the  deeper  part  of  this  layer  the  main  column  of  the 
fibre  frequently  divides  into  two  branches,  which  continue  through  the  fibre 
layer  and  end  in  expanded  pyramidal  or  conical  bases  or  foot-plates.  The 
bundles  of  nerve-fibres  take  advantage  of  the  division  of  the  supporting 
fibre  and  pass  between  the  diverging  limbs  without  deflection.  Within  the 
fibre  layer  the  sustentacular  fibres  give  off  lateral  processes  in  various 
directions,  which,  in  the  form  of  fibres  and  plates,  join  with  one  another 
and  constitute  a  series  of  partitions  which  separate  the  bundles  of  retinal 
nerve-fibres. 

In  the  vicinity  of  the  optic  papilla,  when  the  bundles  of  nerve-fibres 
are  especially  numerous  and  large,  the  sustentacular  fibres  not  infrequently 
divide  into  three  or  more  limbs,  each  of  which  terminates  in  a  conical  base 
or  foot. 

The  expanded  bases  of  the  Miillerian  fibres  lie  in  close  apposition  on  the 
inner  side  of  the  fibre  layer,  and,  when  seen  in  profile,  seemingly  constitute 
a  distinct  lamella,  which  has  long  been  described  as  the  membrana  limitans 
intema.  Surface  views  of  the  bases  of  the  sustentacular  fibres  after  silver 
staining  are  very  striking,  the  deeply  colored  lines  of  cement-substance 
between  the  expanded  ends  of  the  fibres  defining  their  boundaries  with 
great  distinctness  and  producing  a  picture  strongly  recalling  endothelium. 

The  outlines  of  the  individual  bases,  as  exhibited  by  the  silver  lines, 
are  very  irregular  in  both  size  and  form,  the  figures  varying  from  limited 
polyhedral  areas  to  large  irregular  fields.  In  addition  to  the  usual  varia- 

1  Cajal :  Die  Ketina  der  Wirbelthiere,  Wiesbaden,  1894. 


THE   MICROSCOPICAL  ANATOMY   OP  THE   EYEBALL.  323 

tions  in  the  size  and  form  of  the  individual  fields  seen  in  all  parts  of  the 
retina,  silvered  preparations  of  the  peripheral  area  from  the  vicinity  of  the 
ora  serrata  demonstrate  that  the  average  size  of  the  bases  of  the  fibres  within 
this  region  is  markedly  increased,  the  greater  expansion  of  the  base  corre- 
sponding to  the  greater  prominence  of  the  Mullerian  fibres  at  this  point 
General  diminution  of  the  size  of  the  areas,  on  the  other  hand,  is  very  ap- 
parent around  the  optic  entrance,  the  peculiarity  depending,  probably,  upon 
the  more  freely  branched  condition  of  the  fibres  in  this  region  and  the  con- 


FIG.  70. 


Silver  markings  of  surface  of  human  retina  corresponding  to  bases  of  fibres  of  Miiller  ;  from  a  prepa- 
ration of  Professor  Norris.    Magnified  350  diameters. 

sequent  smaller  size  of  the  individual  bases.  Conspicuous  modifications  of 
the  silver  picture  are  presented  within  the  areas  corresponding  to  the  course 
of  the  larger  retinal  blood-vessels.  As  first  pointed  out  by  Schelske,1  and 
later  confirmed  by  the  observations  of  Schwalbe2  and  of  Norris  and 
Shakespeare,3  the  position  of  the  larger  vessels  may  be  inferred  from  the 
notable  modifications  in  the  size  and  disposition  of  the  bases  of  the  fibres, 
bver  the  course  of  the  retinal  blood-vessels — both  arteries  and  veins— the 
basal  areas  become  much  narrowed  and  assume  a  regular  arrangement  in 
which  the  long  dimensions  of  the  fields  are  disposed  at  right  angles  or 
across  the  axis  of  the  blood-vessel.  On  each  side  of  the  vessel  the  usual 

1  Schelske :  Ueber  die  Membrana  limitans  der  menschlichen  Netzhaut,  Virchow's 
Archiv,  Bd.  xxvin.,  1863. 

2  Schwalbe:  Die  Retina,  in  Graefe  u.  Saemisch's  Handbuch,  Bd.  I.,  1874,  S. 

8  Norris  and  Shakespeare :  A  Contribution  to  the  Anatomy  of  the  Human  Retina, 
American  Journal  of  the  Medical  Sciences,  October,  1877. 


324 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


figures  abruptly  give  place  to  the  greatly  narrowed  areas  which  extend 
almost,  but  not  entirely,  to  the  middle  of  the  course  of  the  vessel,  the  inter- 


FIG.  71. 


Superficial  surface  markings  from  silvered  humau  retina.    (Norris  and  Shakespeare.)    Magnified  350 

diameters. 


FIG.  72. 


iiiiinuiiniin»iiiiiiitiHi>nillmi 
i^u^uuiHitlUiMtt      !SS5SiaB'!»«IIM 


*»»?»»» 
injiiijj 


Section  of  human  retina  through  the  macula,  showing  the  disposition  of  the  fibres  of  Miiller.  (Dogiel.) 

vening  space  being  occupied  by  very  small  additional  areas  which  consti- 
tute a  band  of  minute  polygonal  fields  lying  over  the  centre  of  the  vessel. 
The  relation  of  these  less  usual  figures  to  the  Mullerian  fibres  is  suggested 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL.  325 

in  the  modified  disposition  of  the  bases  of  the  latter  at  those  points  where 
they  come  into  contact  with  the  larger  blood-vessels.  At  such  places  the 
fibres  diverge  to  allow  the  vessel  to  pass  between,  their  bases  undergoing 
elongation  to  compensate  for  the  additional  area  to  be  covered.  Norris  and 
Shakespeare  describe  a  double  layer  of  endothelial  markings  in  surface 
preparations  of  silvered  human  retina ;  the  outer  stratum  consists  of  fields 
very  irregular  both  in  size  and  in  form,  the  inner  of  much  smaller  areas 
which  correspond  to  the  outlines  of  the  bases  of  the  Miillerian  fibres. 

At  variance  with  the  usually  accepted  opinion  that  within  the  macula 
the  sustentacular  fibres  are  rudimentary  and  without  bases,  Dogiel l  and 
Bach 2  have  shown  the  correctness  of  the  view  held  by  Merkel 3  that  the 
fibres  of  Miiller  attain  exceptional  length  and  are  especially  prominent 
within  this  area.  In  retinal  preparations  after  the  Golgi  method  these 
fibres  appear  with  great  distinctness,  and  are  shown  to  possess  the  same 
relations  to  the  several  layers  of  nervous  elements,  almost  as  far  as  the 
neuro-epithelial  stratum,  as  they  do  in  other  parts  of  the  retina,  with  the 
single  difference  that  the  fibres  break  up  into  an  unusual  number  of  plate- 
like  septa  within  the  ganglion  layers.  These  partitions  present  depressions 
within  which  the  nerve-cells  are  lodged,  the  niches  varying  in  size  and  in 
form  to  adapt  them  to  the  cells. 

On  reaching  the  vicinity  of  the  inner  extremities  of  the  cone-cells,  how- 
ever, all  sustentacular  fibres  within  the  macular  area  undergo  a  remarkable 
and  characteristic  deflection  in  their  course ;  the  fibres,  on  arriving  at  the 
outer  part  of  the  external  plexiform  layer,  bend  more  or  less  sharply,  some- 
times almost  at  right  angles,  towards  the  fovea,  and  maintain  for  a  variable 
distance  an  oblique  course  towards  the  visual  cells.  On  attaining  the  cone- 
granules,  the  fibres  once  more  change  their  direction  and  reassume  their 
original,  generally  vertical,  course,  which  they  retain  as  far  as  the  external 
limiting  membrane,  giving  off  numerous  plate-like  extensions  for  the 
reception  and  support  of  those  portions  of  the  visual  cells  wrhich  con- 
stitute the  outer  nuclear  layer  at  this  position.  The  macular  sustentacular 
fibres,  therefore,  consist  of  three  portions,  an  inner  and  an  outer  vertical 
and  an  intermediate  obliquely  horizontal  segment.  The  latter  is  much 
compressed,  and  by  no  means  as  much  laterally  expanded  as  the  other  parts 
of  the  fibre. 

In  addition  to  the  fibres  of  Miiller,  which  undoubtedly  constitute 
the  important  sustaining  framework,  the  existence  of  stellate  neuroglia-  or 
spider-cells  has  been  demonstrated  by  the  investigations  of  Borysiekiewicz,4 

1  Dogiel :  Neuroglia  der  Ketina  des  Menschen,  Archiv  f.  mik.  Anat,  Bd.  XLI.,  189 

1  Bach:  Die  menschliche  Netzhaut  nach  Untersuchungen  mit  der  Golgi-Cajal'schen 
Methode,  Archiv  f.  Ophthalmol.,  Bd.  XLI.,  1895. 

s  Merkel :  Ueber  die  Macula  lutea  des  Menschen  und  die  Ora  Serrata  einigcr  Wirbel- 
thiere,  Leipzig,  1870. 

*  Borysiekiewicz :  Untersuchungen  iiber  den  feineren  Ban  der  Netzhaut,  Wi.n, 
1887. 


326 


THE    MICROSCOPICAL   ANATOMY    OF    THE    EYEBALL. 


Cajal,1  Dogiel,2  Greeff,3  and  others,  although  these  elements  had  been  im- 
perfectly observed  years  before  by  Golgi  and  Manfredi 4  and  by  Schwalbe.5 
The  spider-cells  enjoy  but  a  limited  distribution  within  the  retina,  being 
almost  entirely  confined  to  the  layer  of  nerve-fibres  and  its  continuation 
brainward.  Bach,6  however,  records  their  presence  within  the  layer  of 
ganglion-cells.  They  occur  in  locations  where  the  fibre  layer  is  best  de- 
veloped, and  hence  are  particularly  numerous  in  the  vicinity  of  the  optic 
entrance. 

The  neuroglia-cells  appear  as  small  stellate  bodies,  somewhat  flattened, 
and  lodged  between  the  bundles  of  nerve-fibres ;  their  characteristic  ap- 

Fia.  73. 


Portion  of  bundle  of  fibre  layer  of  retina  in  the  vicinity  of  the  optic  papilla,  showing  the  neuroglia- 
cells  (a)  after  Golgi  staining.    (Dogiel.) 

pearance  is  due  to  the  large  number  of  delicate  fibrillar  processes  which  pass 
from  the  cell-body  in  various  directions.  These  fibrillae  are  usually  of 
considerable  length  and  are  at  first  distinctly  grouped ;  the  disposition  of 
the  flattened  cells  between  the  adjacent  bundles  of  nerve-fibres  results  in 
the  enclosure  of  the  latter  by  the  superficial  interfascicular  net-works  formed 
by  the  interwoven  processes  of  the  spider-cells.  The  stellate  neuroglia  ele- 
ments occupying  the  fibre  layer  of  the  retina  or  the  optic  papilla  are  sur- 
passed in  size  by  those  situated  between  the  nerve-bundles  of  the  optic  nerve. 
The  foregoing  description  of  the  retinal  layers  applies  to  their  disposi- 
tion as  found  throughout  the  greater  part  of  the  nervous  tunic  :  two  regions, 
however,  require  particular  consideration,  on  account  of  the  important 
modifications  which  the  layers  undergo  in  these  particular  localities  ;  these 
specialized  areas  are  the  macula  lutea,  with  its  contained  fovea  centralis, 
and  the  ora  serrata. 

1  Cajal:  Die  Ketina  der  Wirbelthiere,  Wiesbaden,  1894,  S.  145. 

2  Dogiel:  Neuroglia  der  Ketina  des  Menschen,  Archiv  f.  mik.  Anat.,  Bd.  XLI.,  1893. 

3  Greeff:  Die  Morphologic  und  Physiologie  der  Spinnenzellen  im  Chiasma,  Sehm-rv 
und  in  der  Retina,  Verhandlungen  der  physiolog.  Gesellschaft  zu  Berlin,  1894. 

4  Golgi  and  Manfredi :  Annotazioni  istologiche  sulla  retina  del  cavallo,  Accad.  di  med. 
di  Torino,  9  Agosto,  1872. 

5  Schwalbe:  Die  Retina,  Graefe  und  Saemisch's  Handbuch,  Bd.  i.  18. 

"Bach:  Die  menschliche  Netzhaut  nach  Untersuchungen  mit  der  Golgi -Cajal'schen 
Methode,  Archiv  f.  Ophthalmol.,  Bd.  XLI.,  1895. 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


327 


THE   MACULA    LUTEA. 

As  already  noted,  the  immediate  vicinity  of  the  posterior  pole  of  the 
visual  axis  is  marked  by  a  specialized  retinal  area,  the  macula  lutea,  or 
the  yellow  spot,  which  surrounds  the  limited  fovea  centralis,  in  which  the 
visual  rays  produce  their  most  accurate  impressions. 

The  macula,  as  observed  after  death,  when  the  retina  no  longer  possesses 
the  transparency  of  the  living  tissue,  appears  as  a  distinctly  pigmented  area, 

FIG.  74. 


Surface  view  of  macular  area  of  human  retina.    (Dogiel.)-o.  fovea  centralis,  containing  a  wide- 
meshed  plexus  of  nerve-fibres ;  ft,  macula  lutea;  c,  macular  bundle;  d,  arched  margini 
sending  nerve-fibres  to  macula ;  e,  nerve-cells. 

the  pronounced  yellowish  tint  of  which  depends  upon  the  presence  of  dif 
fuse  coloring-matter  between  the  retinal  elements  within  the  plane  of  the 
visual  cells,  the  latter,  however,  being  devoid  of  color. 

The  outline  of  the  macular  area  is  almost  circular,  rather  than  elliptu 
as  usually  described,  the  oval  form  so  often  attributed  to  the  yell    •  spol 
depending  more  upon  ophthalmoscopic  appearances  than  upon  anat 


328  THE   MICROSCOPICAL   ANATOMY   OF    THE   EYEBALL. 

examination.  Johnson/  however,  insists  that  when  properly  observed  the 
macula  appears  during  life  as  circular.  The  investigations  of  Schmidt- 
Rimpler,2  as  well  as  the  drawings  of  Merkel3  and  Dogiel,4  sustain  the 
approximately  circular  form  of  the  macula.  The  greatest  diameter  of  the 
yellow  spot  measures  about  two  millimetres,  and  often  does  not  correspond 
accurately  with  the  horizontal  axis  of  the  eye,  making  with  the  latter  an 
angle  of  from  ten  to  fifteen  degrees. 

The  position  of  the  yellow  spot  in  relation  to  the  optic-nerve  entrance 
is  such  that  the  macula  lies  approximately  three  millimetres  external — three 
and  nine-tenths  millimetres,  according  to  Landolt8 — to  the  centre  of  the 
papilla,  and  slightly  lower,  being  about  one  millimetre  below  the  level  of 
the  disk.  The  direct  horizontal  course  of  the  macular  bundles  of  nerve- 
fibres  to  the  optic  papilla,  and  the  arching  disposition  of  the  adjacent  fibres, 
have  already  been  noted. 

The  fovea  centralis  appears  about  the  centre  of  the  macular  area  as  a 
dark-brown,  deeply  pigmented  spot ;  its  deep  color  is  not  due  to  any  special 
pigmentation  of  its  own,  but  to  the  exceptional  thinness  of  this  part  of  the 
retina,  in  consequence  of  which  the  superimposed  pigment  becomes  apparent 
in  an  unusual  degree. 

During  life,  when  the  beautifully  transparent  retina  allows  the  presence 
of  the  highly  vascular  choroid  to  become  apparent  as  the  general  red  reflex 
of  the  eye-ground,  as  seen  with  the  ophthalmoscope,  the  distinctive  color  of 
the  macular  area  is  entirely  masked,  the  fovea  alone  appearing  as  a  brownish- 
red  point.  Owing  to  the  absence  of  rods  within  the  fovea,  the  visual  purple 
is  wanting  in  this  region,  which  therefore  possesses  inherently  a  lighter 
tint  than  the  surrounding  retina,  sometimes  appearing  when  examined  with 
the  ophthalmoscope  as  a  minute  faintly  colored  spot.  The  foveal  reflex 
seen  with  the  mirror  is  due  to  the  direction  and  slope  of  the  sides  of  the 
pit,  the  variations  of  the  reflex  being  attributed  by  Johnson 6  to  changes  in 
the  shape  of  the  fovea.  With  the  opacity  of  the  retina  which  supervenes 
soon  after  death,  the  presence  of  the  characteristic  yellowish  pigment  grad- 
ually becomes  evident  within  the  macula. 

The  size  of  the  fovea  is  usually  stated  as  between  .2  and  .4  milli- 
metre in  diameter  by  various  authors,  including  Kuhnt,7  Schwalbe,8 

1  Johnson  :  Observations  on  the  Macula  Lutea,  Archives  of  Ophthalmology,  New 
York,  1892. 

2  Schmidt-Rimpler :  Die  Macula  lutea  anatomisch  und  ophthalmoscopisch,  Archiv  f. 
Ophthalmol.,  Bd.  xxi.,  1875. 

3  Merkel :  Handbuch  der  topograph.  Anatomie,  Bd.  i.,  1887. 

1  Dogiel :  Ueber  die  nervosen  Elemente  in  der  Retina  des  Menschen,  Archiv  f.  mik. 
Anat.,  Taf.  n.,  Bd.  XL.,  1892. 

5  Landolt:  Die  directe  Entfernung  zwischen  Macula  lutea  und  Nervus  opticus,  Med. 
Centralblatt,  No.  45,  1871. 

6  Johnson  :  loc.  cit. 

7  Kuhnt :  Ueber  den  Bau  der  Fovea  centralis  des  Menschen,  Sitzungsber.  d.  ophthal- 
mol.  Gesellsch.  in  Heidelberg,  1881. 

8  Schwalbe :  Anatomie  der  Sinnesorgane,  1887,  S.  89. 


THE   MICROSCOPICAL   ANATOMY  OP  THE   EYEBALL.  329 

Schafer,1  and  others,  these  measurements  having  been  given  by  H.  Muller  * 
forty  years  ago,  and  even  before  him  by  Michaelis.3  The  later  investiga- 
tions, however,  of  Dimmer,4  and  still  more  recently  of  Golding-Bird  and 
Schafer,5  show  that  the  accepted  foveal  diameters  are  too  small,  since  the 
fovea  measures  at  least  1.1  millimetres,  and  may  approach,  according  to 
Dimmer,  in  exceptional  cases  almost  two  millimetres  in  its  greatest  diameter. 

The  conspicuous  modifications  of  the  retinal  structure  within  the 
macula  and  the  fovea  have  claimed  the  attention  of  the  foremost  histolo- 
gists  from  the  days  of  Heinrich  Muller  to  the  present,  among  those  who 
have  particularly  studied  this  region  and,  in  most  cases,  supplied  drawings 
of  the  fovea  being  Henle,  Hulke,  Merkel,  Max  Schultze,  Krause,  Kuhnt, 
Schwalbe,  and  Cadiat.  While  the  descriptions  of  the  macular  area  given  by 
these  various  authors  differ  materially  as  to  details,  yet  all  agree  in  recog- 
nizing that  the  fundamental  changes  consist  in  a  marked  increase  in  the 
number  of  some  of  the  retinal  elements  within  the  macula,  followed  by  a 
rapid  thinning  out  of  all,  and  the  final  disappearance  of  certain  of  the 
retinal  layers  within  the  foveal  depression. 

In  the  immediate  vicinity  of  the  macula  the  ganglion- cells  are  so 
numerous  that  they  constitute  a  layer  from  two  to  three  cells  deep ;  on 
passing  into  the  yellow  spot  their  number  becomes  rapidly  augmented, 
until  where  best  developed  the  ganglion  layer  contains  from  six  to  eight 
rows  of  nerve-cells  and  constitutes  a  stratum  about  .07  millimetre  in 
thickness. 

The  changes  affecting  the  individual  layers  of  the  retina  within  the 
fovea  have  been  carefully  studied  anew  by  Golding-Bird  and  Schafer,  to 
whom  we  are  indebted  for  additional  accurate  information  concerning  the 
details  of  the  disposition  and  relation  of  the  retinal  elements  within  this 
area. 

On  reaching  the  margin  of  the  circular  foveal  depression,  the  greatly 
thickened  ganglion  layer  rapidly  diminishes  towards  the  centre  of  the 
basin-shaped  pit,  the  cells  becoming  less  closely  packed  and  much  fewer  in 
number  until  they  no  longer  form  a  distinct  zone,  and,  finally,  at  a  point 
corresponding  to  about  one-third  of  the  foveal  radius  from  the  centre  they 
are  no  longer  present.  The  ganglion-cells  within  the  fovea  are  round  or 
pyriform,  and  measure  about  .014  millimetre  in  diameter;  their  peripheral 
processes  are  directed  almost  perpendicularly  outward  towards  the  inner 
plexiform  layer,  within  which  they  probably  end  in  arborizations. 

1  Schafer:  The  Eye,  Quain's  Anatomy,  10th  ed.,  vol.  in.,  Pt.  3,  1894. 

2  H.  Muller :  Anatom.-physiol.  Untersuchungen  fiber  die  Retina  des  Menschen  und 
der  Wirbelthiere,  Zeitschrift  f.  wissensch.  Zoologie,  1856. 

*  Michaelis  :  Ueber  die  Ketina,  besonders  fiber  die  Macula  lutea  und  des  Foramen  <x 
trale,  Verhandl.  d.  Kais.  Leop.-Carolin.  Acad.  d.  Naturforscher,  Bd.  XIX.,  If 

*  Dimmer:  Die  ophthalmoskopischen  Lichtreflex  der  Netzhaut,  Wien,  If 

5  Golding-Bird  and  Schafer:  Observations  on  the  Structures  of  the  Central  Fovea  ol 
the  Human  Eye,  Internationale  Monatsschrift  fur  Anatomie  und  Physiologie,  B 
Heft  1,  1895. 


330 


THE    MICROSCOPICAL    ANATOMY   OF   THE   EYEBALL. 


The  fibre  layer  is  evidently  profoundly  affected  by  the  modifications 
within  the  stratum  of  ganglion-cells  with  which  its  axis-cylinders  are 
directly  continuous.  In  consequence  of  these  changes  the  stratum  of  nerve- 
fibres  very  early  becomes  diminished,  and  at  the  edge  of  the  fovea  meas- 
ures only  about  .015  millimetre  in  thickness,  as  a  continuous  layer  entirely 
disappearing  within  the  fovea  where  the  ganglion-cells  cease.  According 
to  Dogiel,1  a  few  isolated  bundles  of  nerve-fibres  cross  the  fovea  and  consti- 
tute a  wide-meshed  plexus.  (Fig.  75.) 

The  bipolar  nerve-cells,  representing  the  inner  nuclear  layer,  while 
diminishing  greatly  in  numbers  as  they  approach  the  floor  of  the  fovea, 


FIG.  75. 


External  surface 


Diagrammatic  section  of  the  human  fovea.  Magnified  375  diameters.  (Golding-Bird  and  Schafer.) — 
2,  ganglion  layer ;  4,  inner  nuclear  layer ;  6,  outer  nuclear  layer,  the  cone-fibres  forming  the  so-called 
external  fibrous  layer  of  Henle ;  7,  cones ;  v,  section  of  a  blood-vessel ;  M,  membrana  limitans  externa ; 
og,  ig,  outer  and  inner  granules  (cone-nuclei  and  bipolars)  at  the  centre. 

nevertheless  continue  to  the  centre  of  the  pit,  being  there  present  as  an 
irregular  row  of  small  elements  embedded  within  a  finely  reticular  stratum 
which  occupies  the  space  between  the  membrana  limitans  interna  and  the 
layer  of  visual  cells.  This  stratum  probably  represents  the  fused  vestiges 
of  the  outer  and  inner  plexiform  layers.  In  the  recognition  of  the  bipolars 
within  the  centre  of  the  fovea,  Golding-Bird  and  Schafer  deviate  from  the 
usual  descriptions  of  this  area,  although  the  section  of  fovea  figured  by 
Cadiat  suggests  the  presence  of  these  elements.  The  combined  thickness 
of  the  plexiform  layers  and  the  bipolars  is  about  .022  millimetre  at  the 
bottom  of  the  fovea. 

1  Dogiel:  Ueber  die  nervdsen  Elements  der  Retina  des  Menschen,  Archiv  f.   mik. 
Anat.,  Bd.  XL.,  1892. 


THE    MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL.  .Ml 

The  most  conspicuous  stratum  within  the  foveal  area  is  that  formed  by 
the  visual  cells,  which  here  include  cone-elements  exclusively.  This  layer 
at  the  margin  of  the  depression  possesses  a  depth  of  about  .145  millimetre; 
while  undergoing  a  material  reduction  in  its  thickness  in  common  with  the 
layers  already  noted,  the  stratum  of  visual  cells  still  maintains  a  thickness 
of  about  .064  millimetre  at  the  centre  of  the  fovea,  or  almost  three  times 
the  combined  depth  of  the  more  internally  situated  layers. 

The  cone-cells  present  a  striking  modification  due  to  the  change  of  posi- 
tion of  the  nuclei  of  the  fibres.  In  contrast  to  the  usual  position  of  the 
cone-granules,  close  beneath  the  external  limiting  membrane,  these  bodies 
gradually  recede  from  their  normal  situation  and  sink  towards  the  outer 
plexiform  layer,  this  change  being  most  conspicuous  at  the  bottom  of  the 
fovea,  where  the  cone-nuclei  almost  directly  rest  upon  the  reticular  stratum. 
In  consequence  of  these  alterations  in  the  position  of  the  outer  granules, 
the  relative  proportions  of  the  cone-fibre  lying  beyond  and  within  the 
granule  are  reversed,  since  the  outer  part  between  the  granule  and  the  ex- 
ternal limiting  membrane  gradually  lengthens  at  the  expense  of  the  inner 
segment  of  the  fibre  until  the  latter  practically  disappears,  and,  as  in  the 
centrally  situated  cone-fibres,  the  cone-granules  become  "  sessile." 

Golding-Bird  and  Schafer  have  also  called  attention  to  the  difference  in 
the  character  of  the  two  portions  of  the  cone-fibres  belonging  to  the  visual 
cells  within  the  fovea :  while  the  inner  segment  consists  of  an  attenuated 
fibre  which  passes  to  the  outer  plexiform  layer,  in  the  foveal  centre  joining 
the  latter  by  an  expanded  somewhat  triangular  base,  the  external  portion 
possesses  the  same  diameter  and  appearance  as  the  inner  segment  of  the 
cones,  the  direct  continuation  of  which  this  part  of  the  cone-fibre  really  is. 

Owing  to  the  great  thickness  of  the  layer  of  visual  cells  at  the  foveal 
margin,  the  stratum  at  this  point  measuring  .145  millimetre,  and  the 
marked  obliquity  of  their  course  throughout  the  outer  part  of  the  de- 
pression, the  cone-fibres  form  a  zone  of  unusual  distinctness  to  which  the 
name  outer  fibrous  layer  of  Henle  has  been  applied.  On  approaching  the 
floor  of  the  fovea,  the  fibres  are  more  vertically  disposed,  and  at  the  centre 
they  ascend  almost  perpendicularly,  their  granules  being  in  close  proximity 
to  or  even  sessile  upon  the  underlying  plexiform  layer. 

The  characteristic  appearance  of  the  fovea  depends  largely  upon  the 
cupping  of  the  two  limiting  membranes.  The  outward  deflection  of  the 
membrana  limitans  interna  has  long  been  recognized,  the  fovea  interna 
(Schafer)  so  formed  measuring  about  1.1  millimetres  in  diameter  and  about 
.130  millimetre  in  depth. 

The  participation  of  the  outer  limiting  membrane  in  producing  the 
topography  of  the  foveal  area  has  been  less  certainly  recognized ;  but  ac- 
cording to  the  observations  of  Ciaccio1  and  of  Golding-Bird  and  Schafer,1 

1  Ciaccio:  Notizia  sulla  forma  della  fovea  centrale  ch'  d  nella  macchia  lutea  della 
retina  umana,  Kendic.  dell'  accad.  delle  scienze  dell'  istituto  di  Bologna,  1880. 

3  Goldinir-Bird  arid  Schafer:  loc.  cit. 


332  THE    MICROSCOPICAL   ANATOMY    OF   THE    EYEBALL. 

as  well  as  the  drawings  of  Schultze1  and  of  Cadiat,2  a  distinct  cupping 
of  the  membrana  limitans  externa  takes  place  opposite  the  depression  on 
the  inner  surface.  A  number  of  authors,  on  the  other  hand,  including 
Hulke,3  Merkel,4  Henle,5  Kuhnt,6  and  Schwalbe,7  describe  the  outer  limiting 
membrane  as  passing  over  the  foveal  area  without  cupping.  The  outer 
depression,  the  fovea  externa  (Schafer),  while  well  marked,  is  less  extensive 
than  the  inner  pit,  measuring  about  .8  millimetre ;  its  depth,  however, 
equals  that  of  the  fovea  interna,  being  about  .130  millimetre.  Since  the 
layer  of  cones  does  not  sufficiently  compensate  by  increased  thickness  for 
the  depression  of  the  outer  fovea,  it  is  probable  that  the  pigment  layer 
also  dips  inward  at  this  point ;  the  individual  pigmented  elements  of  this 
stratum  are  smaller,  but  somewhat  thickened.  The  increased  space  result- 
ing from  the  inward  deflection  of  the  pigment  layer  is  occupied  by  the  local 
thickening  of  the  choroid,  which,  as  shown  by  Nue'l,8  takes  place  opposite 
the  fovea  in  consequence  of  the  unusual  accumulation  of  capillary  blood- 
vessels which  minister  to  the  nutrition  of  the  special  retinal  area. 

The  layer  of  cones  presents  interesting  modifications  in  the  size  of  its 
component  elements  and  in  its  depth  :  beginning  at  the  foveal  margin  with 
a  thickness  of  about  .04  millimetre,  the  stratum  gradually  increases  until 
at  the  centre  of  the  fovea  externa  it  measures  about  .09  millimetre.  The 
cones  also  exhibit  changes,  becoming  greatly  attenuated  at  the  centre,  the 
diameter  of  the  inner  segment  being  only  about  .0021  millimetre;  the 
length  of  the  inner  segment,  however,  is  markedly  less  than  at  the  margin 
of  the  fovea  or  over  the  rest  of  the  macula,  and  contributes  only  about  one- 
third  of  the  entire  length  of  the  cone  at  the  centre  of  depression.  The 
diameter  of  the  outer  cone-segment  very  closely  approximates  that  of  the 
inner,  measuring  about  .0020  millimetre. 

The  retinal  layers  represented  at  the  centre  of  the  fovea,  therefore,  in 
addition  to  the  well-developed  sustentacular  fibres  and  their  expansions,  the 
limiting  membranes,  are  the  cone  visual  cells,  constituting  the  layer  of  cones 
and  the  outer  nuclear,  here  called  the  external  fibrous  layer,  and  the  fused 
outer  and  inner  plexiform  layers  with  the  included  bipolar  nerve-cells. 
The  centrally  coursing  processes  of  the  last-named  elements  extend  hori- 
zontally to  end  in  relation  with  the  ganglion-cells  which  lie  farther  towards 
the  periphery  of  the  fovea.  The  optic  fibres  do  not  extend  beyond  the 

1  M.  Schultze:  Die  Retina,  Strieker's  Handbuch,  1872. 

*  Cadiat:  Traite  d'anatomie general e,  t.  n.,  1881,  p.  479. 

8  Hulke :  On  the  Anatomy  of  the  Fovea  Centralis  of  the  Human  Retina,  Philosoph. 
Transactions,  vol.  CLVII.,  1867. 

*  Merkel:  Ueber  die  Macula  lutea  des  Menschen,  Leipzig,  1870. 

5  Henle  :  Handbuch  der  Anatomie  des  Menschen,  Bd.  n.,  1873,  S.  690. 

'  Kuhnt:  Ueber  den  Bau  der  Fovea  centralis  des  Menschen,  Sitzungsber.  d.  ophthal- 
molog.  Gesellsch.  in  Heidelberg,  1881. 

1  Schwalbe:  Anatomie  der  Sinnesorgane,  1887,  S.  110. 

8  Nuel :  De  la  vascularisation  de  la  choroide  et  de  la  nutrition  de  la  retina  principale- 
ment  au  niveau  de  la  fovea  centralis,  Archives  d'ophthalmol.,  t.  xii.,  1892. 


THE   MICROSCOPICAL    ANATOMY   OF   THE   EYEBALL.  :j:j:{ 

position  of  the  presiding  ganglion-cells,  and  hence,  together  with  the  latter, 
as  a  distinct  layer,  are  absent  in  the  fovea  centralis. 

THE   ORA   SERRATA. 

The  ora  serrata,  marking  as  it  does  the  extreme  peripheral  limit  of  the 
visual  portion  of  the  retina,  is  distinguished  by  the  abrupt  diminution 
and  disappearance  of  the  highly  specialized  structures  concerned  in  the 
perception  of  the  light-stimuli,  the  pigment  layer  alone  of  all  the  retinal 
strata  maintaining  its  integrity.  The  regular  diminution  in  the  thickness 
of  the  retina  proceeds  gradually  from  the  fundus  towards  the  periphery,  so 
that  at  this  point  the  nervous  coat  measures  but  about  one-third  of  its  width 
at  the  posterior  pole  of  the  eyeball ;  on  reaching  the  ora  serrata,  the  ter- 
mination of  so  many  layers  within  a  limited  zone  only  .1  millimetre  in 
breadth  results  in  an  abrupt  still  further  reduction,  so  that  the  continuation 
of  the  retina  beyond  the  ora  serrata,  as  the  pars  ciliaris,  possesses  a  thick- 
ness of  only  from  .04  to  .05  millimetre. 

FIG.  76. 


Section  of  human  retina  at  the  ora  serrata,  showing  the  abrupt  termination  of  the  usual  retinal 
layers  and  the  continuation  of  the  retinal  sheet  as  the  pars  ciliaris.— o,  pigment  layer ;  6,  rods  and 
cones ;  c,  outer  nuclear  layer ;  d,  outer  plexiform ;  e,  inner  nuclear ;  /,  inner  plexiform ;  g,  ganglion- 
cells  ;  h,  point  of  transition  into  inner  stratum  (fc)  of  pars  ciliaris ;  i,  section  of  cyst.  Magnified  165 
diameters. 

The  sudden  decrease  in  the  retinal  thickness  depends  especially  upon 
the  rapid  reduction  and  termination  of  the  plexiform  layers,  the  strata  of 
nerve-fibres  and  ganglion-cells  having  faded  away  before  reaching  the 
limits  of  the  ora.  The  layer  of  rods  and  cones  earliest  loses  its  integ- 
rity as  a  distinct  stratum,  the  rods  first  disappearing,  while  the  remaining 
scattered  and  irregularly  disposed  cones,  imperfectly  formed  and  lacking 
outer  segments,  are  continued  somewhat  farther.  Of  the  plexiform  strata, 
the  outer  earliest  disappears,  in  consequence  of  which  obliteration  the  fusion 
of  the  outer  and  inner  nuclear  layers  takes  place.  The  inner  nuclear  layer, 
of  all  the  constituents  of  the  nervous  portion  of  the  retina,  is  continued 
farthest  forward,  usually  losing  its  characteristic  appearance  near  the  an- 
terior margin  of  the  ora,  where  it  seemingly  passes  into  the  single-celled 
layer  of  columnar  elements  which,  together  with  the  external  stratum  coi 
tinued  from  the  pigment  zone,  constitute  the  layer  of  the  pars  ciliari! 
retinae. 


334  THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 

The  supporting  radial  fibres  of  Muller  are  unusually  conspicuous  within 
the  ora  serrata,  since  they  are  here  exceptionally  well  developed  and  nu- 
merous. The  round  or  oval  openings,  often  of  considerable  size,  which 
are  not  infrequently  encountered  near  the  periphery  of  the  retina,  or  within 
the  ora,  are  probably  to  be  referred,  as  pointed  out  by  Kuhnt,1  Blessig,2 
and  others,  to  senile  changes,  since  it  is  doubtful  whether  such  cysts  occur 
in  perfectly  normal  tissue. 

The  rudimentary  anterior  retinal  segment  includes  the  pars  ciliaris  and 
the  pars  iridica,  both  of  which  are  composed  of  a  double  layer  of  cells 
representing  the  atrophic  layers  of  the  optic  vesicle. 

The  outer  lamina  consists  of  the  pigmented  elements  continued  from 
the  corresponding  layer  of  the  posterior  segment  of  the  retina.  The  cells, 
however,  are  lower  and  devoid  of  the  irregular  processes  which,  in  the 
visual  area,  extend  between  the  outer  segments  of  the  rods  and  cones. 

The  inner  lamina  consists  of  the  single  row  of  columnar  cells,  the 
finely  granular  or  faintly  striated  protoplasm  of  which  in  the  ciliary  seg- 
ment is  devoid  of  pigment  particles,  while  in  the  iridial  segment,  on  the 
contrary,  it  is  densely  crowded  with  pigment.  In  the  immediate  vicinity 
of  the  ora  their  height  is  from  .040  to  .050  millimetre,  but  this  gradu- 
ally becomes  reduced  to  .015  millimetre  over  the  ciliary  processes.  The 
additional  variations  which  the  elements  of  this  layer  present  in  their 
transformation  from  the  low  columnar  type  over  the  ciliary  zone  to  the 
dark  fusiform  cells  covering  the  iris  have  been  more  fully  described  in  con- 
nection with  the  latter  structure. 

The  exact  relation  of  the  inner  lamina  of  the  pars  ciliaris  to  the  retinal 
layers  has  been  variously  estimated,  the  columnar  cells  being  by  some  re- 
garded as  the  continuation  of  the  inner  nuclear  layer,  by  others  as  the  repre- 
sentatives of  modified  fibres  of  Muller.  Their  true  nature,  however,  is  in- 
dicated by  their  development ;  these  cells  must  be  regarded,  as  maintained 
by  Schwalbe,  as  the  continuation  of  no  one  particular  layer,  but  rather  as 
independent  indifferent  elements  which  have  always  retained  the  character 
of  undifferentiated  formative  retinal  elements  composing  the  innermost 
layer  of  the  optic  vesicle. 

The  delicate  glassy  membrane,  the  limitans  ciliaris,  already  described 
as  covering  the  internal  surface  of  the  inner  cells,  is  not  homologous  with 
the  limitans  interna  of  the  visual  segment  of  the  retina,  since  it  is  neither 
a  continuation  of  this  structure  nor  a  product  of  the  specialization  of 
the  primary  retinal  elements,  as  are  the  radiating  fibres  of  Muller.  The 
irregular  projections,  however,  which  the  limitans  ciliaris  sends  into  the 
clefts  between  the  bases  of  the  cylindrical  cells  suggest,  at  first  sight,  a 
closer  relation  between  the  two  structures  than  really  exists.  The  simi- 


1  Kuhnt:    Ueber  ein   neues  Endothelhautchen  im  Auge,  Bericht  iiber  die  12.  Ver- 
samml.  der  Ophthalm.  Gesellschaft  in  Heidelberg,  1879. 

2  Blessig:  De  retinae  textura disquisitiones  microscopicae,  Dorpat,  1885. 


THE   MICROSCOPICAL   ANATOMY  OP  THE   EYEBALL.  335 

larity  of  their  chemical  constituents,  as  determined  by  Berger1  has  also 
been  misleading  to  those  authors  who  have  regarded  the  cuticulaV  covering 
of  the  pars  ciharis  and  iridica  as  homologous  with  the  supporting  fibres 
of  Muller. 

THE  OPTIC  PAPILLA. 

In  addition  to  the  two  regions  characterized  by  marked  modifications  in 
the  retinal  layers,  already  considered,— the  macula  lutea  and  the  ora  ser- 
rata,— the  position  of  the  optic  nerve  presents  a  third  area  of  variation. 

The  optic  papilla,  or  optic  entrance,  as  it  is  very  generally  called,  indi- 
cates the  place  towards  which  the  centrally  directed  axis-cylinders  of  the 
fibre  layer  converge  to  form  the  optic  nerve  and  escape  from  the  interior 

Fio.  77. 


d     a  'p 

Section  of  optic  entrance.— n,  n,  longitudinally  cut  bundles  of  optic  nerve,  surrounded  by  the 
dural  (d),  arachnoidal  (a),  and  pial  (p)  sheaths;  v,  t/,  central  retinal  vessels;  I,  lamina  cribrow; 
e,  physiological  excavation  ;  /,  fibre  layer  of  retina  (r) ;  c,  choroid.;  s,  sclera.  Magnified  20  diameters. 

of  the  eyeball  in  their  course  towards  the  cerebral  centres.  Viewed  from 
the  inner  surface,  as  seen  with  the  ophthalmoscope  or  by  inspection  of  the 
eye  of  the  recently  dead  subject,  the  papilla  appears  as  a  circular  light- 
colored  area,  from  one  and  a  half  to  one  and  seven-tenths  millimetres  in 
diameter,  situated  about  four  millimetres  to  the  inner  or  nasal  side  and 
slightly  above  the  posterior  pole  of  the  eyeball,  as  marked  by  the  fovea 
centralis.  The  surface  of  the  yellowish-,  sometimes  bluish-,  white  optic 
disk  is  broken  by  the  main  divisions  of  the  central  retinal  vessels,  which 
emerge  from  the  centre  and  pass  over  the  periphery  of  the  papilla  to  gain 
the  fibre  layer  of  the  surrounding  area. 


1  Berger:    Beitrage  zur  Anatomic  der  Zonula  Zinnii,  Archiv  f.   Ophthalmol.,  Bd. 
xxviii.,  1882. 


336  THE    MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

Viewed  in  section,  the  margin  of  the  disk  appeal's  slightly  higher  than 
the  adjacent  retina,  hence  the  appropriate  name  "  papilla  :"  this  projection 
depends  upon  the  unusual  thickness  of  the  converging  bundles  of  nerve- 
fibres,  which,  as  they  approach  their  point  of  exit,  arch  over  the  abruptly 
interrupted  remaining  retinal  layers  and  assume  their  places  as  the  parallel 
bundles  taking  part  in  the  formation  of  the  optic  nerve.  In  consequence 
of  the  rapid  arching  of  the  fibres,  as  well  as  of  the  entrance  of  the  retinal 
blood-vessels,  the  central  portion  of  the  optic  disk  is  usually  occupied  by  a 
funnel-shaped  depression  known  as  the  physiological  excavation,  in  contra- 
distinction to  that  produced  by  pathological  processes. 

The  position  of  the  physiological  excavation  usually  is  central,  the  de- 
pression occupying  sometimes  as  much  as  two-thirds  of  the  entire  papilla, 
but  always  leaving  a  ring  of  nerve-fibres  at  the  periphery  of  the  disk. 
The  depth  of  the  depression  also  is  subject  to  much  variation ;  the  shallow 
concavity  present  in  some  cases  in  others  is  replaced  by  a  deep  funnel-like 
recess,  the  most  dependent  point  of  which  may  sink  to  the  level  of  the 
choroid.  The  steepness  of  the  walls  of  the  excavation  is  obviously  related 
to  the  depth  of  the  depression ;  the  deeper  the  latter  the  more  perpendicular 
its  sides.  In  general,  the  descent  of  that  part  of  the  wall  next  the  macula  is 
more  precipitous  than  in  other  portions  of  the  excavation,  on  account  of  the 
retinal  layers  here  terminating  nearly  at  the  same  distance  from  the  centre 
of  the  papilla ;  on  the  mesial  side,  on  the  contrary,  they  end  at  different 
distances  from  the  centre  of  the  disk,  the  outer  layers  extending  farthest  in- 
ward, in  consequence  of  which  the  slope  of  the  excavation  is  more  gradual. 

The  wedge  of  retinal  tissue  included  between  the  arching  bundles  of 
nerve-fibres  and  the  choroid  which  results  from  the  convergence  of  the 
fibre  layer  towards  the  papilla  contains  the  terminal  portions  of  the  retinal 
layers  from  the  stratum  of  ganglion-cells  to  the  pigmented  epithelium 
inclusive.  The  layer  of  ganglion-cells  terminates  first,  but  its  cessation  is 
soon  followed  by  that  of  the  succeeding  plexiform  and  inner  nuclear  zones. 
The  stratum  of  visual  cells,  in  consequence  of  its  more  favorable  situation, 
stretches  somewhat  farther  towards  the  centre  of  the  papilla,  but  at  the 
same  time  presents  decided  modifications  in  the  disposition  of  its  com- 
ponent parts,  the  rod  and  cone  and  the  outer  nuclear  layer.  The  most 
conspicuous  of  these  alterations  is  the  obliquity  of  the  cone-  and  rod-fibres, 
the  external  ends  of  which  are  directed  inward  towards  the  correspondingly 
obliquely  placed  percipient  elements,  while  the  internal  ends  point  outward. 
The  outer  granules — nuclei  of  the  visual  cells — also  evince  a  tendency  to 
recede  from  their  usual  situation  beneath  the  limitans  exterua  and  to 
assume  instead  a  position  closer  to  the  subjacent  plexiform  layer  ;  in  conse- 
quence of  this  modification  the  external  portion  of  the  cone-  and  rod-fibres 
becomes  conspicuous  as  a  non-nucleated  obliquely  striated  zone,  the  outer 
fibrous  layer  of  various  authors,  Avhich  resembles  the  similarly  constituted 
band  within  the  fovea  bearing  a  like  name,  and  measures  about  .022  milli- 
metre in  thickness. 


THE    MICROSCOPICAL   ANATOMY    OF   THE   EYEBALL. 


887 


Schwalbe l  has  called  attention  to  the  existence  of  a  narrow  wedge  of 
reticulated  tissue  which  separates  the  nerve-fibres  as  they  arch  into  the 
optic  nerve  from  actual  contact  with  the  other  retinal  layers.  This  inter- 
mediate tissue,  as  named  by  Kuhnt,2  is  present  in  somewhat  greater  amount 
on  the  median  than  on  the  lateral  side,  and  consists  of  a  reticulum  possessing 
plate-like  cells  ;  the  tissue  apparently  is  continuous  with  the  framework  of 
the  iuner  retinal  layers,  being  limited  externally  by  the  limitans  ezterna. 
The  terminal  margin  of  the  choroid  marks  the  external  point  to  which  the 
intermediate  tissue  extends. 

During  foetal  life,  and  sometimes  in  later  years,  the  excavation  of  the 
optic  disk  is  occupied  by  a  peculiar  embryonal  connective  tissue  which  is 
closely  connected  with  the  central  hyaloid  artery  of  the  foetus  which  it 
ensheathes.  Remains  of  this  structure  are  often  encountered  in  later  years 
as  irregular  masses  of  delicate  texture  lying  within  and  partially  filling  the 

excavation. 

FIG.  78. 


Diagram  of  the  blood-vessels  of  the  human  retina.    (Leber,  after  Jaeger.)-<MW,  mu,  superior  nwal 
artery  and  vein;  ats,  vt*,  superior  temporal  artery  and  vein;  ant,  mi.  i 
inferior  temporal  vessels ;  ame,  vme,  median  vessels;  am,vm,  macular  vessels. 

The  Blood-Vessels  of  the  Retina.— The  retinal  blood-vessels  constitute  a 
distinct  and  closed  circulation,  of  which  the  arteria  central™  retinae  and  the 
accompanying  vein   are  the  chief  stems;    in  the  vicinity  of  the  opti 
entrance  alone  does  this  isolated  system  communicate  with  the  circul 
formed  by  the  ciliary  vessels  supplying  the  remaining  tunics  of  the  . 

i  Schwalbe  :  Der  Sehnerv,  Graefe  u.  Saemisch's  Handbuch  d.  Augenheilkunde,  Bd.  I., 

» Kuhnt:  Zur  Kenntniss  des  Sehnerven  und  der  Netzhaut,  Archiv  f.  Ophthal.,  Bd. 

xxv.,  1879. 

VOL.  T.— 22 


338  THE    MICROSCOPICAL    ANATOMY    OF    THE    EYEBALL. 

At  a  distance  of  from  fifteen  to  twenty  millimetres  from  the  eyeball, 
somewhat  below  and  to  the  outer  side,  the  central  artery  and  vein  obliquely 
penetrate  the  optic  nerve  to  gain  an  axial  position  which  is  maintained  as 
far  as  the  optic  papilla.  On  reaching  the  latter  position,  the  central  artery 
divides  into  two  principal  stems,  the  superior  and  inferior  papillary  branches, 
which  are  directed  respectively  almost  vertically  upward  and  downward. 
The  papillary  branches  each  subdivide  into  twigs  which  pass  mesially  and 
laterally  and  constitute  the  arterise  nasalis  et  temporalis  superior  and  inferior. 
Additional  small  but  important  direct  lateral  branches  pass  outward,  as 
the  arterise  macularis  superior  and  inferior,  for  the  especial  supply  of  the 
macular  region,  these  vessels,  as  noted  by  Kunn  l  and  others,  sometimes 
reaching  almost  to  the  fovea  centralis,  and  hence  being  of  moment  in  aiding 
in  the  nutrition  of  this  important  area. 

After  the  establishment  of  the  principal  stems  division  rapidly  takes 
place  into  smaller  vessels,  which  in  turn  subdivide,  but  have  no  communi- 
cation with  one  another,  belonging  to  the  non-anastomosing  "  end-arteries." 
The  arterioles  have  a  more  tortuous  course  than  the  corresponding  veins, 
and  break  up  into  an  arborization  of  smaller  branches,  which  terminate  in 
a  dense  capillary  net-work  extending  throughout  the  retina.  The  capilla- 
ries immediately  associated  with  the  arteries  differ,  according  to  Musgrove,2 
in  their  arrangement  from  those  connected  with  the  venous  radicles,  since 
the  former  constitute  very  irregular  meshes,  giving  the  appearance  of  a 
confused  net-work,  in  marked  contrast  to  the  regular  converging  rhom- 
boidal  spaces  formed  by  the  capillaries  connected  with  the  veins.  The 
small  arteries  are  separated  from  their  accompanying  veins  by  an  interval 
occupied  by  the  capillary  plexus. 

As  established  by  the  observations  of  Leber,3  Hesse,4  His,9  and  others, 
and  reaffirmed  by  Musgrove,  the  retinal  arteries  have  no  anastomotic  con- 
nections, the  capillary  net-work  being  the  sole  means  of  communication ; 
the  veins  likewise  do  not  anastomose.  The  exceptional  connections  estab- 
lished between  the  otherwise  closed  retinal  circulation  and  that  formed  by 
the  ciliary  vessels  will  be  considered  together  with  the  blood-vessels  of  the 
optic  nerve. 

The  capillaries  expand  within  two  planes,  as  the  inner  and  outer  vascular 
plexuses.  The  exact  position  of  the  inner  capillary  net-work  has  been  a 
matter  of  some  uncertainty,  Hesse,  His,  and  Schwalbe  describing  it  as  lying 

1  Kunn  :  Ein  Fall  von  Astembolie  der  Art.  centralis  retinae  nebst  Bemerkungen  iiber 
d.  Verlauf  der  macularen  Arterien,  Wiener  med.  Wochensch.,  Bd.  XLIV.,  No.  36,  1894. 

2  Musgrove :   The  Blood- Vessels  of  the  Ketina,  Journal  of  Anat.  and  Physiology, 
New  Series,  vol.  vi.,  1892. 

3  Leber  :  Die  Circulations-  und  E  mannings  verhaltniss  des  Auges,  Graefe  u.  Saemisch's 
Handbuch  d.  Augenheilkunde,  Bd.  n.,  1876. 

4  Hesse :  Ueber  die  Vertheilung  der  Blutgefasse  in  der  Netzhaut,  Archiv  f.  Anat.  u. 
Physiolog.,  1880. 

5  His  :  Abbildungen  iiber  das  Gefass-system  der  menschlichen  Netzhaut  und  derjeni- 
gen  des  Kaninchens,  Archiv  f.  Anat.  u.  Physiolog.,  1880. 


THE    MICROSCOPICAL    ANATOMY    OF   THE   EYEBALL. 


889 


within  the  fibre  layer,  while  Musgrove  finds  it  within  the  ganglion-cell 
stratum.  The  sections  of  human  retina  examined  by  the  writer  show  the 
capillaries  to  be  more  numerous  within  the  layer  of  ganglion-cells  than 
within  the  fibre  zone.  The  outer  capillary  plexus  lies  within  the  inner 
nuclear  layer,  for  the  nutrition  of  whose  bipolar  nerve-cells  it  is  particularly 
destined.  In  rare  instances  the  capillaries  extend  into  the  outer  plexiform 
zone ;  never,  however,  as  far  as  the  outer  nuclear  layer.  According  to 
Hesse  and  His,  the  inner  capillary  plexus  is  distinguished  by  the  coarseness 
of  its  meshes ;  the  outer,  lying  within  the  inner  nuclear  layer,  possessing 
close  meshes  which  surround  the  small  ganglion-cells. 

Fio.  79. 


Accurate  drawing  of  the  blood-vessels  supplying  the  macular  region  of  the  human  retina:  from  an 
injected  preparation  by  Heinrich  Miiller.    (Becker.)    Magnified  45  diameters. 

The  retinal  veins,  beginning  in  the  capillary  mesh-works,  follow  the 
general  course  and  arrangement  of  the  corresponding  arterial  vessels  quite 
closely,  the  venous  branches  possessing  generally  a  somewhat  greater  diam- 
eter. All  the  veins  and  the  capillaries  are  surrounded  by  delicate  adven- 
titious sheaths ;  between  the  latter  and  the  wall  of  the  blood-vessel  a  cleft 
exists  which  represents  a  perivascular  lymph-space  surrounding  the  blood- 
channel. 

As  already  noted,  the  retinal  circulation  does  not  penetrate  beyond  the 


340  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

ganglion  retinae,  the  stratum  of  visual  cells  being  nourished  more  especially 
by  the  fluids  conveyed  by  the  dense  capillary  net-work  within  the  choroid 
coat  constituting  the  chorio-capillaris,  which  lies  in  close  proximity  to  the 
outer  pigmented  cells  and  the  underlying  percipient  elements. 

The  fact  that  the  dense  net- work  of  capillary  vessels  was  wanting  at 
the  area  of  the  retinal  sheet  most  highly  endowed  with  acute  vision  has 
long  been  established  ;  it  has  likewise  been  assumed  that  the  non-vascular 
area  corresponded  closely  with  the  fovea,  the  remaining  part  of  the  yellow 
spot  being  well  supplied  with  capillaries  of  the  retinal  system.  The  ap- 
pearance of  the  paper  by  Johannides,1  based  upon  the  examination  of  the 
injected  retina  of  a  child  of  four  years,  in  which  the  presence  of  blood- 
vessels within  the  macula  was  strongly  questioned,  called  forth  contribu- 
tions to  the  accurate  knowledge  of  the  distribution  of 
FlG'  80'  _—_  the  blood-vessels  in  the  communications  by  Leber,2 
Becker,3  Gerlach,4  Reuss,6  Ayres,6  and  Mayerhausen.7 
These  authors  agree  in  regarding  the  fovea  cen- 
tralis  as  devoid  of  retinal  blood-vessels,  while  the  re- 
maining part  of  the  macular  region  is  richly  supplied. 
Mayerhausen  estimates  the  surface  of  the  entire 
macular  area  at  2.356  square  millimetres,  of  which 
2.205  square  millimetres  are  markedly  vascular,  while 
the  remaining  .151  square  millimetre  represent  the 
non-vascular  tract.  According  to  the  same  observer, 
the  vessels  of  the  macula  terminate  at  .087  to  .137 
millimetre  from  the  edge  of  the  fovea. 
Retinal  blood  -  vessel  The  lymphatics  of  the  retina  are  represented  chiefly 

surrounded  by  perivascu-      1,1  i          i  i 

lar  lymph-sheath.  Drawu  b7  the  penvascular  lymph-channels,  already  men- 
from  photograph  of  prepa-  tioned,  which  surround  the  veins  and  the  capillaries 

ration  of  Professor  Norris.  i/>i  •  •  i        i  •  '  •  »    i 

and  freely  communicate  with  the  subpial  lymph- 
spaces  of  the  optic  nerve.  (Schwalbe.)  Additional  lymphatic  clefts  prob- 
ably exist  between  the  larger  nerve-bundles  in  the  vicinity  of  the  papilla. 
Injections  of  the  subpial  space  not  infrequently  also  pass  between  the 
retina  and  the  pigmented  epithelium,  as  well  as  between  the  hyaloid  mern- 

1  Johannides :  Die  gefasslose  Stelle  der  mensch.  Ketina  und  deren  Verwerthung  zur 
Bestimmung  der  Ausdehnung  der  Macula  lutea,  Archiv  f.  Ophthalmol.,  Bd.  xxvi.,  1880. 

2  Leber :  Bemerkung  uber  das  Gefass-system  der  Netzhaut  in  der  Gegend  der  Macula 
lutea,  Archiv  f.  Ophthalmol.,  Bd.  xxvi.,  1880. 

'Becker:  Die  Gefasse  der  menschlichen  Macula  lutea,  Archiv  f.  Ophthalmol.,  Bd. 
xxviii.,  1881. 

*  Gerlach :  Ueber  die  Gefasse  der  Macula  lutea,  Sitzungsber.  der  physik.-medicin. 
Societal  zu  Erlangen,  1881. 

5  Reuss :  Notiz  uber  die  Netzhautgefasse  im  Bereiche  der  Macula  lutea  bei  Embolia 
art.  cent,  ret.,  Archiv  f.  Ophthalmol.,  Bd.  xxvn.,  1881. 

6  Ayres :  Der  Blutlauf  in  der  Gegend  des  gelben  Fleckes,  Archiv  f.  Augenheilkunde, 
Bd.  xiii.,  1883. 

7  Mayerhausen  :  Noch  einmal  der  gefasslose  Bezirk  der  menschlichen  Retina,  Archiv 
f.  Ophthalmol.,  Bd.  xxix.,  1883. 


THE   MICROSCOPICAL   ANATOMY  OF  THE   EYEBALL. 


341 


brane  and  the  limitans  interna,  which  extensions  of  the  injection  fluid  have 
been  regarded  by  Schwalbe  as  indicating  the  presence  of  lymph-spaces  in 
these  positions. 

THE   OPTIC  NERVE. 

The  optic  nerve,  owing  to  the  variations  in  its  relations,  presents  three 
distinct  segments,  the  intra-cranial,  the  intro-orbital,  and  the  intra-ocular. 
The  first  of  these  is  considered  in  connection  with  the  optic  tract,  to  which 
chapter  the  reader  is  referred ;  the  description  of  the  remaining  parts  alone 
claims  attention  in  this  place. 

The  optic  nerve  within  the  orbit,  in  addition  to  the  obliquity  of  its 
course  from  the  optic  foramen  to  the  eyeball,  presents  distinct,  although 


Transverse  section  of  optic  nerve.— n,  n,  bundles  of  nerve-fibres  separated  by  vascular  septa  (i,  i)  of 
connective  tissue.    Magnified  165  diameters. 

inconspicuous,  curvatures  which  render  its  axis  somewhat  sigmoid  rather 
than  straight.  Although,  as  found  by  Weiss,1  great  variation  in  length 
and  curvature  undoubtedly  exists,  yet  usually,  upon  entering  the  orbit,  the 
nerve  bends  outward  and  downward,  almost  touching  the  inner  surface  of 
the  external  rectus  muscle  at  the  point  of  greatest  deflection ;  this  curve  is 
followed  by  one  less  marked  in  the  opposite  direction,  beyond  which  the 
nerve  continues  straight. 

In  addition  to  the  foregoing  S-like  curvature,  it  is  generally  assumed, 
based  on  the  investigations  of  Vossius,2  that  the  optic  nerve  in  the  course 

1  Weiss:  Anatomie   der  Eintrittsstelle  der  Sehnerven,   Notiz   in   Munchner  med. 
Wochenschrift,  No.  6,  1889. 

2  Vossius:  Beitrage  zur  Anatomie  des  Nervus  Opticus,  Archiv  f.  Opbthalmol.,  Bd. 
xxix.,  1883. 


342 


THE    MICROSCOPICAL    ANATOMY    OF    THE    EYEBALL. 


of  its  development  undergoes  a  torsion  about  its  longitudinal  axis  in  such 
manner  that  the  surface  which  primarily  is  directed  downward  closer  to 
the  eyeball  attains  an  upward  and  lateral  position  and  conies  to  occupy  the 
temporal  quadrant.  The  torsion  first  appears  at  a  point  corresponding  to 
the  union  of  the  posterior  and  middle  third  of  the  nerve,  and  becomes  more 
pronounced  towards  its  anterior  extremity.  The  recent  observations  of 
Deyl,1  however,  do  not  support  this  view ;  the  last-mentioned  author  em- 
phatically states  that  "the  described  rotation  of  ninety  degrees  of  the 
embryonic  eyeball  does  not  exist,"  since  the  arteria  centralis  retinae  pene- 
trates the  optic  nerve  always  in  the  median  inferior  quadrant.  This  being 

established,    it   follows   that    the 

FIG. 82-  assumed    inner    rotation   of    the 

primary  fissure  does  not  take  place, 
and,  hence,  that  the  suggested  close 
morphological  relations  of  the 
choroidal  fissure  and  the  macula 
lutea  and  the  fovea  centralis  must 
likewise  be  abandoned. 

The  optic  nerve,  as  seen  in 
transverse  sections,  consists  of 
about  eight  hundred  distinct  bun- 
dles of  medullated  nerve-fibres, 
separated  from  one  another  by 
connective-tissue  trabeculae  and 
septa  which  are  derived  as  exten- 
sions from  the  general  pial  sheath. 
The  entire  nerve  corresponds 
in  its  structure  to  a  gigantic  funi- 
culus,  the  penetrating  pial  tissue 
forming  the  septa  representing  the 
endoneurium  and  the  pial  sheath 
proper  the  perineurium.  The 
optic  nerve,  furthermore,  must  be 

regarded  as  part  of  the  central  nervous  mass,  being,  according  to  Monro,2 
comparable  to  the  posterior  columns  of  the  spinal  cord. 

The  bundles  are  composed  of  medullated  nerve-fibres  of  an  average 
diameter  of  about  .002  millimetre ;  the  extremes  of  the  thickness  of  the 
individual  fibres,  however,  include  a  wide  range,  many  fibres  being  so  fine 
that  their  width  is  inappreciable,  while  others  of  less  frequency  possess  a 
diameter  of  from  .005  to  .010  millimetre. 

The  entire  number  of  nerve-fibres  contained  within  the  optic  nerve  has 

1  Deyl :  Ueber  den  Eintritt  der  Arteria  centralis  retinae  in  den  Sehnerv  beim  Menschen, 
Anatom.  Anzeiger,  Bd.  XL,  No.  22,  1896. 

•  Monro :  The  Optic  Nerve  as  a  Part  of  the  Central  Nervous  System,  Journal  of 
Anatomy  and  Physiology^  N.  S.,  vol.  x.,  1895. 


Longitudinal  section  of  optic  nerve.— n,  n,  bun- 
dles of  nerve-fibres  separated  by  septa  of  connective 
tissue  (i,  i).  Magnified  165  diameters. 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


341 


been  variously  stated.  The  estimates  of  Salzer l  and  of  Krause,8  in  which 
the  number  of  measurable  fibres  is  placed  at  about  four  hundred  and  twenty- 
five  thousand,  may  be  regarded  as  approximately  correct. 

The  close  relation  already  noted  which  exists  between  the  retina  and  its 
stalk  and  the  more  centrally  situated  nervous  masses  is  still  further  empha- 
sized by  the  correspondence  in  the  structure  of  the  fibres  composing  the 
optic  nerve  and  those  of  the  brain  and  the  spinal  cord ;  as  are  the  latter 
so  are  the  former  devoid  of  a  neurilemma,  the  medullated  axis-cylinders 
being  held   in   place   by  the 
interstitial    supporting    sub- 
stance, the  netiroglia.    Robin- 
son3  formulates    the   genetic 
relations  of  the  optic  nerve  as 
follows :    "  In   mammals   the 
optic  stalk  becomes  converted 
into  the  optic  nerve  by  the 
transformation  of  its   proto- 
plasmic  substance   into   reti- 
form  sustentacular  tissue,  and 
by  the  passage  of  nerve-fibres 
through  its  walls,  the  nerve- 
fibres    being    protected    and 
supported    by    the    external 
limiting    membrane    of    the 
stalk,   and   by   the    reticular 
framework    formed    by    the 
modification  of  its  walls,  the 
transformation  from  the  stalk 
to  the  nerve  being  associated 
with  the  disappearance  of  the 
cavity  of  the    stalk."      The 
recent  investigations  of  Do- 
giel,  Cajal,  Greeff,  and  Bach, 
to  which  reference  has  already  been  made,  conclusively  show  the  presence' 
of  large  neuroglia-cells  within  the  supporting  tissue  of  the  optic  bundles. 
These  spider-cells,  when  stained  by  the  Golgi  or  other  improved  method, 
are  very  conspicuous   elements,  resembling   the  similar  though   smaller 
bodies  found  within  the  fibre  layer  of  the  retina.    Additional  flattened  celli 
lie  within  the  connective  tissue  separating  the  nerve-bundles,  their  deeply 
stained  nuclei  contrasting  strongly  with  the  feebly  colored  nervous  tissue. 


Longitudinal  section  of  optic  nerve  stained  by  the  Golgi 
method  to  display  the  neuroglia-cells.  (Dogiel.)— «,  bun- 
dles of  nerve-fibres ;  6,  neuroglia-cells;  c,  similar  cells  near 
the  optic  papilla. 


1  Salzer:  Uebcr  die  Anzahl  der  Sehnervenfasern  und  der  Retinazapfen  im  Auge  d 
Menschen,  Sitzungsber.  der  k.  Akad.  d.  Wissensch.,  Bd.  LXXXI.,  1880. 

2  Krause :  Ueber  die  Fasern  des  Sehnerven,  Archiv  f.  Ophthalmol.,  B< 
«  Robinson :    Formation  and  Structure  of  the  Optic  Nerve,  and 

Optic  Stalk,  Journal  of  Anatomy  and  Physiology,  vol.  xxx.  3,  1896. 


344 


THE    MICROSCOPICAL    ANATOMY    OF   THE    EYEBALL. 


On  reaching  the  intraocular  portion  of  the  optic  nerve,  the  nerve-fibres 
sooner  or  later  undergo  a  conspicuous  change,  since  they  usually  lose  the 
medullary  substance  in  their  passage  through  the  lamina  cribrosa,  or,  to 
state  it  more  accurately,  gain  the  white  substance  of  Schwann  on  emerging 
from  the  cribriform  lamella  of  the  sclera  in  their  course  from  the  retina 
brainward. 

In  consequence  of  the  disappearance  of  the  medullary  coat,  as  well  as 
of  a  large  part  of  the  interfibrillar  supporting  tissue,  the  diameter  of  the 
optic  nerve  becomes  rapidly  diminished  on  approaching  its  retinal  expan- 

FIG.  84. 


o 


Section  of  the  lamina  cribrosa  showing  the  passage  of  the  bundles  of  optic  fibres  (o,  o)  through  the 
felt-work  formed  by  the  scleral  tissue.— r,  r,  bundles  of  nerve-fibres  emerging  from  the  retina.  Magnified 
165  diameters. 

sion ;  the  diameter  of  about  three  millimetres  which  it  possesses  at  the 
exterior  of  the  ball  is  reduced  to  about  one  and  five-tenths  millimetres 
during  its  passage  through  the  lamina  cribrosa.  The  nerve  presents  its 
least  diameter  within  the  zoue  corresponding  to  the  inner  fourth  of  the 
sclera  and  the  choroid  ;  the  relation  of  the  latter  coat,  however,  is  variable, 
since  in  some  cases  the  choroidal  ring  marks  the  most  constricted  part  of 
the  nerve,  while  in  others  the  optic  trunk  is  distinctly  wider  at  this  point 
than  where  embraced  by  the  scleral  constriction. 


THE   MICROSCOPICAL   ANATOMY  OF  THE   EYEBALL.  :',lf> 

Tliis  latter  structure  consists  of  a  series  of  several— from  four  to  eight- 
horizontal  interlacements  of  scleral  fibres  which  extend  between  the  bundles 
of  the  optic  nerve,  as  the  latter  traverses  the  outer  tunic  of  the  eyeball,  and 
collectively  constitute  the  lamina  cribrosa.  The  bridging  fibres  originate 
mostly  within  the  inner  third  of  the  scleral  coat,  but  receive  additional  support 
by  their  close  connection  with  the  septa  within  the  optic  nerve,  particularly 
with  the  central  mass  of  connective  tissue  surrounding  the  retinal  vessels. 

The  several  horizontal  mesh-works  are  supplemented  by  trabeculie  which 
extend  obliquely  or  longitudinally  and  still  further  unite  the  individual 
reticula  with  one  another.  The  trabeculse  which  pass  from  the  scleral  tissue 
towards  the  centre  of  the  nerve  usually  support  blood-vessels,  in  conse- 
quence of  which  arrangement  the  optic  nerve,  at  the  base  of  the  papilla, 
is  traversed  by  a  vascular  net-work  of  exceptional  richness,  whereby  the 
important  communication  between  the  ciliary  and  the  retinal  vessels  is 
established. 

In  addition  to  the  scleral  bands,  delicate  trabeculae  from  the  choroid 
coat  also  penetrate  among  the  optic  nerve  bundles  and  constitute  what  has 
been  termed,  in  accordance  with  the  investigations  of  Kuhnt l  and  of  Hoff- 
mann,2 the  choroidal  portion  of  the  lamina  cribrosa,  a  part  which  in  early 
life  represents  a  relatively  more  important  constituent.  Delicate  exten- 
sions of  the  blood-vessels  of  the  choroid  are  also  converged  within  these 
trabeculse,  by  means  of  which  the  anastomosis  between  the  retinal  and 
choroidal  circulations  is  effected. 

Mention  has  been  incidentally  made  of  the  communications  which  exist 
between  the  circulation  established  by  the  ciliary  vessels,  including  the 
scleral  and  choroid  plexuses,  and  that  formed  by  the  arteria  centralis  retinae. 
The  earliest  literature  concerning  the  cilio-retinal  anastomosis  includes  men- 
tion of  the  existence  of  such  communication  by  Hovius  (1716),  Zinn 
(1753),  Waller  (1754),  Tiedemann  (1824),  and  Sommering  (1844);  of 
these,  the  description  of  the  relations  of  the  ciliary  vessels  around  the  optic 
nerve  given  by  Zinn  is  the  most  complete,  in  recognition  of  which  the  vas- 
cular wreath  encircling  the  optic  nerve  has  been  termed  the  circulus  arteri- 
osus  Zinnii.  Jaeger,3  however,  particularly  called  attention  to  the  im- 
portance of  this  communication,  and  supplied  additional  data  concerning  its 
details.  Subsequently  Leber4  emphasized  this  supplementary  source  of 
nutrition  of  the  nerve  and  a  limited  retinal  area,  to  which  Czermak  *  later 

1  Kuhnt:  Zur  Kenntniss  des  Sehnerven  und  der  Netzhaut,  Archiv  f.  Ophthalmol., 
Bd.  xxv.,  1879. 

2  Hoffmann  :  Zur  vergleichenden  Anatomie  der  Lamina  cribrosa  nervi  optu 
Archiv  f.  Ophthalmol.,  Bd.  xxix.,  1883. 

'  Jaeger:  Ueber  die  Einstellungen  des  dioptrischen  Apparates  im  mem 
"Wien,  1861,  Anmerkung,  S.  52. 

*  Leber:  Bemerkungen  iiber  die  Circulations-Verbaltnissedes  Optic 

Archiv  f.  Ophthalmol.,  Bd.  xvni.,  1872. 

*  Czermak:  Beitrag  zur  Kenntniss  der  sogenan.  cilio-retinalen  GeC 
nische  Wochenschrift,  No.  11,  1888. 


346 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


added  testimony  to  show  that  the  cilio-retinal  vessels  supply  especially  the 
macular  region. 

The  vascular  circuit  of  Zinn  which  encircles  the  optic  nerve  entrance  is 
formed  by  branches  from  the  short  posterior  ciliary  arteries ;  from  this 
vascular  wreath  robust  branches  are  sent  to  the  choroid,  while  delicate 
twigs  pass  to  the  lamina  cribrosa  and  indirectly  communicate  with  the 
central  vessels  of  the  retina  by  means  of  the  intervening  capillaries  with 
which  both  sets  of  vessels  anastomose.  Likewise,  delicate  twigs  derived 

FIG.  85. 


Longitudinal  section  through  the  optic  entrance  showing  blood-vessels  injected  from  ophthalmic 
artery.  (Leber.)— S, sclera  ;  Ch,  choroid;  R,  retina;  Ve,  outer,  Vi,  inner  optic  sheath;  A,  arteria  cen- 
tralis  retinae ;  V,  vena  centralis  retinae ;  Lc,  lamina  cribrosa ;  Aci,  short  post-ciliary  arteries,  giving  off 
twig  to  optic  nerve ;  c,  anastomosis  between  choroidal  and  retinal  vessels. 

from  the  choroidal  plexus  penetrate  along  the  trabeculse  constituting  the 
innermost  part  of  the  lamina  cribrosa  and  thus  effect  a  capillary  union 
between  the  central  retinal  and  the  choroidal  vessels.  As  an  exceptional 
arrangement  larger  trunks  belonging  to  the  cilio-retinal  vessels  may  pass  to 
the  optic  head  and  the  retina. 

The  practical  importance  of  these  communications  has  been  emphasized 
by  the  clinical  and  experimental  observations  of  Rumschewitsch,1  Wagen- 
mann,2  Adamiik,3  Randall,4  and  others ;  by  these  it  was  shown  that  after 
the  usual  source  of  blood-supply  from  the  central  retinal  artery  had  been 

1  Rumschewitsch :  Ueber  die  Anastomosen  der  hinteren  Ciliargefasse  mit  denen  des 
Opticus  und  der  Retina,  Klinische  Monatsblatter  f.  Augenheilkunde,  Bd.  xxvu.,  1889. 

2  Wagenmann  :  Experimentelle  Untersuchungen  iiber  den  Einfluss  der  Circulation  in 
der  Netzhaut  und  Aderhautgefassen  u.  s.  w.,  Archiv  f.  Ophthal.,  Bd.  xxxvi.,  1890. 

3  Adamiik  :  Zur  Frage  ueber  den  Einfluss  der  Chorioidea  auf  die  Ernahning  der  Netz- 
haut, Archiv  f.  Augenheilkunde,  Bd.  xxvu.,  1893. 

4  Randall :  Cilio-Retinal  or  Aberrant  Vessels,  Trans.  American  Ophthalmol.  Society, 
1887. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL.  34? 

cut  off,  either  by  embolism  or  by  ligation,  the  collateral  circulation  estab- 
lished from  the  choroidal  system  sufficed  to  maintain  the  nutrition  of  the 
posterior  retinal  area. 

The  explanation  of  the  communications  between  the  retinal  and  ciliary 
arteries  around  the  papilla  is  to  be  sought  in  the  early  distribution  of  the 
arteries  of  this  region,  since,  as  pointed  out  by  Eversbusch,1  the  optic  head 
with  the  lamina  cribrosa  is  supplied,  in  addition  to  the  central  artery,  by  a 
dense  capillary  net-work  derived  from  the  sclera  and  choroid.  While  the 
older  view,  by  which  the  vitreous  vessels  were  credited  with  an  active  r6le 

FIG.  86. 


Transverse  section  of  optic  nerve  with  its  sheaths.— n,  bundle  of  nerve-fibres;  f,  interfascicular 
connective  tissue ;  d,  a,  p,  respectively  dural,  subarachnoidal,  and  pial  sheaths  euclosing  lymph-spaces, 
I,  I.  Magnified  20  diameters. 

in  the  vascularization  of  the  young  retina,  is  no  longer  tenable  in  the  light 
of  the  careful  researches  of  O.  Schultze,2  who  has  shown  that  the  retinal 
vessels  develop  entirely  independently  of  those  of  the  choroid,  the  early 
intimate  relations  between  the  retinal  vessels  and  those  of  the  surrounding 
choroid  and  sclera  persist  to  a  limited  degree  and  are  represented  by  the 
minute  branches  of  communication  found  in  the  adult  organ.  Exception- 
ally one  of  these  embryonic  vessels  undergoes  greater  development  than 
usual,  and  then  appears  as  a  cilio-retinal  branch  seen  with  the  ophthal- 
moscope. 

1  Eversbusch :    Klin.-anatom.  Beitrage  zur  Embryologie  und  Teratologie  des  Glaa- 
korpers,  Mittheilungen  d.  konigl.  Univ.-Augenklinik  zu  Munchen,  Bd.  I.,  1882. 

2  O.  Schultze :  Zur  Entwickelungsgeschichte  des  Gefass-Systems  im  Saugetier-Auge, 
Kolliker's  Festschrift,  1892. 


348  THE   MICROSCOPICAL    ANATOMY   OF   THE    EYEBALL. 

The  Sheaths  of  the  Optic  Nerve. — The  intra-orbital  portion  of  the  optic 
nerve  is  enveloped  by  three  distinct  sheaths,  the  direct  prolongations  of 
the  brain-membranes.  These  envelopes  begin  at  the  cranial  surface  of  the 
optic  foramen,  where  they  spring  from  the  meninges,  and  terminate  in  the 
fibrous  tunic  of  the  eyeball,  with  which  they  blend. 

The  outer  or  dural  sheath,  the  direct  continuation  of  the  dura  mater  of 
the  brain,  is  a  thick  fibrous  covering  which  loosely  envelops  the  nerve-trunk 
within  the  orbit,  but  closely  embraces  it,  together  with  the  other  sheaths,  at 
the  optic  foramen.  On  reaching  the  eyeball,  the  dural  sheath  bends  sharply 
outward  and  becomes  continuous  with  the  outer  part  of  the  sclerotic  coat. 

The  surface  of  the  optic  nerve  is  invested  by  the  pial  sheath,  the  exten- 
sion of  the  pia  mater,  which  closely  resembles  the  brain-membrane  in  its 
rich  vascularity  and  is  an  essential  factor  in  maintaining  the  nutrition  of 
the  peripheral  portions  of  the  nerve-trunk.  The  intervaginal  lymph-space 
which  separates  the  outer  from  the  inner  sheaths  of  the  nerve  is  unequally 
divided  by  a  delicate  septum,  the  arachnoidal  sheath,  continued  from  the 
central  arachnoid. 

The  arachnoidal  sheath  is  so  closely  connected  with  the  outer  dural 
covering  by  means  of  numerous  trabeculse  that  only  a  narrow,  irregularly 
interrupted  cleft  separates  the  two  sheaths ;  this  interstice  constitutes  the 
subdural  space,  and  is  continuous  with  the  corresponding  intra-cranial 
lymphatic  space.  The  wider  cleft  between  the  arachnoidal  and  pial  sheaths 
constitutes  the  subarachnoidal  space,  continuous  with  the  corresponding 
space  surrounding  the  brain,  across  which  stretch  the  irregular  trabeculse 
and  bands  of  the  arachnoidal  tissue ;  these,  as  well  as  the  lymph-spaces  of 
the  nerve  in  general,  are  more  or  less  perfectly  invested  by  a  covering  of 
endothelial  plates. 

The  sheaths  of  the  optic  nerve,  as  well  as  the  included  subdural  and 
subarachnoidal  spaces,  end  within  the  fibrous  coat  of  the  eyeball.  The 
dural  sheath,  as  already  mentioned,  terminates  within  the  outer  two-thirds 
of  the  solera  by  sharply  bending  over  into  the  fibrous  tissue  of  this  coat. 
The  arachnoidal  sheath  fuses  with  the  dural  envelope  shortly  before  ending, 
the  termination  of  its  fibres  being  traceable  into  the  middle  third  of  the 
sclera.  The  greater  part  of  the  pial  sheath  likewise  fades  away  in  the  scle- 
rotic coat,  blending  with  the  inner  third  ;  the  most  internal  fibres,  however, 
probably  extend  still  farther  and  become  connected  with  the  choroid.  The 
subdural  and  subarachnoidal  compartments  of  the  intervaginal  space  likewise 
terminate  within  the  sclera,  the  fusion  of  the  arachnoidal  and  pial  sheaths 
with  the  fibrous  tunic  marking  the  points  at  which  these  lymph-spaces 
respectively  end.  In  exceptional  cases,  as  noted  by  Michel,1  the  spaces 
may  be  somewhat  prolonged  as  clefts  which  extend  parallel  to  the  surfaces 
of  the  sclera  between  its  inner  and  middle  thirds. 


1  Michel :  Beitrage  zur  naheren  Kenntniss  der  hinteren  Lymphbahnen  des  Auges, 
Archiv  f.  Ophthal.,  Bd.  xvin.,  1872. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL.  349 

The  pial  sheath  is  intimately  related  to  the  optic  nerve  bundles  not  only 
at  the  periphery  but  throughout  the  trunk.  The  pial  covering  gives  off 
numerous  septa  which  penetrate  in  all  directions  within  the  nerve  and  sub- 
divide the  nervous  tissue  into  the  hundreds  of  bundles  of  nerve-fibres  al- 
ready noted,  thus  corresponding  to  the  coarser  trabecul®  of  the  endoneurium 
of  other  nerves.  At  the  point  where  the  central  retinal  blood-vessels 
abruptly  pierce  the  periphery  to  gain  the  interior  of  the  optic  nerve,  a 
robust  extension  of  the  pial  sheath  accompanies  the  vessels  and  thenceforth 
occupies  the  centre  of  the  nerve  as  the  axial  perivascular  connective  tissue. 

The  blood-vessels  supported  by  the  larger  pial  trabecuhe  are  of  im- 
portance in  nourishing  the  nerve-bundles  ;  after  the  entrance  of  the  central 
retinal  vessels,  the  minute  twigs  directly  given  off  from  these  branches 
constitute  additional  sources  for  the  supply  of  the  axial  portions  of  the 
optic  nerve.  According  to  Krause,1  the  pial  tissue  surrounding  the  central 
retinal  vessels  also  contains  a  delicate  nervous  plexus  which  accompanies 
the  blood-vessels. 

The  lymphatics  of  the  optic  nerve,  in  addition  to  the  lymph-spaces 
within  the  sheaths  already  noted,  are  represented  by  clefts  situated  between 
the  nerve-bundles  and  the  intervening  pial  septa.  These  channels,  as  well 
as  the  intra-fascicular  interstices  between  the  groups  of  nerve-fibres,  commu- 
nicate, according  to  Schwalbe,2  with  the  intra-vaginal  lymphatic  tracts. 

THE   CRYSTALLINE    LENS. 

The  lens,  or  lens  erystallina,  is  the  most  important  part  of  the  dioptric 
system  of  the  eye,  since  the  refractive  changes  necessary  for  accommodation 
are  mainly  effected  by  the  alterations  of  its  curvatures,  and  hence  of  its 
power  of  focussing  rays  upon  the  retina  proceeding  from  objects  situated 
at  various  distances. 

The  lens  of  the  human  eye  is  a  transparent  biconvex  body,  with  cir- 
cular outline  and  rounded  margin,  which  measures  about  four  millimetres 
in  its  sagittal  and  from  nine  to  ten  millimetres  in  its  transverse  diameter. 
Seen  in  profile,  the  anterior  surface  is  less  convex  than  the  posterior,  the 
respective  radii  of  curvature  of  the  two  surfaces  during  accommodation  for 
distant  objects  being  ten  millimetres  and  six  millimetres ;  during  accommo- 
dation for  near  objects  the  lens  thickens  and  its  surfaces  become  distinctly 
more  convex.  In  this  latter  condition  the  radii  of  curvature  increase  to 
six  millimetres  and  five  and  a  half  millimetres  for  the  anterior  and  poste- 
rior surfaces  respectively ;  the  change,  moreover,  affects  the  anterior  to  a 
much  greater  degree  than  the  posterior  curvature.  When  critically  ex- 
amined, the  lens  curvatures  are  found  not  to  be  quite  spherical,  Brucke* 

1  Krause  :  Die  Nerven  der  Arteria  centralis  retinae  u.  8.  w.,  Archiv  f.  Ophthal.,  Bd. 
xxi.,  1875. 

»  Schwalbe :  Ueber  Lymphbahnen  der  Netzhnut  und  des  Glaskorpers,  B 
konigl.  sachs.  Gesellschaft  der  Wissensch.,  1872. 

s  Brucke  :  Anatomische  Beschreibung  des  menschlichen  Augapfels,  1847. 


350 


THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 


FIG.  87. 


having  shown  long  ago  that  the  anterior  surface  corresponds  to  the  segment 
of  an  ellipsoid,  the  posterior  to  a  paraboloid.  The  form  of  the  lens  also 
varies  at  different  periods  of  life.  •  The  lens  of  early  childhood  is  relatively 
thicker  and  more  convex  than  that  of  the  adult ;  old  age,  on  the  other 
hand,  is  attended  by  a  diminution  of  the  curvatures,  the  senile  lens  being 
distinctly  flatter  than  normal.  The  foetal  lens  approaches  the  spherical 
form,  and  by  the  end  of  gestation  has  attained,  according  to  Sappey l  and 
Jaeger,2  almost  its  fully  developed  sagittal  diameter.  The  transverse 
diameter,  on  the  contrary,  is  subject  to  augmentation  after  birth,  since 
from  a  measurement  of  seven  millimetres,  which  it  possesses  in  the  new- 
born, it  increases  to  eight  millimetres  by  the 
twelfth  year,  and  reaches  its  full  size  by  about 
the  eighteenth  year. 

Lying  between  the  iris  in  front  and  the 
vitreous  behind,  the  relations  of  the  lens  are 
such  that  its  anterior  surface  fills  the  pupillary 
opening  and  supports  the  pupillary  margin  of 
the  iris,  which  rests  upon  the  lens  for  a  varia- 
ble distance ;  the  peripheral  zone  of  the  lens 
lies  behind  and  separated  from  the  iris  by  the 
intervening  posterior  chamber,  to  the  posterior 
boundary  of  which  space  the  anterior  lens-sur- 
face contributes.  The  posterior  surface  of  the 
lens  lies  within  a  depression,  the  patellar  fossa, 
situated  in  the  anterior  surface  of  the  vitreous 
body  ;  its  margin  is  in  relation  with  the  ciliary 
body,  to  which  it  is  attached  by  means  of  the 
supporting  fibres  of  the  suspensory  ligament. 

The  lens-substance  during  life  is  doubly 
refracting,  perfectly  transparent,  and  colorless,  acquiring,  however,  a  light 
straw  tint  in  advanced  age.  It  exhibits  a  differentiation  into  a  harder,  less 
elastic  core,  the  nucleus  lentis,  which  occupies  the  centre,  and  the  superficial, 
softer,  compressible  peripheral  layer,  the  substantia  corticalis  ;  the  transition 
from  one  to  the  other,  however,  is  very  gradual  and  without  distinct  de- 
marcation. The  contrast  between  the  superficial  and  deeper  layers  depends 
not  upon  inherent  structural  differences,  but  upon  the  larger  amount  of 
tissue-juice  contained  within  the  cortex  in  consequence  of  its  more  favorable 
situation  for  the  imbibition  of  the  nutritive  fluids  by  means  of  which  alone 
the  vitality  of  the  non-vascular  lens  is  maintained. 

The  nucleus  obviously  enjoys  less  opportunity  for  active  nutrition  than 
does  the  cortex,  and  in  consequence  is  the  part  which  earliest  undergoes 
changes  resulting  in  the  characteristic  hardness  and,  in  late  life,  slight 

'Sappey:  Traite  d'anatomie  descriptive,  1889,  t.  in.  p.  761. 

2  Jaeger :  Ueber  die  Einstellung  des  dioptrischen  Apparatus  im  menschlichen  Auge, 
1861. 


Meridional  section  through  human 
lens.    (Babuchin.) 


THE   MICROSCOPICAL  ANATOMY   OP  THE  EYEBALL.  351 

coloration  and  even  opacity  observed  within  the  central  portion  of  the  lens. 
The  fact  that  the  most  recently  formed,  and  hence  least  resistant.  l.-n<- 
substance  lies  superficially  also  must  be  borne  in  mind  when  considering 
the  causes  of  the  softness  of  the  cortex.  The  refractive  index  of  the  lens- 
substance,  as  determined  by  Helmholtz,1  is  from  1.44  to  1.45;  according 
to  Krause,2  the  cortex  is  less  highly  refracting  than  the  nucleus,  the  in- 
dices of  the  two  being  1.4053  and  1.4541  respectively.  The  crystalline 
lens,  together  with  its  envelope,  presents  histologically  three  structures  for 
consideration, — the  lens-capsule,  the  suboapsular  epithelium,  and  the  letu- 
fibres. 

A  knowledge  of  the  development  of  the  lens  is  so  essential  for  an 
appreciation  of  the  true  significance  of  its  structure  that  a  brief  recapitu- 
lation of  the  manner  of  its  formation  may  be  here  presented  with  advantage, 
the  reader  being  referred  to  the  section  treating  of  the  Development  of  the 
Eye  for  the  details  of  the  somewhat  intricate  histogenesis. 

It  will  be  recalled  that  the  lens  develops  from  an  invagination  of  the 
ectoderm,  which  rapidly  becomes  converted  into  the  closed  lens-sac  by  the 
approximation  and  subsequent  union  of  the  lips  of  the  epithelial  recess. 
Soon  all  connection  with  the  surface  ectoderm  is  lost,  the  lens  then  being 
represented  by  a  sac  the  epithelial  walls  of  which  are  no  longer  of  uniform 
thickness,  but  already  present  a  differentiation  into  a  broader  posterior 
and  a  thinner  anterior  layer.  The  contrast  between  these  layers  rapidly 
becomes  more  marked  as  the  lens  'develops,  in  consequence  of  the  great 
increase  in  the  growth  and  length  of  the  cells  constituting  the  posterior 
wall  and  their  conversion  into  the  lens-fibres.  At  first  of  large  size,  the 
cavity  of  the  lens-sac  gradually  becomes  reduced  by  the  encroachment  of 
the  rapidly  thickening  posterior  wall,  until  finally  it  is  entirely  obliterated 
by  the  apposition  of  the  lens-fibres  against  the  cells  of  the  anterior  wall, 
which  meanwhile,  notwithstanding  their  slight  increase  in  numbers,  retain 
the  character  of  epithelial  elements. 

The  primary  lens-substance,  therefore,  is  the  product  at  first  entirely  of 
the  growth  and  differentiation  of  the  elements  of  the  posterior  layer  of  the 
lens-sac,  the  anterior  cells  remaining  largely  passive.  With  the  increasing 
dimensions  of  the  lens,  however,  the  original  posterior  cells  no  longer 
suffice,  and  it  becomes  necessary  to  produce  new  and  additional  lens-fibres ; 
these  augmentations  take  place  at  the  periphery  of  the  now  solid  lens  by 
the  conversion  of  the  most  peripherally  situated  elements  of  the  anterior 
layer.  This  region,  the  so-called  transitional  zone,  henceforth  until  the 
completion  of  the  growth  of  the  lens  becomes  the  seat  of  a  constant  meta- 
morphosis whereby  the  short  columnar  marginal  cells  of  the  anterior  layer 
are  converted  into  the  meridionally  arranged  lens-fibres.  The  elemente  of 

1  Helmholtz:  Ueber  die  Accommodation  des  Auges,  Archiv  f.  Ophthalmol.,  Bd.  I., 
1855. 

2  Krause:  Die  Brechungsindices  der  durchsichtigen  Medien  des  menscl 

1855. 


352 


THE    MICROSCOPICAL    ANATOMY   OF    THE    EYEBALL. 


ch 


the  anterior  stratum  which  are  never  needed  for  the  production  of  lens-fibres 
persist  as  a  single  layer  of  polygonal,  mostly  hexagonal,  cells  situated 
beneath  the  capsule,  and  constitute  the  epithelium  of  the  anterior  capsule, 
or  subcapsular  epithelium. 

The  capsule  of  the  lens  is  entirely  different  in  origin,  being  probably 
largely,  if  not  entirely,  the  derivative  of  the  mesodermic  tissue  which  at 
an  early  period  surrounds  the  primitive  lens  as  an  extension  of  the  vitreous 

mesoderm    occupying    the 

FIG.  88.  cavity    of   the    secondary 

optic  cup.  The  primitive 
vitreous  tissue  immediately 
surrounding  the  posterior 
surface  of  the  lens  is  highly 
vascular  in  consequence  of 
the  rich  net-work  of  radi- 
ally coursing  capillaries  de- 
rived from  the  breaking 
up  of  the  hyaloid  artery 
in  the  vicinity  of  the  pos- 
terior pole  of  the  lens.  The 
vascular  sheet  of  vitreous 
mesoderm  thus  differen- 
tiated constitutes  the  mem- 
brana  capsularis. 

At  the  margin  of  the 
lens  the  capsular  membrane 
passes  to  the  anterior  sur- 
face of  the  lens,  which  it 
completely  covers,  though 
designated  by  different 
names.  Before  reaching 
the  centre  of  this  surface 
the  radially  arranged  capil- 
laries continued  from  the 
posterior  net-work  are 
joined  by  supplementary 
vessels  proceeding  from  the 
pupillary  margin  of  the 
iris  ;  the  additional  vessels 

so  derived  are  venous  in  character  and  aid  in  carrying  off  the  blood  con- 
veyed by  the  branches  of  the  hyaloid  artery. 

The  part  of  the  anterior  vascular  stratum  closing  the  pupillary  opening 
constitutes  the  membrana  pupillaris,  the  remaining  portion,  occupying  the 
peripheral  zone,  the  membrana  capsulo-pupillaris.  While  receiving  dis- 
tinctive names,  it  must  be  remembered  that  the  pupillary  and  capsulo- 


Vertical  section  through  the  eye,  at  an  early  stage,  of  an 
embryo  mouse.  Enlarged  130  times.  (After  Kessler,  from  Hert- 
wig's  Lehrbuch.)— p,  pigmented  epithelium,  forming  the  outer 
layer  of  the  secondary  optic  cup ;  r,  thickened  inner  layer,  the 
future  retina;  TO,  marginal  zone,  or  border  of  secondary  optic 
vesicle,  which  develops  later  into  the  non-sensory,  ciliary,  and 
iritic  portions  of  the  retina  (uveal  tract) ;  v,  vitreous  body,  with 
blood-vessels;  1v,  tunica  vasculosa  lentis;  be,  blood-corpuscles; 
ch,  choroid  ;  If,  lens-fibres ;  le,  anterior  epithelium  of  lens ;  I,  nu- 
clear zone  of  lens-fibres ;  c,  connective-tissue  layer  (corlum)  of 
cornea;  ce,  outer  corneal  epithelium. 


THE    MICROSCOPICAL    ANATOMY   OF   THE    EYEBALL.  353 

pupillary  membranes  are  really  parts  of  the  same  vascular  mesodermic 
envelope  that  constitutes  the  membrana  capstilaris. 

With  the  advanced  development  of  the  lens,  the  important  services  of 
the  vascular  capsular  membrane  in  supplying  the  rapidly  growing  lens  with 
nourishment  terminate,  and  in  the  human  eye  the  structure  undergoes 
atrophy  and  disappearance  before  birth  ;  as  well  known,  in  some  mammals, 
on  the  contrary,  the  membrana  capsularis  persists  for  a  short  time  after 
birth,  the  still  conspicuous  pupillary  membrane  of  the  new-born  kitten 
affording  a  familiar  example. 

The  foregoing  intimate  relations  between  the  lens  and  the  surrounding 
vascular  envelope  at  once  suggest  a  mesodermic  origin  for  the  lens-capsule, 
the  only  difficulty  in  accepting  such  derivation  for  the  latter  structure  being 
the  fact,  as  established  by  the  observations  of  Kolliker,1  Kessler,2  and  others, 
that  the  immediate  lens-capsule  exists  before  the  appearance  of  the  vascular 
mesodermic  membrana  capsularis.  This  precedence  is  interpreted  by  these 
authorities  as  indicating  that  the  lens-capsule  must  be  regarded  as  a  cuticu- 
lar  structure  formed  through  the  agency  of  the  ectodermic  elements  of  the 
lens-sac,  a  view  already  accepted  by  H.  Muller.3 

Notwithstanding  these  observations,  the  genetic  relation  of  the  meso- 
dermic tissue  to  the  capsule  seems  undeniable,  and  is  accepted  by  a  majority 
of  anatomists,  including  Zernoff4  and  Waldeyer.5  A  possible  twofold 
origin,  from  the  mesoderm  posteriorly  and  from  the  ectodermic  cells  ante- 
riorly, has  been  suggested  by  Manz  : 6  while  this  view  is  untenable,  in  con- 
sideration of  the  continuity  of  all  parts  of  the  capsule,  which  undoubtedly 
possess  a  common  origin,  the  manifestly  close  relation  of  the  surrounding 
mesodermic  tissue  and  the  intimate  attachment  of  the  suspensory  fibres  to 
the  somewhat  differentiated  superficial  zonular  lamella  have  led  Schwalbe7 
to  entertain  the  opinion  that  possibly  the  innermost  portion  of  the  cap- 
sule is  a  cuticular  formation  produced  by  the  lens-cells,  the  external  layer 
of  the  capsule  being  a  product  of  the  surrounding  mesoderm.  In  view 
of  the  variance  of  opinion,  it  must  be  admitted  that  the  derivation  of  the 
lens-capsule  is  still  uncertain,  and  awaits  renewed  study  of  developmental 
stages  ;  the  writer,  however,  from  the  examination  of  the  developing  eyes 
of  rabbit  embryos,  inclines  to  the  opinion  that  the  entire  capsule  is  of  meso- 
dermic origin. 

The  Capsule  of  the  Lens. — The  capsule  completely  envelops  the  lens, 

1  Kolliker:  Entwickelungsgeschichte  des  Menschen  und  der  hoheren  Thiere,  1879. 

*  Kessler  :  Zur  Entwickelung  des  Auges  der  Wirbelthiere,  1877. 

8  H.  Muller:  Anatomische  Beitrage  zur  Ophthalmologie,  Archiv  f.  Ophthalmol.,  Bd. 
II.,  1856. 

*  Zernoff:    Zur  Entwickelung  des  Auges,  Centralblatt  f.  d.  med.  Wissenschafl.,  No. 
13,  1872. 

5  Waldeyer  :  Entwickelungsgeschichte  des  Auges,  Jahresbericht  f.  Ophthalmol.,  1872. 

6  Manz :  Entwickelungsgeschichte  des  menschlichen  Auges,  Handbuch  d.  Augenheil- 
kunde,  von  Graefe  u.  Saemisch,  Bd.  II.,  1876. 

7  Schwalbe  :  Lehrbuch  der  Anatomic  der  Sinnesorgane,  1887,  S.  127. 

VOL.  I.— 23 


354 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


and  by  its  intimate  attachment  with  the  suspensory  fibres  proceeding  from 
the  ciliary  region  forms  an  important  part  of  the  supporting  apparatus  of 
the  lens.  It  is  a  transparent,  highly  elastic  membrane,  which,  while  con- 
tinuous in  all  parts,  is  conventionally  divided  into  the  anterior  and  posterior 
capsule  covering  the  respective  surfaces  of  the  lens.  The  capsule  varies  in 
thickness,  being  considerably  stouter  in  the  central  area  of  its  anterior  seg- 
ment, where  it  attains  a  thickness  of  from  .010  to  .015  millimetre,  and 
thinner  at  the  periphery ;  the  most  attenuated  portion  corresponds  to  the 
middle  of  the  posterior  surface,  at  which  point  the  membrane  is  reduced  to 
from  .005  to  .007  millimetre  in  thickness.  In  its  chemical  composition 
and  reactions  the  lens-capsule  corresponds  neither  to  elastic  nor  to  white 

FIG.  89. 


.1,  section  of  crystalline  lens  embracing  capsule  (a),  anterior  epithelium  (b),  and  most  super- 
ficial fibres  (c)  of  cortex ;  B,  portion  of  lens-capsule  (d)  seen  from  the  surface,  with  a  number  of  anterior 
epithelial  cells  (e)  still  adherent.  Magnified  500  diameters. 

fibrous  tissue,  but  approaches  most  closely  to  the  sarcolemma  and  the  base- 
ment membrane  of  glands. 

The  histological  details  of  the  capsule  are  negative,  the  membrane  pos- 
sessing no  cells,  and  at  most  exhibiting  an  indication  of  lamellae  as  expressed 
by  a  fine  parallel  striation  seen  in  sections  when  examined  under  high  mag- 
nification. After  maceration  in  a  solution  of  potassium  permanganate, 
Berger l  succeeded  in  splitting  up  the  capsule  into  lamella?  corresponding  with 
the  parallel  markings  seen  in  section ;  the  separation  of  the  most  superficial 
stratum,  the  zonular  lamella,  most  readily  takes  place,  the  more  deeply 
situated  portion  of  the  capsule  stoutly  resisting  cleavage.  The  pronounced 
tendency  to  undergo  this  division  is  regarded  by  some  authorities,  among 


1  Berger:  Bemerkungen  uberdie  Linsenkapsel,  Centralblatt f.  prakt.  Augenheilkunde, 


1882. 


THE    MICROSCOPICAL,   ANATOMY    OF   THE    EYEBALL. 


355 


Fro.  90. 


whom  is  Schwalbe,1  as  strong  evidence  in  favor  of  a  double  origin  of  the 
capsule,  the  outer  layer  being  mesodermic,  and  the  inner  derived  from  the 
ectoderm  as  a  cuticular  formation  produced  by  the  epithelium  of  the  lens-sac. 

The  Epithelium  of  the  Lens. — The  epithelium  constitutes  the  most  ante- 
rior layer  of  the  lens  proper,  and,  as  already  pointed  out,  represents  the 
remains  of  the  front  wall  of  the  primary  lens-sac.  It  consists  of  a  single 
layer  of  polyhedral,  mostly  six-sided,  flattened 
cells,  about  .020  millimetre  in  diameter;  in  early 
life  the  cells  are  smaller  and  cuboidal,  measuring 
about  .010  millimetre  in  each  dimension.  The 
finely  granular  protoplasm  contains  usually  a 
spherical  nucleus  about  .005  millimetre  in  diameter, 
often  nucleoli,  and  sometimes  vacuoles.  The  epi- 
thelial cells  extend  with  their  anterior  ends  into 
the  thin  layer  of  subcapsular  stratum  of  albumi- 
nous material  which  serves  to  connect  the  capsule 
with  the  epithelium ;  the  embedded  ends  of  the 
cells  sometimes  present  irregular  projections  which 
give  an  irregular  contour  to  the  epithelial  elements. 

On  approaching  the  margin  of  the  lens,  the 
epithelial  cells,  in  addition  to  becoming  more 
granular,  undergo  important  change  in  form,  as- 
suming more  and  more  markedly  the  columnar 
type ;  the  columnar  cells,  in  turn,  are  replaced  in 
the  equatorial  region  by  the  extended  lens-fibres 
into  which  they  elongate.  The  elements  within 
the  transition-zone  at  first  present  a  slightly  sinuous 
axis;  as  the  curve  becomes  more  marked,  how- 
ever, its  concavity  is  directed  outward.  Coin- 
cidently  with  the  rapid  increase  in  the  length  of 
the  transforming  cells,  their  axes  gradually  become 
straight  and  then  bowed  in  the  opposite  direction, 
the  concavities  looking  outward.  As  a  result 
of  these  variations  in  the  axes  of  the  developing 
lens-fibres,  the  marginal  portions  of  the  lens  in 
meridional  sections  present  the  peculiar  appear- 
ance which  has  been  termed  the  lens-whorl  by 
O.  Becker.2 

When  critically  examined,  the  increase  which  takes  place  in.  the  length 
of  the  cylindrical  cells  during  their  conversion  into  the  lens-fibres  is  seen 
to  be  especially  dependent  upon  the  rapid  growth  of  the  extra-nuclear  half 
of  the  cell, — that  is,  of  the  part  lying  between  the  nucleus  and  the  anterior 

1  Schwalbe  :  loc.  cit. 

3  O.  Becker  :  Ueber  den  Wirbel  und  Kernbogen  in  der  menschlichen  Linse,  Archiv  f. 
Augenheilkunde,  Bd.  xn.,  1883. 


Meridional  section  through 
equatorial  region  of  young  lens, 
showing  zone  of  transforma- 
tion (K],  in  which  cells  of  ante- 
rior epithelium  (6)  are  con- 
verted into  lens-fibres  (d) ;  c, 
nucleated  young  fibres ;  a,  lens- 
capsule.  Magnified  240  diam- 
eters. 


356 


THE    MICROSCOPICAL    ANATOMY    OF    THE    EYEBALL. 


capsule.  In  consequence  of  this,  the  nuclei  of  the  young  leus-fibres  become 
progressively  farther  removed  from  the  equator,  their  position  within  the 
so-called  nuclear  zone  being  indicated  by  the  conspicuous  nuclear  arch, 
which  is  marked  out  by  the  sequence  of  the  deeply  staining  nuclei. 

The  nuclear  zone  includes  but  a  limited  peripheral  area  near  the  margin 
of  the  lens,  since  the  nuclei  disappear  from  the  more  deeply  situated  fibres. 
The  lens-whorl  and  nuclear  arch  depend  upon  the  arrangement  of  the 
young  developing  lens-fibres  derived  from  the  elongating  elements  of  the 
anterior  epithelium,  and  are,  therefore,  most  conspicuous  during  the  years 
of  active  growth ;  in  subjects  of  advanced  years,  on  the  contrary,  they 
are  but  feebly  marked. 

The  delicate  subepithelial  stratum  of  albuminous  material  which,  in  the 
fresh  lens,  lies  between  the  posterior  surface  of  the  epithelium  and  the  ante- 

FIG.  91. 


Fragments  of  isolated  lens-fibres.— A,  from  the  superficial  layers ;  B,  from  a  more  centrally  situated 
zone ;  C,  parts  of  younger  fibres,  containing  nuclei.    Magnified  300  diameters. 

rior  ends  of  the  lens-fibres,  occupies  the  space  which  represents  the  remains 
of  the  cavity  of  the  primary  lens-sac.  During  life  it  is  probable  that  this 
substance  is  semi-fluid ;  it  rapidly  liquefies  after  death,  when  its  presence 
may  be  demonstrated,  on  puncture  of  the  anterior  capsule  and  epithelium, 
by  the  extrusion  of  a  droplet  of  the  so-called  liquor  Morgagni.  The  latter, 
according  to  O.  Becker,  never  exists  during  life.  Abnormal  dilation  of 
the  capillary  subepithelial  space,  owing  to  imbibition  of  fluid,  and  vacuoli- 
zation  of  its  albuminous  contents  are  also  frequent  post-mortem  changes. 
The  substance  of  the  lens  consists  of  an  aggregation  of  the  lens-fibres 
united  by  the  intervening  cement-substance.  The  lens-fibres  represent  the 
greatly  elongated,  specialized  epithelial  cells,  primarily  of  the  posterior  wall 
of  the  lens-sac,  supplemented  by  those  derived  from  the  anterior  epithelium. 
The  individual  elements,  as  seen  after  isolation  by  means  of  boiling,  pro- 
longed maceration  in  acids,  or  other  methods,  are  long,  flattened,  ribbon-like 


THE   MICROSCOPICAL  ANATOMY   OF   THE   EYEBALL.  357 

fibres  which  present  in  transverse  section  a  compressed  hexagonal  outline. 
The  longer  parallel  sides  face  outward  and  inward,  while  the  shorter  boun- 
daries converge  at  either  edge. 

The  length  of  the  lens-fibres  composing  the  superficial  layers  is  much 
greater  than  that  of  those  centrally  situated,  since  the  former  extend  about 
two-thirds  of  the  meridional  distance  from  pole  to  pole,  while  the  latter  cor- 
respond closely  to  the  length  of  the  lens-axis ;  the  peripheral  fibres  measure, 
therefore,  about  eight  millimetres,  and  the  central  ones  four  millimetres,  or 
about  half  as  much.  Additional  variations  in  the  breadth  and  the  thick- 
ness of  the  fibres  are  exhibited  by  the  superficial  and  axial  elements :  the 
former  measure  from  .010  to  .012  millimetre  in  width  and  about  .005 
millimetre  in  thickness,  the  latter  only  about  .0075  millimetre  and  .0025 
millimetre  for  the  corresponding  dimensions. 

Further  differences  between  the  central  and  the  superficial  lens-fibres 
are  emphasized  by  the  nucleus,  which  structure  is  present  in  the  peripheral 
fibres,  but  absent  in  those  axially  placed.  The  position  of  the  nucleus, 
when  present,  is  indicated  in  profile  views  of  the  elements  by  a  localized 
increase  in  the  thickness  of  the  fibre ;  the  width  of  the  fibre,  on  the  other 
hand,  is  sufficient  to  accommodate  the  nucleus  without  modifying  the  con- 
tour. The  superficial  fibres  are  further  distinguished  from  those  occupying 
the  centre  of  the  lens  by  being  softer  and  containing  more  fluid,  the  axial 
fibres  exhibiting  the  results  of  a  process  of  hardening  somewhat  analogous 
to  that  observed  in  the  outer  layers  of  the  epidermis. 

The  peripheral  lens-fibres  present  a  smooth  outline  in  contrast  to  the 
serrated  edges  of  those  forming  the  middle  and  inner  portions  of  the  lens. 
The  finely  toothed  borders  of  the  fibres  are  not 
uniformly  developed  on  all  edges,  but  are  usually  FIG.  92. 

best  marked  on  the  diagonally  opposed  blunter 
edges.  The  serrations  are  so  disposed  that  they 
do  not  interlock,  but  are  only  in  apposition  along 
the  apices  of  the  minute  projections,  the  union  of 
the  lens-fibres  being  effected  by  the  interfibrillar 
cement-substance.  The  minute  intervals  left  be- 
tween the  individual  lens-fibres  in  consequence  of  Lens-fibres  seen  tt L  trww- 

«  verse  section.     Magnified  350 

the  arrangement  of  the  abutting  serrations  are  oc-    diameters. 

cupied  by  the  semi-fluid  cement-substance.     While 

these  tracts  constitute  passage-ways  of  importance  for  the  transmission  of 

nutritive  juices  to  the  tissue  of  the  lens,  they  do  not  constitute  a  system 

of  interfibrillar  canals  as  described  by  v.  Becker,1  but  correspond  to  the 

intercellular  clefts  found  in  other  epithelial  structures.      Schlosser2  not 

only  regards  these  spaces  as  lymph-paths,  which  in  principle  they  indeed 

1  F.  v.  Becker :  Untersuchung  iiber  den  Bau  der  Linse  bei  den  Menschen  und  den 
Wirbelthieren,  Archiv  f.  Ophthalmol.,  Bd.  ix.,  1863. 

2  Schlosser:    Ueber  die  Lymphbahnen  der  Linse,  Munchner  med.  Wochenschrift, 
Bd.  xxxvi.,  1889. 


358 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


are,  but  ascribes  to  their  contents  a  definite  current,  quoting  the  clinical 
observations  of  Samelsohn  on  the  dissemination  of  rust  particles  along  the 
interfibrillar  clefts  in  support  of  his  views.  The  bond  of  union  between 
the  individual  lens-elements  differs  in  its  stability  on  the  several  sides  of 
the  fibres,  since  the  layer  of  cement-substance  is  thinnest  between  the 
broader  surfaces  of  the  fibres  and  thickest  over  the  narrow  facets. 

The  cement-substance  resembles  that  occurring  in  other  epithelial  struc- 
tures, being  semi-fluid  in  consistence  and  albuminous  in  nature ;  in  addition 
to  uniting  the  individual  lens-fibres  with  one  another,  it  exists  in  other 
localities,  as  beneath  the  anterior  epithelium  and  between  the  ends  of  the 


FIG.  93. 


FIG.  94. 


Central  part  of  the  anterior  lens-star, 
showing  close  relations  of  lines  of  juncture  in 
perfectly  preserved  tissue.  (Babuchin.) 


Central  part  of  the  posterior  lens-star, 
showing  union  of  lines  of  juncture  of  lens- 
fibres.  (Babuchin.) 


abutting  fibres  along  the  so-called  sutures.  The  presence  of  considerable 
quantities  of  interfibrillar  substance  in  the  form  of  a  stellar  mass,  and  along 
its  diverging  limbs,  constituting  the  lens-stars,  later  to  be  noted,  as  described 
by  the  older  anatomists  and  represented  in  the  classic  drawings  of  Arnold,1 
depends  upon  post-mortem  changes,  no  such  accumulations  of  inter- 
fibrillar substance  existing  within  the  living  lens  or  within  the  perfectly 
preserved  tissue.  The  investigations  of  Zernoff,2  Babuchin,3  O.  Becker,4 
and  others  show  conclusively  the  fallacy  of  the  older  descriptions. 

In  addition  to  the  variation  in  the  consistence  of  the  peripheral  and  of 
the  centrally  situated  fibres,  already  noted,  each  lens-element  exhibits  indi- 
vidual differences  in  the  softness  and  greater  affinity  for  stains  possessed  by 

1  F.  Arnold:  Tabulae  anatomicae,  Fasciculus  n. ;  Icones  organorum  sensum,  Tab.  in., 
1838. 

2  Zernoff:  Zum  mikroskopischen  Bau  der  Linse  beim  Menschen  und  bei  den  Wirbel- 
thieren,  Archiv  f.  Ophthalmol.,  Bd.  xin.,  1867. 

3  Babuchin :  Die  Linse,  Strieker's  Handbuch  der  Lehre  von  den  Geweben,  Bd.  n., 
1872. 

*  O.  Becker :  Ueber  den  Wirbel  und  Kernbogen  in  der  menschlichen  Linse,  Archiv 
f.  Augenheilkunde,  Bd.  xn.,  1883. 


THE   MICROSCOPICAL   ANATOMY   OP  THE   EYEBALL. 


359 


Fio.  96. 


the  extremities  of  the  fibres,  as  compared  with  the  middle  portions ;  like- 
wise by  the  axial  portion  of  the  fibre  as  contrasted  with  the  denser  super- 
ficial zone.     A  distinct  demarcation  between  these  parts  of  the  fibre  how- 
ever, does  not  exist,  these  changes  in  the  physical 
character  of  the  lens-elements  occurring  gradually 
and  hot  abruptly.    While  the  peripheral  or  super- 
ficial substance  of  each  fibre  is  somewhat  con- 
densed and  thereby  constitutes  a  boundary  zone, 
no   distinct   membrane   envelops    the    lens-fibre. 
Interesting  changes  in  the  contour  of  the  lens- 
fibres  due  to  variations  in  their  consistence  are 
figured  in  the  paper  of  Ritter.1 

After  hardening  and  removal  of  the  capsule, 
the  lens  may  be  readily  separated  into  a  number 
of  concentric  lamellae  in  a  manner  somewhat  simi- 
lar to  the  separation  taking  place  in  the  cleavage 
of  the  coats  of  an  onion.  The  laminae  thus  ob- 
tained, however,  are  not  continuous  sheets  in- 
cluding the  entire  lens,  but  break  up  into  seg- 
ments which  at  most  include  only  about  one-third,  not  infrequently  distinctly 
less,  of  the  lens-surface. 

The  fracture  of  the  lamellae  in  a  definite  manner  depends  upon  the 
arrangement  of  the  lens-fibres,  and  corresponds  to  the  lines  along  which 
these  elements  abut,  as  presently  to  be  described.  The  lines  of  apposition 

FIG.  96. 


Crystalline  lens  of  new-born 
child,  seen  from  the  side,  show- 
ing the  course  of  the  lens-fibres. 
(Arnold.)  Magnified  6  diameters. 


Adult  crystalline  lens,  showing  lens-stars.  (Arnold.)— A,  anterior  surface;  B,  posterior  surface.  The 
radiating  lines  of  juncture  meet  at  the  central  area  apparently  occupied  by  an  indifferent  granular 
substance.  Magnified  6  diameters. 

of  the  fibres,  or  suture-lines,  appear  as  faintly  marked,  whitish,  radiating 
strife,  which  depend  upon  the  local  accumulations  of  the  interfibrillar 
cement-substance  and  constitute  the  lens-stars. 

The  lens-stars  are  best  seen   in  the  foetal  lens  or  in  the  centre  of  the 


1  Ritter  :  Zur  Histologie  der  Linse,  Archiv  f.  Ophthalmol.,  Bd.  xxn.  u.  xxm.,  1876, 


1877. 


360 


THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 


Fio.  97. 


adult  lens,  where  they  appear  on  both  surfaces  as  triradiate  figures  the 
arms  of  which  extend  from  the  centre  towards  the  periphery.  They  are 
symmetrically  disposed,  and  diverge  from  one  another  at  a  uniform  angle 
of  120°.  The  rays  of  the  two  surfaces  do  not  correspond  in  position,  but 
are  so  arranged — as  well  seen  in  the  lens  of  the  new-born  child  and  less 
distinctly  in  the  nucleus  of  the  adult  lens — that  on  the  anterior  surface  the 
upper  ray  is  vertical,  while  on  the  posterior  surface  the  lower  ray  stands 
perpendicularly.  It  follows  from  this  relation  that  the  rays  of  the  ante- 
rior star  have  undergone  a  torsion  of  60°  in  reaching  the  posterior  surface, 
or,  in  other  words,  that  the  position  of  the  points  of  contact  of  the  lens- 
fibres  changes  in  each  successive  stratum,  so  that  their  lines  of  apposition 
become  spiral. 

In  the  adult  lens  the  star  figures  become  less  symmetrical  and  more 
complicated  by  the  introduction  of  secondary  rays ;  these  are  so  placed  that 
the  lens-stars  possess  five,  six,  or  more  arms.  The  exact  arrangement, 
both  as  to  number  and  disposition  of  the  radii,  is  variable,  although  the 

basis  of  the  figures,  as  shown  by 
the  lens-stars  of  the  nuclear  por- 
tion of  the  lens,  still  represents 
the  now  masked  primary  plan. 
Friedenberg l  found  the  five-rayed 
star  most  common  in  the  adult 
lens.  The  rays,  in  the  fully 
matured  condition,  do  not  extend 
as  far  as  the  centre  of  the  poles, 
but  join  in  a  common  area,  the 
stellar  mass,  which  appears  granu- 
lar, as  usually  seen,  and  is  some- 
what irregular  in  outline.  The 
appearance  of  the  secondary  radii 
is  due  to  the  juncture-line  of  the 
additional  lens-fibres  necessitated 
by  the  increased  circumference 
of  the  growing  lens,  the  number 
sufficing  to  cover  in  the  periphery  of  the  fcetal  lens  no  longer  possessing  the 
requisite  space. 

The  complicated  disposition  and  course  of  the  lens-fibres  have  already 
been  mentioned  in  connection  with  the  description  of  the  lens-stars :  it 
remains  here  to  consider  the  arrangement  of  the  fibres  more  in  detail. 

With  the  exception  of  those  forming  the  immediate  core  of  the  lens, 
the  direction  of  which  corresponds  closely  with  the  lens-axis,  the  gen- 
eral course  of  the  fibres  is  meridional,  the  ribbon-like  elements  extending 


Diagram  showing  course  of  lens-fibres  from  ante- 
rior to  posterior  stars.  (Testut.)— E,  E,  equatorial 
axis ;  the  dark  lines  represent  the  anterior  star,  the 
dotted  lines  the  posterior;  the  course  of  the  indi- 
vidual fibres  A-D  is  indicated  to  a'-d'. 


1  Friedenberg :  Ueber  die  Sternfigur  der  Krystalllinse,  Inaug.  Dissertation,  Strassburg, 


1891. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL. 


361 


Pio.  98. 


from  the  anterior  to  the  posterior  surface.  The  anterior  end  of  the  fibre 
lies  within  some  part  of  the  star  of  the  corresponding  surface,  extends  to 
the  periphery  of  the  lens,  around  which  it  sharply  bends,  and  passes  to 
some  point  within  the  system  of  the  stellar  figure  of  the  posterior  sur- 
face. Those  fibres  which  begin  near  the  anterior  pole  proceed  to  the 
more  peripheral  parts  of  the  posterior  star,  and,  conversely,  those  lying 
near  the  margin  anteriorly  reach  farther  towards  the  posterior  pole. 

The  lens  being  formed  by  the  superposition 
of  consecutive  strata  of  fibres,  it  follows  that 
the  curvature  as  well  as  the  length  of  the  ele- 
ments composing  the  various  layers  must  differ, 
those  constituting  the  more  superficial  portions 
of  the  lens  being  more  sharply  bent  and  longer 
than  those  occupying  a  position  nearer  the 
nucleus.  The  elements  of  the  same  stratum, 
however,  possess  the  same  length  and  curve. 
The  curvature  and  length  of  the  fibres  gradu- 
ally decrease  towards  the  centre ;  the  elements 
occupying  the  middle  layers,  or  about  the 
outer  limit  of  the  nucleus,  present  almost  the 
curvature  of  a  sphere,  those  lying  axially  pos- 
sessing progressively  diminishing  flexure  and 
length  until  the  central  fibres,  as  already  stated, 
are  almost  straight  and  of  equal  length  with 
the  antero-posterior  diameter  of  the  lens. 

The  course  of  the  lens-fibres  is  not  strictly 
meridional  throughout,  since  the  fibres  join 
the  suture-lines  at  angles  approaching  ninety 
degrees ;  the  fibres,  therefore,  usually  undergo 
additional  bending  just  before  they  join  the 
rays,  in  consequence  of  which  their  course  in 
situ  often  approaches  an  S-like  curve.  As  the 
result  of  this  bending  towards  the  lines  of 
cement-substance,  the  fibres  turn  in  such  manner 
that  collectively  they  form  peculiar  figures  be- 
tween the  radii,  to  which  the  term  vortex  lentis 
has  been  applied. 

It  follows  from  the  arrangement  of  the 
elements  above  described  that  the  subcapsular 

epithelium  comes  in  contact  with  the  broader  surface  of  the  lens-fibres, 
and  nowhere,  except  throughout  a  limited  equatorial  area,  with  the  ends 
of  the  fibres,  since  these  abut  within  the  star-rays  against  one  another. 
Meridional  sections  of  the  lens,  therefore,  invariably  show  the  superficial 
lens-fibres  running  parallel  with  the  anterior  epithelium,  to  which  they  are 
united  by  the  intervening  cement-substance.  At  one  point,  however,  where 


Meridional  section  of  equatorial 
region  of  crystalline  lens  of  a  woman 
aged  seventy-five  years.  (Becker.) 
The  inactivity  of  the  transformation 
zone  is  conspicuous  in  contrast  with 
that  of  the  young  lens. 


362  THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 

the  epithelial  cells  undergo  transition  into  the  elongated  lens-fibres  at  the 
equatorial  margin,  the  ends  of  the  young  fibres  come  in  contact  with  the 
anterior  epithelium  within  a  limited  area ;  this  disposition  is  soon  replaced 
by  the  usual  relation,  in  consequence  of  the  rapid  growth  of  the  lens-fibres 
and  their  increasing  convexity. 

In  preparations  of  silvered  lens  and  capsule,  in  addition  to  the  some- 
what complicated  mosaic  produced  by  the  cells  on  the  anterior  capsule,  the 
posterior  capsule,  as  noted  by  Barabaschew,1  also  exhibits  the  impressions 
of  the  lens-fibres,  as  well  as  aberrant  markings  due  to  the  silver  reduction 
effected  by  extruded  fluid  substances. 

The  growth  of  the  lens  after  its  primary  formation  takes  place  by  the 
addition  of  strata  of  new  fibres,  principally  in  the  equatorial  region,  in 
correspondence  with  the  great  cellular  activity  within  the  transition  zone. 
The  production  of  the  new  elements  necessary  to  maintain  the  continual 
transformation  of  the  epithelial  cells  into  lens-fibres  is  found  within  the 
anterior  epithelium  alone,  the  elements  of  which,  as  shown  by  the  presence 
of  karyokinetic  figures,  undergo  division  and  produce  new  cells  during  the 
growth  of  the  lens.  The  new  cells  are  gradually  displaced  towards  the 
equator  by  the  still  younger  elements,  and  in  turn  become  transformed  into 
fibres.  There  is  no  evidence  of  division  and  multiplication  directly  of  the 
lens-fibres,  these  elements  being  in  a  condition  of  high  specialization  and 
beyond  the  stage  of  reproductive  activity.  According  to  the  investigations 
of  Harting,2  the  embryonal  lens  increases  by  interstitial  growth,  the  indi- 
vidual fibres  gaining  in  size ;  after  birth,  however,  as  shown  originally  by 
Kolliker3  and  by  Henle,4  the  further  growth  of  the  lens  is  by  apposition, 
the  new  fibres  being  added  by  the  transformation  of  the  anterior  epithelium 
at  the  equatorial  transition  zone. 

THE   VITREOUS   BODY. 

The  large  space  included  between  the  lens  in  front  and  the  retina  behind 
and  at  the  sides,  embracing  approximately  four-fifths  of  the  capacity  of  the 
eyeball,  is  occupied  by  the  vitreous  body. 

The  vitreous  body,  or  humor  vitreus,  when  examined  in  the  perfectly  fresh 
condition,  is  a  beautifully  transparent,  semi-fluid  mass,  the  general  form  of 
which  resembles  a  sphere  flattened  in  its  antero-posterior  axis ;  the  anterior 
pole  is  further  modified  by  a  depression,  the  fossa  patellaris,  which  receives 
the  posterior  surface  of  the  lens.  An  important  function  of  the  vitreous 
body  is  the  support  of  the  retina,  against  which  it  closely  lies,  but  from 
which  it  is  readily  separable  after  appropriate  treatment;  at  one  point, 

1  Barabaschew:  Beitragzur  Anatomic derLinse,  Archiv  f.  Ophthalmol.,  Bd.  xxxvm., 
1892. 

*  Harting  :  Recherches  micrometriques,  1847. 

5  Kolliker:  Ueber  die  Entwickelung  der  Linse,  Zeitschrift  f.  wiss.  Zoologie,  Bd.  vi., 
1855. 

*  Henle :  Zur  Entwickelungsgeschichte  der  Krystalllinse  und  zur  Theilung  des  Zell- 
kerns,  Archiv  f.  mik.  Anatomic,  Bd.  xx.,  1882. 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL.  363 

however, — namely,  over  the  optic  entrance, — the  vitreous  and  the  retina 
are  more  intimately  connected  than  elsewhere,  in  consequence  of  the  close 
association  established  by  the  early  entrance  of  the  hyaloid  in  this  location. 

This  remarkable  and  conspicuous  constituent  of  the  eyeball  early 
attracted  the  attention  of  anatomists,  and  from  the  observations  of  Petit, 
Demours,  Zinn,  Cloquet,  Fr.  Arnold,  Brucke,  Hannover,  and  Bowman  to 
the  investigations  of  Schvvalbe,  Iwanoff,  H.  Virchow,  Straub,  and  Retzius, 
a  period  of  almost  one  and  three-quarters  centuries,  the  structure  of  the 
vitreous  body  has  been  the  subject  of  widely  divergent  opinions,  and  even 
at  the  present,  it  must  be  admitted,  conflicting  views  remain  to  be  reconciled. 

The  descriptions  of  the  vitreous  by  the  earlier  anatomists,  handicapped 
as  they  were  by  the  inadequacy  of  the  methods  at  their  disposal,  pertained 
chiefly  to  the  macroscopic  appearances :  the  papers  of  Bowman,1  in  1848, 
and  of  Virchow,2  in  1852,  may  be  said  to  mark  the  beginning  of  the 
studies  really  devoted  to  the  minute  structure  of  the  vitreous. 

Examination  of  the  fresh  tissue  was  soon  supplemented  by  inspection 
of  frozen  eyes,  and  later  by  study  of  the  vitreous  after  treatment  with  so- 
lutions of  various  chromic  acid  salts.  Among  the  most  important  results 
of  the  early  investigations  was  the  establishment  by  F.  Arnold 3  of  the 
existence  of  a  distinct  limiting  membrane,  the  hyaloidea,  bounding  the 
exterior  of  the  vitreous,  the  presence  of  which  had  been  surmised  by 
Demours 4  and  Zinn.5  Arnold  also  accepted  the  teachings  of  these  authors, 
that  the  external  membrane  sent  numerous  prolongations  inward  which 
served  to  divide  the  vitreous  into  thin-walled  compartments  or  "cells" 
containing  the  watery  constituents. 

Pappenheim 6  and  Brucke,7  from  consideration  of  the  effects  of  certain 
chemical  solutions  (potassium  bichromate  and  lead  acetate),  advanced  the 
view  that  the  vitreous  body  consisted  of  a  number  of  concentrically  dis- 
posed lamellae.  Hannover,8  about  the  same  time,  while  accepting  a  some- 
what similar  concentric  lamellation  for  the  vitreous  of  mammals,  described 
that  of  man  as  composed  of  radially  arranged  sectors.  Two  rival  views — 
the  concentric  or  "onion"  and  the  radial  or  "orange"  theory — were  thus 
offered  the  student  at  the  close  of  the  first  half  of  the  present  century,  each 
being  warmly  supported  by  its  own  adherents.  Bowman,9  Virchow,10  and 

1  Bowman  :  Observations  on  the  Structure  of   the  Vitreous  Humor,  Dublin  Quart. 
Journal  of  Med.  Science,  1848. 

2  Virchow:  Ueber  den  menschlichen  Glaskorpers,  Virchow's  Archiv,  Bd.  iv.  u.  y., 
1852-53. 

3  F.  Arnold :  Anatomische  und  physiologische  Untersuchungen  iiber  das   Auge  des 
Menschen,  1832. 

4  Demours  :  Observation  anatomique  sur  la  structure  cellulaire  du  corps  vitre,  Mem.  de 
Paris,  1741. 

5  Zinn  :  Descriptio  anatornica  oculi  humani,  1765. 

6  Pappenheim  :  Die  specielle  Gewebelehre  des  Auges,  1842. 

7  Brucke  :  Ueber  den  inneren  Bau  des  Glaskorpers,  Muller's  Archiv,  1843. 

8  Hannover:  Entdeckung  des  Baues  des  Glaskorpers,  Muller's  Archiv,  1846. 

8  Bowman  :  loc.  cit.  10  Virchow  :  loc.  cit. 


364  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

Kolliker,1  a  few  years  later,  questioned  the  correctness  of  both  these 
theories,  and  advanced  the  opinion,  based  upon  microscopical  examination 
of  foetal  as  well  as  adult  tissue,  that  the  vitreous  contained  structural  ele- 
ments represented  by  fibres  and  cells ;  Virchow  further  maintained  that 
the  vitreous  in  general  corresponded  to  a  gelatinous  or  mucoid  connective 
tissue,  resembling  the  jelly  of  Wharton  of  the  umbilical  cord  in  the  pos- 
session of  irregular  cells  and  a  homogeneous  intercellular  substance. 

The  fact  that  the  vitreous  body  consists  of  two  parts — the  fluid  and  the 
firmer  constituents — is  readily  demonstrated  by  filtration,  the  substances 
remaining  upon  the  filter  representing  the  morphological  elements.  The 
latter,  however,  comprise  but  an  insignificant  part  of  the  entire  mass,  since, 
as  shown  by  Lohmeyer,2  of  one  hundred  parts  by  weight  of  vitreous  sub- 
stance, only  from  .021  to  .07  part  remain  on  the  filter.  The  readiness  with 
which  the  fluid  portion  of  the  vitreous  drains  away  also  shows  the  fallacy 
of  the  older  views  that  the  vitreous  fluid  was  contained  within  membranous 
compartments. 

In  chemical  composition  the  vitreous  substance  consists  almost  entirely 
of  water,  which,  according  to  Berzelius,3  Frerichs,4  and  Lohmeyer,  consti- 
tutes from  98.40  to  98.64  per  cent,  of  the  whole.  The  remaining  small 
proportion,  composed  of  solids,  includes  salts,  extractives,  and  traces  of 
proteids  and  nucleo-albumin. 

The  soft,  semi-fluid,  gelatinous  substance  of  the  vitreous  is  enclosed 
within  a  distinct  envelope,  the  hyaloid  membrane,  throughout  a  large  part 
of  its  extent,  which  defines  the  relations  of  the  collapsible  vitreous  mass  to 
the  surrounding  parts :  the  vitreous  body,  therefore,  may  be  regarded  as 
consisting  of  two  portions,  the  substance  proper  and  its  capsule. 

The  Vitreous  Substance. — The  investigations  of  Virchow  and  of  Kol- 
liker,  to  which  reference  has  been  already  made,  called  attention  to  the 
presence  of  fibres  and  cells  within  the  apparently  homogeneous  vitreous 
substance.  The  reticulated  character  of  the  vitreous  framework  described 
by  the  first  of  these  authorities  was  accepted  by  Weber,5  but  Ciaccio 6  and 
H.  Virchow7  first  strongly  emphasized  the  richness  of  the  fibrillation 
existing  throughout  this  structure,  although  both  Henle 8  and  Blix 9  had 

1  Kolliker:  Mikroscopische  Anatomic,  fid.  n.,  1864. 

2  Lohmeyer:  Beitrage  zur  Histologie  und  Aetiologie  der  erworbenen  Linsenstaare, 
Zeitsch.  f.  ration.  Medicin,  N.  F.,  Bd.  v.,  1854. 

3  Berzelius:  Lehrbuch  der  Chemie,  Bd.  ix.,  1831. 

4  Frerichs:  TJeber  Linsenstaare,  Hannover'sche  Annalen,  1848. 

5  C.  O.  Weber :  Ueber  den  Bau  des  Glaskorpers  und  die  pathologischen,  namentlich 
entzundlichen  Veranderungen  desselben,  Virchow's  Archiv,  Bd.  xix.,  1860. 

8  Ciaccio :  Beobachtungen  iiber  den  inneren  Bau  des  Glaskorpers  im  Auge  des  Men- 
schen  und  der  Wirbelthiere  im  Allgemein,  Moleschott's  Untersuchungen  zur  Naturlehre, 
Bd.  x.,  1870. 

T  H.  Virchow  :  Die  morphologische  Natur  des  Glaskorpergewebes,  Bericht  iiber  d.  17. 
Versamml.  d.  Ophthalmologischen  Gesellschaft,  Heidelberg,  1885. 

8  Henle:  Eingeweidelehre,  Handbuch  d.  Anatom.  d.  Menschen,  Bd.  n.,  1866. 

9  Blix:  Studier  ofver  glaskroppen,  Medicinskt  Archiv,  Bd.  IV.,  1868. 


THE   MICROSCOPICAL   ANATOMY   OP  THE   EYEBALL. 


365 


recorded  the  observation  of  net-works  of  delicate  fibres.  Other  observers 
accepting  more  or  less  provisionally  the  existence  of  a  supporting  frame- 
work of  fibres  are  Merkel,1  Schiefferdecker,2  Rauber,3  and  Schafer.4 

Divergent  views,  however,  have  not  been  wanting,  among  which  those 
of  Stilling,5  Iwanoif,6  Schwalbe,7  and  Toldt8  in  common  recognize  a  differ- 
entiation of  the  vitreous  body  into  a  peripheral  or  cortical  portion,  con- 
centrically lamellated,  and  a  central  or  nuclear  portion  which  possesses  a 

FIG.  99. 


Portion  of  vitreous  substance  of  six  months'  human  foetus,  showing  stellate  cellular  elements  which 
contribute  to  form  the  framework.    (Retzius.)    Magnified  450  diameters. 

radial  arrangement.  These  authors,  however,  differ  from  the  older  anato- 
mists in  their  interpretation  of  the  significance  of  the  peripheral  lamellation, 
since  they  regard  the  cleavage  as  dependent  upon  minute  interlamellar 
clefts,  without  distinct  walls,  rather  than  upon  the  presence  of  membranous 
partitions.  In  his  subsequent  descriptions,9  Schwalbe  aptly  compares  the 
vitreous  substance  to  a  sponge  saturated  with  fluid,  and  declares  that  his- 
tologically  the  adult  vitreous  substance  is  to  be  regarded  neither  as  a  form 

1  Merkel :  Handbuch  der  topographischen  Anatomic,  Bd.  I.,  1887. 

2  Schiefferdecker  und  Kossel:  Gewebelehre,  Bd.  u.,  1891. 

3  Rauber:  Lehrbuch  der  Anatomic  des  Menschen,  4te  Aufl.,  Bd.  n.,  1894. 

4  Schafer:  The  Sense  Organs,  Quain's  Anatomy,  10th  ed.,  vol.  in.,  Pt.  3,  1894. 

5  Stilling:  Eine  Studie  iiber  den  Bau  des  Glaskorpers,  Archiv  f.  Ophthalmol.,  Bd. 
xv.,  1869. 

6  Iwanoff :  Der  Glaskorper,  Strieker's  Handbuch  der  Lehre  von  den  Geweben,  Bd.  II., 
1872. 

7  Schwalbe;  Der  Glaskorper,  Graefe  u.  Saemisch's  Handbuch  der  gesammten  Augen- 
heilkunde,  Bd.  I.,  1874. 

8  Toldt:  Lehrbuch  der  Gewebelehre,  3te  Aufl.,  1888. 

9  Schwalbe  :   Lehrbuch  der  Anatomic  der  Sinnesorgane,  1887,  S.  140. 


366  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

of  raucoid  tissue  nor  as  a  transudate,  "  but  as  a  connective  tissue  exception- 
ally rich  in  water,  the  fixed  cells  of  which  have  been  lost,  and  whose  inter- 
fibrillar  substance,  largely  infiltrated  with  water,  is  invaded  by  migratory 
cells." 

Study  of  the  foetal  condition  of  the  vitreous  body  is  of  much  im- 
portance as  affording  trustworthy  information  as  to  the  primary  arrange- 
ment and  the  morphological  significance  of  its  structure  in  the  adult.  Ex- 
amination of  rabbit  embryos  of  from  thirteen  to  fifteen  days  shows  the 
primary  vitreous  to  be  represented  by  an  irregular  reticulum  of  branched 
mesodermic  cells  lodged  within  a  relatively  large  amount  of  homogeneous 
matrix  or  ground-substance.  These  elements  are  undoubted  embryonal 
forms  of  connective-tissue  cells  the  ancestors  of  which  gained  en-trance  into 
the  interior  of  the  eye  by  the  ingrowth  of  the  perivesicular  mesoderm. 
While  Keibel1  denies  that  the  ingrowth  of  any  mesoderm  beyond  the 
blood-vessels  takes  place,  other  observers,  including  Kolliker,2  Bonnet,3 
Hertwig,4  Minot,5  and  Schenk,6  agree  in  recognizing  that  such  primary 
intra-ocular  inclusion  of  the  mesoderm  does  occur,  in  which  opinion  the 
observations  of  the  writer  lead  him  to  share. 

The  primary  condition  of  the  vitreous,  therefore,  represents  an  embry- 
onal connective  tissue ;  the  later  stages  of  this  structure  are  marked  by 
conspicuous  changes,  among  which  the  almost  complete  disappearance  of 
the  cells  and  the  infiltration  of  the  ground-substance  by  a  large  amount 
of  fluid  are  prominent.  Coincidently  with  the  suppression  of  the  cellular 
elements  the  formation  of  delicate  fibrillse  takes  place,  so  that  it  is  possible, 
although  by  no  means  easy,  even  within  the  fresh  vitreous  substance,  to 
distinguish  cells  and  fibres  as  the  morphological  constituents  of  the  beauti- 
fully transparent  tissue. 

After  treatment  with  suitable  preservatives,  however,  as  Miiller's  fluid, 
solutions  of  chromic  acid,  Flemming's  solution,  or  sublimate  solution,  and 
subsequent  staining,  the  demonstration  of  these  elements  becomes  far  more 
certain  and  satisfactory.  In  preparations  of  such  character  the  fibrillae, 
which  are  particularly  conspicuous,  are  seen  to  constitute  in  general  a  sup- 
porting felt-work  in  the  interspaces  of  which  are  lodged  the  more  fluid 
parts  of  the  vitreous  substance. 

The  fibrittar  framework  of  the  adult  human  vitreous  consists  of  an 
interlacement  of  delicate  threads  which  vary  somewhat  in  thickness  and  in 
contour  according  to  the  method  of  preservation  of  the  tissue.  While 

1  Keibel:  Zur  Entwickelung  des  Glaskorpers,  Archiv  f.  Anat.  u.  Physiolog.,  1886. 

2  Kolliker:  Grundriss  der  Entwickelungsgeschichte  der  Menschen  und  der  hoheren 
Tiere,  2te  Aufl.,  1884. 

3  Bonnet:  Grundriss  der  Entwickelungsgeschichte  der  Haussaugethiere,  1891. 

4  Hertwig :    Lehrbuch  der  Entwickelungsgeschichte  des  Menschen  und  der  Wirbel- 
thiere,  3te  Aufl.,  1890. 

5  Minot :  Human  Embryology,  1892. 

6  Schenk  :  Lehrbuch  der  Embryologie  des  Menschen  und  der  Wirbelthiere,  2te  Aufl., 
1896. 


THE   MICROSCOPICAL   ANATOMY   OF  THE   EYEBALL. 


367 


FIG.  100. 


fibrillse  may  undoubtedly  be  demonstrated  in  the  fresh  vitreous,  their  dis- 
play in  tissue  hardened  in  solutions  containing  chromic  acid  or  its  salts 
is  much  more  certain  and  com- 
plete. 

The  extent,  however,  to 
which  these  preserving  fluids 
may  affect  the  albuminous,  al- 
most fluid,  matrix  must  be 
taken  into  account  in  estimating 
the  richness  of  the  fibrillar  net- 
work during  life,  since  it  is 
possible,  and  the  writer  believes 
probable,  that  the  action  of  the 
reagent  is  responsible  for  the 
production  of  at  least  a  part 
of  the  densely  felted,  delicate 
fibrillfe  seen  in  certain  prepara- 
tions. The  experiments  of  the  Portion  of  vitreous  substance  from  adult,  showing  the 
writer1  on  the  effect  of  treating  dense  felt-work  of  fibres  and  the  absence  of  cells:  only 
_  . ..  .  .  3  atrophic  traces  of  the  latter  are  seen.  (Retzius.)  Magni- 

albliminoUS  fluids  With  Solutions   fled  450  diameters. 

of  potassium  bichromate,  chromic 

acid,  etc.,  emphasize  the  production  with  certainty  of  fibrillar  structures  of 

great  perfection  and  complexity  under  such  conditions. 

The  vitreous  fibrillae,  as  described  by  Retzius,2  who  regards  the  appear- 
ances seen  in  preparations  treated  with  Flemming's  solution  as  entirely 
trustworthy  as  representing  the  normal  structure,  occur  throughout  all 
parts  of  the  vitreous  substance,  and  form  by  their  interlacement,  but  not 
true  anastomosis,  a  felting  in  which  the  interspaces  are  often  so  narrow  as 
scarcely  to  equal  the  diameter  of  a  red  blood-cell.  "  Under  high  amplifi- 
cation the  entire  tissue  is  resolved  into  a  felt-work  of  exceptional  intricacy 
composed  of  the  finest  fibres,  which  cross  one  another  in  various  directions 
and  here  and  there  join  in  narrow  nodal  points,  without,  however,  consti- 
tuting net-works." 

According  to  Retzius,  many  fibres  are  beset  with  peculiar  minute  glis- 
tening spherules  or  granules  of  varying  size ;  regarding  the  nature  of  these 
bodies  the  distinguished  observer  remains  in  doubt,  although  he  rejects  as 
untenable  the  supposition  that  their  presence  is  attributable  to  the  action 
of  the  reagents  employed,  since  the  intervening  ground-substance  remains 
homogeneous.  The  experiences  of  the  writer,  already  noted,  lead  him, 
however,  to  regard  these  bodies  as  due  to  the  effect  of  the  reagents  em- 
ployed, since  almost  identical  appearances  may  be  artificially  produced. 

1  Piersol :  Keview  of  Heitzmann's  Microscopical  Morphology,  Amer.  Journal  of  the 
Med.  Sciences,  January,  1883. 

2  Eetzius  :  Uber  den  Bau  des  Glaskorpers  und  der  Zonula  Zinnii  in  dem  Auge  des 
Menschen  und  einiger  Thiere,  Biolog.  Untersuch.,  N.  F.,  Bd.  vi.,  1894. 


368  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

With  the  advance  of  age  from  childhood  to  adult  life  the  intricacy  of 
the  fibrillar  framework  may  also  undergo  modification,  since,  as  shown  by 
Retzius,  partial  absorption  usually  takes  place,  especially  in  certain  locali- 
ties, thereby  producing  a  framework  of  less  density. 

The  central  part  of  the  vitreous  body  is  modified  by  the  presence  of  a 
distinct  channel,  the  canalis  hyaloideus,  which  extends  from  the  optic 
papilla  forward  almost  to  the  posterior  surface  of  the  lens-capsule,  and 
represents  the  remains  of  the  passage-way  occupied  by  the  foetal  hyaloid 
vessels  in  their  transit  from  the  optic  entrance  to  the  vascular  capsule  of 
the  lens  which  they  supply. 

The  hyaloid  canal,  canal  of  Stilling,  canal  of  Cloquet,  or  central  canal, 
as  seen  in  the  adult  human  eye,  is  a  slightly  compressed  tubular  channel, 
from  one  to  two  millimetres  in  diameter,  which  begins  at  the  optic  papilla 
with  a  slight  enlargement,  the  area  Martegiani,  of  equal  width  with  the 
disk,  and  continues  towards  the  posterior  pole  of  the  lens,  in  the  vicinity 
of  which  it  terminates  in  a  blind,  often  somewhat  dilated,  extremity.  In 
the  foetus  the  anterior  termination  of  the  canal  is  distinctly  funnel-shaped, 
since  the  loose  vascular  connective  tissue  which  it  contains  becomes  con- 
tinuous with  the  vascular  lens-capsule.  After  the  disappearance  of  the 
hyaloid  artery,  the  canal  contains  the  remains  of  the  vascular  structures 
and  the  surrounding  delicate  connective-tissue  trabeculee,  which  in  later 
years  are  represented  alone  by  the  shrunken  mass  attached  to  the  retina 
within  the  physiological  excavation  of  the  optic  disk. 

The  limits  of  the  hyaloid  canal  are  defined  by  delicate  membranous 
structures,  which,  however,  are  to  be  regarded,  according  to  Retzius,1  rather 
as  the  result  of  condensation  of  the  fibrillar  tissue  of  the  vitreous  substance 
than  as  a  part  of  the  hyaloid  membrane,  as  frequently  assumed.  Haensell 2 
recognizes  three  zones  within  that  portion  of  the  vitreous  body  of  the 
newly-born  child  situated  outside  of  the  hyaloid  canal, — («)  an  inner  con- 
densed stratum  contributing  the  wall  of  the  central  channel,  (6)  the  vitreous 
substance  proper,  (c)  an  external  condensed  stratum,  the  future  hyaloid 
membrane. 

While  the  existence  of  membranous  lamellae  throughout  the  vitreous 
substance  cannot  be  accepted  in  the  sense  of  the  descriptions  of  the  older 
anatomists,  local  thickenings  and  condensations  of  the  fibrillar  framework 
occur  at  various  levels  within  the  vitreous,  as .  especially  emphasized  in  the 
recent  investigations  of  Retzius.  Such  thickenings  of  the  usual  vitreous 
tissue  are  particularly  distinct,  and  often  numerous,  without,  however,  being 
regularly  disposed  in  the  anterior  and  lateral  parts  of  the  ball. 

The  cellular  elements  of  the  vitreous  substance  are  of  two  kinds,  the 
true  connective-tissue  cells  and  the  migratory  leucocytes  :  from  the  fore- 
going account  of  the  origin  of  the  vitreous  it  is  evident  that  the  con- 

1  Ketzius:  loc.  cit.,  S.  82. 

*  Haensell :  Kecherches  de  la  structure  et  1'histogenese  du  corps  vitre  normal  et 
pathologique,  Paris,  1888. 


THE   MICROSCOPICAL   ANATOMY  OF  THE   EYEBALL. 

nective-tissue  elements  are  far  more  numerous  and  conspicuous  during  the 
early  stages  of  the  tissue  than  later.     In  the  vitreous  of  the  new-born, 
Haensell l  describes  a  pro- 
toplasmic net-work  com-  FIG.  101. 
posed      of     the      united 
processes    of  the    young 
con  nective-tissue 
while     Retzius2 
richly   branched 


Portions  of  adult  vitreous  substance,  showing  remains  of 
vitreous  cells  in  various  stages  of  atrophy  and  fibrous  metamor- 
phosis. (Retzius.)  Magnified  450  diameters. 


cells, 
figures 
stellate 

elements     in     the     foetal 
tissue. 

Examination  of  the 
adult  vitreous,  on  the  con- 
trary, shows  the  connec- 
tive-tissue cells  to  be 
meagre  and  sparsely  dis- 
tributed and  so  incon- 
spicuous that  their  pres- 
ence is  readily  entirely  overlooked.  The  sole  representatives  of  the  fair- 
sized  connective-tissue  cells  of  the  early  stages  are  minute,  irregularly  stellate 
or  branched  elements  the  protoplasm  of  which  often  exhibits  cleavage  into 
fine  fibrillse  which  take  part  in  the  constitution  of  the  general  fibrous  frame- 
work. These  vitreous  cells  are  irregular  in  their  distribution,  and  give  the 
impression  of  being  in  various  stages  of  a  retrogressive  process. 

The  migratory  leucocytes,  or  wandering  cells,  occur  within  the  adult 
vitreous  in  variable  number,  especially,  however,  immediately  beneath  the 
hyaloid  membrane,  where  they  constitute  the  so-called  subhyaloid  cells. 
That  these  elements  are  leucocytes  which  have  invaded  the  vitreous  sub- 
stance is  shown  by  their  morphological  identity,  including  amoeboid  move- 
ment, with  the  colorless  cells  of  the  circulation,  and  by  the  fact  that  they 
occur  in  particular  profusion  in  those  localities,  as  in  the  vicinity  of  the 
ora  serrata  and  the  optic  entrance,  where  the  proximity  of  blood-vessels 
favors  the  entrance  of  leucocytes  into  the  vitreous  tissue. 

Iwanoff3  divided  the  wandering  cells  within  the  vitreous  into  three 
groups :  1,  round  cells,  resembling  the  peripheral  subhyaloid  elements ; 
2,  stellate  or  spindle-cells,  with  a  single  or  several  nuclei  and  extended 
processes ;  3,  vacuolated  cells,  which,  in  addition  to  one  or  more  nuclei,  con- 
tained a  vacuole,  sometimes  two,  filled  with  fluid.  While  Iwanoff  regarded 
these  elements  as  distinct  varieties,  other  observers,  including  Pagenstecher 
and  Schwalbe,  recognized  that  all  the  migratory  cells  were  but  different 

1  Haensell :  Recherches  sur  le  corps  vitre,  Bulletin  de  la  clinique  nationale  ophthal- 
mique  de  1'hospice  des  Quinze-Vingts,  t.  iv.,  1886. 

2  Retzius:  loc.  cit,  Tafel  xxix.,  Fig.  3. 

3  Iwanofi':  Zur  normalen  und  pathologischen  Anatomic  des  Glaskorpers,  Archiv  f. 
Ophthalmol.,  Bd.  XL,  1865. 

VOL.  I.— 24 


370 


THE    MICROSCOPICAL    ANATOMY    OF   THE    EYEBALL. 


FIG.  102. 


forms  and  conditions  of  the  same  elements,  the  leucocytes.  Schwalbe1 
experimentally  demonstrated  the  correctness  of  his  interpretation  by  intro- 
ducing portions  of  mammalian  vitreous  into  the  lymph-sacs  of  frogs,  and 
subsequently  noting  the  entrance  of  the  amphibian  leucocytes,  previously 
marked  by  the  inclusion  of  particles  of  pigment,  into  the  vitreous  sub- 
stance. The  migratory  cells  so  introduced  assumed  all  the  various  forms 
and  appeared  identical  with  the  elements  usually  encountered  in  the  vitre- 
ous body. 

The  Hyaloid  Membrane. — The  posterior  and  lateral  portions  of  the  vit- 
reous mass  are  invested  by  a  distinct 
limiting  structure,  the  hyaloid  mem- 
brane, which  appeal's  as  an  extremely 
delicate  structureless  envelope  closely 
applied  to  the  inner  surface  of  the 
retina.  In  the  earliest  years  this 
limiting  membrane  is  wanting,  and, 
indeed,  its  presence  even  in  the  adult 
eye  has  been  questioned  by  some  anato- 
mists, among  whom  is  Merkel,2  who 
regards  the  structure  as  artificially  pro- 
duced. 

The  presence  of  the  hyaloid  can  be 
readily  demonstrated  in  eyes  which 
have  lain  for  a  couple  of  days  after 
death,  or  been  preserved  for  several 
days  in  dilute  alcohol.  In  such  pre- 
parations the  vitreous  body  may  be 
readily  detached  from  the  surrounding 
nervous  tunic  without  mutilation,  and 
on  careful  inspection  the  existence  of 
a  delicate  investment  be  shown  by  the 
definite  boundary  of  the  separated  mass 

and  the  minute  folds  on  its  free  surface.  The  separation  thus  effected  also 
emphasizes  the  fact  that  the  hyaloid  membrane  belongs  to  the  vitreous  body 
and  not  to  the  retina,  as  maintained  by  Iwanoff3  and  others,  although,  as 
shown  by  O.  Schultze,4  the  hyaloid  is  primarily  genetically  related  to  the 
retinal  tunic. 

The  immediate  formation  of  the  hyaloid  is  effected  by  the  metamor- 

1  Schwalbe  :  Der  Glaskorper,  Graefe  und  Saemisch's  Handbuch  d.  gesammten  Augen- 
heilkunde,  1874,  Bd.  i.  S.  474. 

3  Merkel:  Handbuch  der  topographischen  Anatomic,  1887,  Bd.  i.  S.  266. 

s  Iwanoff:  Der  Glaskorper,  Strieker's  Handbuch  der  Lehre  von  den  Geweben,  Bd.  n., 
1871. 

4  O.  Schultze  :    Zur  Entwickelungsgeschichte  des  Gefass-Systems  im  Saugethierauge, 
Kolliker's  Festschrift,  1892. 


Surface  view  of  fragment  of  hyaloid  mem- 
brane from  adult  eye,  showing  a  number  of 
adherent  subhyaloid  cells.  (Retzius.)  Magni- 
fied 330  diameters. 


FIG.  103. 


THE   MICROSCOPICAL   ANATOMY  OP  THE  EYEBALL.  371 

phosis  of  the  layer  of  mesodermic  tissue  covering  the  periphery  of  the 
vitreous  substance.  According  to  Haensell,1  a  transient  endothelial  structure 
precedes  the  completed  hyaloid  envelope. 

Concerning  the  relations  between  the  hyaloid  membrane  and  the  anterior 
surface  of  the  vitreous  body  opinions  by  no  means  accord,  since  this  question 
depends  so  closely  for  its  solution 
on  the  broader  and  greatly  mooted 
one    regarding    the     details  of  the 
participation  of  the  hyaloid  mem- 
brane   in     the    formation    of     the 
suspensory  apparatus  of  the  lens. 

Anticipating  briefly  for  our 
present  purpose  some  of  the  facts 
to  be  discussed  more  at  length  in 
connection  with  the  zonula,  two 
opposed  views  as  to  the  behavior 
of  the  hyaloid  in  the  vicinity  of 
the  ora  serrata  must  be  noted.  Ig- 
noring the  individual  views  of  the 
older  anatomists,  it  may  be  recalled 
that  it  was  generally  accepted  that 
beyond  the  ora  serrata  the  hyaloid 
splits  into  an  outer  and  an  inner 
layer  which  respectively  are  attached 
to  the  anterior  and  the  posterior 
lens-capsule,  the  entire  hyaloid  mem- 
brane being  devoted  to  the  formation 
of  the  suspensory  apparatus  of  the 
lens. 

In  opposition  to  the  preceding 
and  generally  accepted  opinion, 
Henle2  maintained  that  the  zonula 

was  the  continuation  of  the  outer  layer  into  which  the  hyaloidea  separates 
at  the  border  of  the  ciliary  body,  the  inner  lamella  following  the  anterior 
surface  of  the  vitreous  body  and  investing  the  patellar  fossa.  Among  the 
more  recent  writers,  Stuart 3  and  Schafer 4  also  accept  the  existence  of  a 
lamella  of  the  hyaloid  membrane  over  the  anterior  surface  of  the  vitreous, 
while  opposed  to  this  view  the  opinion  of  Retzius5  is  especially  emphatic. 


Portion  of  anterior  boundary  layer  of  vit- 
reous body  of  adult.  (Retzius.)— g.r.,  distinctly 
laminated  boundary  zone ;  g.L,  adjoining  vitreous 
substance ;  I,  portion  of  posterior  lens-surface, 
p,  lies  within  the  so-called  Petit's  canal.  Magni- 
fied 330  diameters. 


1  Haensell :  Recherches  de  la  structure  et  1'histogenese  du  corps  vitre  normal  et  patho- 
logique,  Paris,  1888. 

2  Henle  :  Eingeweidelehre,  Anatomie  des  Menschen,  2te  Aufl.,  Bd.  n.,  1873,  S.  697. 

8  Stuart :  On  a  Membrane  lining  the  Fossa  Patellaris  of  the  Corpus  Vitreum,  Pro- 
ceedings of  the  Royal  Society,  vol.  XLIX.,  1891. 

4  Schiifer :  The  Eye,  Quain's  Anatomy,  10th  ed.,  vol.  in.,  Pt.  3,  1894. 

5  Retzius:  loc.  cit.,  S.  83. 


372 


THE   MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


That  the  anterior  portion  of  the  periphery  of  the  vitreous  substance, 
uncovered  by  the  hyaloid  membrane,  possesses  a  definite  boundary  is  not  to 
be  questioned,  since  the  presence  of  such  a  lamella  is  readily  demonstrated 
in  suitably  prepared  sections  :  critical  study  of  this  limiting  structure,  how- 
ever, shows  it  to  be  formed,  as  maintained  by  Retzius,  and,  indeed,  long 
before  by  Iwanoff,  by  a  condensation  of  the  vitreous  substance,  and  not  as  a 
derivative  of  the  laterally  situated  hyaloid.  Merkel l  likewise  recognizes  a 
peripheral  thickening  of  the  vitreous  tissue,  although  he  rejects  the  presence 
of  a  distinct  hyaloid  membrane  at  any  point.  According  to  Retzius,  the 
limiting  layer  lining  the  patellar  fossa  consists  of  a  condensation  of  the 
fibrillar  framework  of  the  vitreous  substance  :  a  similar  thickening,  although 
not  to  such  a  conspicuous  degree,  takes  place  in  the  production  of  the  wall 
of  the  hyaloid  canal. 

THE   SUSPENSORY   APPARATUS   OF   THE   LENS. 

Reference  to  the  accompanying  diagrammatic  section  of  the  human  eye 
shows  that  the  periphery  of  the  lens  is  connected  with  the  adjacent  annular 

Fio.  104. 

fe^ 

Canal  x>f  Schlemm 

^N^*t^      ^  Sp*068  °f  Fontana 
*^^>^^  Ant.  Ciliary  Veins 

Conjunctiva 
Posterior  Chamber  "W^"^ Sc!era 


Diagram  of  anterior  segment  of  eye,  drawn  to  accurate  scale.    (Plemming.) 

series  of  ciliary  processes  by  means  of  delicate  bands  which  pass  from  the 
vicinity  of  the  ora  serrata  over  the  ciliary  body  to  the  margin  of  the  lens. 
These  trabeculse  collectively  constitute  the  suspensory  ligament  or  zone  of 
Zinn,  a  structure  of  much  importance  in  maintaining  the  position  of  the 
lens  and  in  effecting  changes  in  its  curvature. 

The  zone  of  Zinn,  or  zonula  eiliaris,  appears  as  a  delicate  annular 
structure,  about  six  millimetres  in  width,  encircling  the  border  of  the  lens 
and  stretching  from  the  latter  point  to  the  ora  serrata.  As  seen  in  meridi- 


Merkel :  Handbuch  der  topographischen  Anatomic,  Bd.  I.,  1887,  S.  266. 


THE    MICROSCOPICAL    ANATOMY   OF   THE    EYEBALL.  373 

onal  sections  of  the  human  eye,  the  zonula  is  composed  of  an  interlacement 
of  conspicuous  fibres  which  span  at  various  angles  the  interval  between  the 
lens  and  the  ciliary  processes. 

The  origin  of  these  fibres  and  the  relations  of  the  zonula  to  the  adjacent 
parts  of  the  eye  have  long  engaged  the  attention  of  anatomists,  but,  notwith- 
standing the  numerous  investigations  undertaken  by  competent  observers, 
opinions  are  far  from  settled  as  to  many  details  in  the  constitution  of  these 
parts.  As  already  incidentally  noted,  the  question  concerning  the  anterior 
relations  of  the  hyaloid  membrane  is  inseparably  connected  with  that  of 
the  origin  of  the  zontilar  fibres. 

The  older  view,  according  to  which  the  hyaloid  membrane  beyond  the 
ora  serrata  splits  into  two  lamella  which  are  continued  as  the  zonula  to  the 
anterior  and  posterior  surfaces  of  the  lens-capsule,  was  for  a  long  time  very 
generally  accepted,  and  even  yet  Schwalbe l  regards  the  zone  of  Zinn  as  the 
direct  membranous  prolongation  of  the  thickened  hyaloid. 

It  is  especially  due  to  the  observations  of  Merkel,2  Gerlach,3  Czermak,4 
Topolanski,5  and  Gamier6  that  our  conceptions  regarding  the  constitution 
of  the  zonula  have  been  modified,  since  they  emphasized  the  fact  that  the 
zone  of  Zinn,  formerly  regarded  as  a  membrane,  is  entirely  composed  of 
fibres  which  are  independent  structures  and  not  parts  of  the  hyaloid  mem- 
brane. The  latter,  while  closely  associated  with  the  zonula  and  giving 
origin  to  many  of  the  zonular  fibres,  is  continued  over  the  orbiculus  ciliaris 
on  to  the  ciliary  body  as  the  extremely  delicate  homogeneous  cuticular  or 
glassy  membrane  covering  these  parts. 

In  estimating  the  closeness  of  the  relations  between  the  vitreous  sub- 
stance and  the  zonular  fibres,  it  is  of  importance  to  recall  the  intimate 
genetic  association  between  the  two.  It  may  be  assumed  as  established 
that  the  primitive  zonular  fibres  are  earliest  formed  in  the  anterior  portion 
of  the  developing  vitreous  substance,  from  which  they  proceed  towards  the 
immature  ciliary  body.  As  pointed  out  by  Retzius,7  the  tissue  occupying 
the  funnel-shaped  area  surrounding  the  hyaloid  canal  is  especially  concerned 
in  the  production  of  the  zonular  fibres.  These  are  at  first  very  numerous 
and  of  extreme  delicacy,  later,  as  shown  by  Czermak,8  becoming  reduced  in 
number  and  increased  in  size.  With  the  limitation  of  the  vitreous  sub- 
stance proper,  the  loose  connective  tissue  associated  with  the  hyaloid  artery 

1  Schwalbe  :  Lehrbuch  der  Anatomie  der  Sinnesorgane,  1887. 

2  Merkel:  Die  Zonula  ciliaris,  Habilitationsschrift,  1870. 

8  Gerlach :  Beitrage  zur  normalen  Anatomie  des  menschlichen  Auges,  1880. 
*  Czermak :  Zur  Zonulafrage,  Archiv  f.  Ophthalmol.,  Bd.  xxxi.,  1885. 

5  Topolanski :  Ueber  den  Bau  der  Zonula  und  Umgebung  nebst  Bemerkungen  uber 
das  albinotische  Auge,  Archiv  f.  Ophthalmol.,  Bd.  xxxvn.,  1891. 

6  Gamier  :  Ueber  den  normalen  und  pathologischen  Zustand  der  Zonula  Zinnii,  Archiv 
f.  Augenheilkunde,  Bd.  xxiv.,  1892. 

7  Ketzius  :  Ueber  den  Bau  des  Glaskorpers  und  der  Zonula  Zinnii  in  dem  Auge  der 
Menschen  und  einige  Thiere,  Biologisch.  Untersuch.,  N.  F.,  vi.,  1894,  S.  84. 

8  Czermak  :  loc.  cit. 


374  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

and  its  expansion  over  the  lens-capsule  undergoes  absorption,  the  young 
zonular  fibres  alone  remaining. 

The  immature  fibres  thus  formed  converge  towards  the  position  of  the 
future  orbicular  zone,  where  they  later  become  intimately  attached  to  the 
anterior  extension  of  the  hyaloid  membrane  when  that  structure  appears. 
Genetically,  therefore,  the  earliest  zonular  fibres  may  be  regarded  as  a 
product  of  the  primitive  vitreous  connective  substance.  Subsequent  to  the 
limitation  of  the  vitreous  mass  bv  the  formation  of  the  anterior  boundarv 

*>  »f 

layer  already  described,  the  fibres  constitutiug  the  zone  of  Zinn  lose  their 
connection  with  the  vitreous  tissue  and  apparently  have  their  exclusive 
origin  from  the  inner  surface  of  the  orbicular  zone  in  the  immediate 
vicinity  of  the  ora  serrata. 

The  last-named  structure,  according  to  the  recent  studies  of  Schoen,1 
plays  a  rdle  of  unsuspected  importance  in  the  origin  of  the  zonular  fibres. 
This  writer  describes  the  ora  serrata  as  differing  essentially  at  various 
periods  of  life,  since  he  finds  that  in  early  childhood,  before  the  accommo- 
dative function  is  established,  the  ora  serrata,  as  usually  described,  is 
wanting,  its  place  being  occupied  by  a  "  transition  border,"  within  which 
the  visual  portion  of  the  retina  gradually  gives  place  to  the  pars  ciliaris. 
In  the  eye  of  the  young  child,  therefore,  the  ora  serrata  is  represented  by 
a  smooth  line  marking  the  anterior  limit  of  the  proper  retinal  area; 
microscopical  examination,  however,  shows  that  this  zone  is  beset  with 
very  minute  serrations,  between  six  hundred  and  eight  hundred  in  number, 
from  each  of  which  springs  a  delicate  fibre  whicli  takes  part  in  the  forma- 
tion of  the  zonula. 

The  macroscopic  appearances  usually  described  as  the  normal  condition 
of  the  ora  serrata,  in  which  about  forty  well-marked  teeth  are  present, 
Schoen z  regards  as  the  result  of  secondary  functional  changes  induced  by 
the  continual  pull  exerted  by  the  zonular  fibres  which  have  their  origin  in 
the  minute  serrations.  A  second  group  of  zonular  fibres,  more  deeply 
situated  than  those  derived  directly  from  the  ora  serrata,  originate,  ac- 
cording to  this  author,  as  extensions  of  the  epithelial  cells  which  constitute 
the  inner  layer  of  the  pars  ciliaris  retinae.  The  tall  columnar  elements  of 
this  stratum  terminate  in  delicate  fibres  which  appear  as  the  direct  exten- 
sions of  the  protoplasm  of  the  retinal  elements.  Assuming  that  the  origin 
of  the  zonular  fibres  has  been  correctly  interpreted  by  Schoen,  the  zone  of 
Zinn  must  be  regarded  as  related  to  the  sustentacular  retinal  tissue  and  as 
the  specialized  extension  of  the  supporting  fibres  of  Miiller.  Considered 
in  this  light,  the  retinal  sheet  is  not  only  continued  as  the  epithelium  of 
the  pars  ciliaris,  but  is  represented  as  far  as  the  angle  of  the  ciliary  body 
and  the  lens-capsule  by  the  prolongation  of  its  supporting  tissue  as  the 
zonular  fibres. 

1  Schoen :    Der   Uebergangssaum   der   Netzhaut    oder  die   sogenannte   Ora   serrata, 
Archiv  f.  Anat.  u.  Physiol.,  1895. 

2  Schoen:  Zonula  und  Ora  serrata,  Anatom.  Anzeiger,  Bd.  x.,  1895. 


THE   MICROSCOPICAL    ANATOMY   OF   THE   EYEBALL.  375 

Most  observers,  however,  will  be  slow  to  disregard  the  strong  evidence 
as  to  the  close  genetic  relations  between  the  zonula  and  the  mesodermic 
tissue  of  the  early  vitreous,  as  shown  by  the  mode  of  development  of  the 
structures  under  discussion,  and  will,  therefore,  consider  the  view  attrib- 
uting the  origin  of  the  zouular  fibres  to  the  retinal  tissue  as  not  beyond 
question. 

Depending  upon  their  grouping  and  course,  the  zonular  fibres  in  the 
adult  eye  may  be  divided  into  two  varieties,  the  chief  and  the  accessory. 
The  chief  zonular  fibres,  which  include  the  orbiculo-capsular  and  cilio- 
capsular  subgroups  suggested  by  Czermak  and  adopted  by  Topolanski  and 
by  Gamier,  constitute  the  principal  bond  of  union  between  the  crystalline 
lens  and  the  surrounding  ciliary  body,  and  are  the  chief  constituents 
forming  the  zonnla  Zinnii. 

Following  the  classification  of  Gamier,1  the  chief  accessory  fibres  may 
be  grouped  as — 1,  Orbiculo-antero-capsular  ;  2,  Orbiculo-postero-capsular  ; 
3,  Cillo-postero-capsular ;  4,  Cilio-equatorial. 

The  orbiculo-postero-capsular  fibres  include  those  occupying  the  most 
posterior  and  internal  position  and  lying  in  close  relation  with  the  anterior 
boundary  layer  of  the  vitreous,  without,  however,  taking  origin  from  the 
latter.  The  general  relations  of  the  zonular  fibres  at  their  origin  to  the  ora 
serrata  and  the  pars  ciliaris  retinae  have  been  subjects  of  much  discussion. 
Both  Iwanoff2  and  Berger3  regard  the  fibres  as  taking  origin  behind  the 
ora  from  the  vitreous ;  Schoen 4  upholds  that  their  point  of  beginning  cor- 
responds to  the  ora  serrata  itself;  Dessauer5  declares  that  the  fibres  never 
reach  the  ora  serrata;  Czermak  maintains6  that  they  begin  shortly  in  ad- 
vance of  the  ora  by  intra-cellular  attachment  with  the  epithelium  of  the 
pars  ciliaris  retinae ;  while  Topolanski 7  regards  their  zone  of  origin  as 
beginning  from  one  to  one  and  a  half  millimetres  in  advance  of  the  ora 
serrata  and  extending  over  the  entire  remaining  surface  of  the  ciliary  body, 
including  the  elevations  of  the  ciliary  processes  as  well  as  their  sides  and 
intervening  valleys.  These  latter  conclusions  correspond  most  closely  with 
the  results  of  study  of  the  author's  sections. 

Regarding  the  particular  fibres  in  question, — those  constituting  the 
orbiculo-postero-capsular  group, — it  may  be  accepted  that  they  spring  from 
the  prolongation  of  the  hyaloid  membrane  investing  the  ciliary  ring,  the 
innermost  arising  almost  immediately  in  front  of  the  ora  serrata.  The  long 

1  Gamier:  loc.  cit.,  S.  35. 

2  Iwanoff:  Das  Glaskorper,  Strieker 's,Handbuch  der  Lehre  von  den  Geweben,  Bd. 
ii.,  1872. 

3  Berger:   Beitrage  zur   Anatomic   der  Zonula  Zinnii,  Archivr   f.  Ophthalmol.,   Bd. 
xxvm.,  1882. 

4  Schoen  :  loc.  cit. 

6  Dessauer:  Zur  Zonulafrage,  Klinische  Monatsblatter  f.  Augenheilkunde,  Bd.  xxi., 
1883. 

6  Czermak  :  loc.  cit. 

7  Topolanski :   loc.  cit. 


376  THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL. 

zonular  fibres  have,  without  exception,  a  meridional  course,  and  when  of 
considerable  thickness  are  resolvable  into  bundles  of  delicate  fibrillae.  The 
close  association  of  the  posterior  fibres  with  the  vitreous  impresses  upon 
them  a  slight  convexity,  although  the  general  direction  of  the  bands  is  nearly 
straight,  particularly  throughout  that  part  of  their  course  as  they  pass 
from  the  ciliary  processes  to  their  insertion  in  the  posterior  lens-capsule. 
When  about  to  join  the  capsule,  the  zonular  fibres  break  up  into  groups  of 
radiating  fibrillse  which  meet  the  capsule  within  an  area  of  local  thickening 
and  continue  for  some  distance  generally  parallel  to  the  surface  of  the  lens, 
becoming  more  and  more  attenuated  and  finally  lost  in  the  substance  of  the 
capsule.  Retzius1  regards  the  minute  elevations  on  the  surface  of  the 
lens  to  which  the  fibrillse  pass  as  indicative  of  the  presence  of  a  pericapsular 
membrane;  in  the  latter  structure  he  recognizes  an  external  zone  of  the 
capsule  the  differentiation  of  which  is  to  be  referred  to  initial  genetic  rela- 
tions of  the  vascular  mesodermic  tissue  enclosing  the  young  lens.  Czermak 
and  Topolanski  have  shown  that  the  zonular  fibres  are  more  numerous  in  the 
very  young  eye  than  at  any  later  time,  the  fibres  becoming  fewer  with  the 
advance  in  age. 

The  orbiculo-antero-capsular  fibres,  as  indicated  by  their  name,  extend 
from  the  orbicular  region  to  the  anterior  surface  of  the  lens-capsule,  and 
constitute  the  thickest  and  strongest  bands  of  the  zonula.  They  take 
origin  from  the  smooth  part  of  the  ciliary  body  behind  the  processes,  and 
receive  numerous  accessory  fibres  throughout  their  course  over  the  ciliary 
body.  Their  relation  to  the  valleys  between  the  ciliary  processes  is  such 
that  they  form  bundles  of  considerable  size,  which  are  maintained  in  close 
union  with  the  sides  of  the  valleys  by  means  of  numerous  delicate  acces- 
sory bands.  In  the  posterior  part  of  their  course  these  fibres  lie  intimately 
united  to  the  glassy  lamella  of  the  ciliary  region,  the  latter  membrane  oc- 
cluding the  interfibrillar  clefts,  and  thereby  producing  the  appearance  of  a 
continuous  sheet  or  membranous  zonula,  as  described  by  Schwalbe.  That 
the  zonula,  however,  is  composed  of  distinct  fibres  is  shown  by  careful 
inspection  of  surface  views  of  the  structures  in  question.  Serial  sections 
passing  through  the  ciliary  processes  in  planes  at  right  angles  to  the  course 
of  the  chief  zonular  fibres  are  of  interest  as  showing  the  relation  of  the 
fibres  to  the  elevations  and  valleys.  Such  preparations  show  that  the  fibres 
are  usually  associated  into  bundles  of  varying  size,  which,  while  following 
the  general  contour  of  the  depression  lodging  them,  are  not  closely  applied 
to  the  adjacent  walls,  but  are  separated  from  them  by  a  more  or  less  con- 
spicuous cleft ;  the  fibres  occupying  the  summit  of  the  process,  on  the 
contrary,  are  usually  in  intimate  relation  with  the  elevations.  Retzius, 
who  has  furnished  a  number  of  careful  figures  showing  the  transversely 
sectioned  fibres  in  situ,  calls  attention  to  these  relations,  as  well  as  to  the 
proximity  of  the  posterior  group  of  zouular  fibres  to  the  anterior  boundary 

1  Retzius:  loc.  cit.,  S.  86. 


THE    MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 


377 


of  the  vitreous  body,  this  relation  being  naturally  most  marked  in  the 
orbicular  zone,  where  the  space  between  the  wall  of  the  eyeball  and  the 


Fio.  105. 


Meridional  section  through  the  ciliary  region  of  an  adult  human  eye.  (Retzius.)— /,  equatorial 
periphery  of  lens ;  g.L,  vitreous  substance ;  g.r.,  anterior  boundary  of  same ;  o,  orbicular  space ;  i,  root 
of  iris ;  a,  short,  robust  attachment-fibres  of  the  posterior  zonular  trabeculse ;  6,  posterior  fibres  related 
to  vitreous;  c,  anterior  fibres  from  ciliary  processes;  d,  ciliary  fibres  crossing  and  adhering  to  the 
zonular  trabeculae;  e,  clefts  between  the  pericapsular  membrane  and  the  lens-capsule.  Low  amplifi- 
cation. 

vitreous  is  narrow.  The  study  of  incomplete  sections  of  the  ciliary  body, 
in  which  the  fibres  occupying  the  valleys  are  seen  passing  to  the  anterior 
capsule,  is  misleading  in  so  far  that  the  antero-capsular  fibres  encountered  do 


378  THE    MICROSCOPICAL   ANATOMY   OP   THE    EYEBALL. 

not  take  their  primary  origin  from  the  valleys,  as  seemingly  is  the  case, 
but  from  more  posteriorly  lying  points.  The  distinction  sometimes  made, 
therefore,  that  the  zonular  fibres  arising  within  the  valleys  pass  to  the 
anterior  capsule,  while  those  inserted  into  the  posterior  capsule  are  derived 
from  more  elevated  parts  of  the  ciliary  processes,  is  not  well  founded, 
since  the  origin  within  the  valleys  is  only  seeming,  the  true  beginning 
of  the  zouular  fibres  in  both  cases  being  the  more  posterior  parts  of  the 
ciliary  body. 

The  eilio-postero-capsular  fibres  constitute  a  group  comprising  a  large 
number  of  bands  of  moderate  size.  These  fibres — according  to  Gamier  the 
most  numerous  variety — spring  from  the  summits  and  the  sides  of  the 
ciliary  processes ;  after  pursuing  a  somewhat  backwardly  directed  course, 
during  which  they  cross  the  longer  zonular  bands  destined  for  the  anterior 
surface  of  the  lens,  they  reach  the  lens-capsule  and  find  insertion  on  the 
posterior  surface  in  advance  of  the  attachment  of  the  orbicular  fibres 
passing  to  that  surface. 

The  cilio-equatorial  fibres  are  best  represented  in  young  eyes,  where 
they  may  be  occasionally  seen  passing  from  the  summits  of  the  ciliary 
processes  to  the  equator  of  the  lens.  When  well  developed,  they  are  con- 
spicuous as  occupying  the  angular  space  included  between  anterior  and 
posterior  groups  of  zonular  fibres  which  pass  to  the  lens-capsule. 

The  accessory  fibres  constitute  an  important  source  of  additional  strength 
within  the  zone  of  Zinn,  since  they  brace  the  longer  bands  and  give  fixation 
to  their  points  of  ciliary  attachment.  The  accessory  fibres  are  usually  short, 
and  pass  from  parts  of  the  ciliary  body  to  adjacent  bands  of  the  larger 
series  or  to  more  or  less  remotely  situated  portions  of  the  ciliary  processes 
themselves. 

Since  the  evident  object  of  these  fibres  is  to  give  greater  security  to  the 
suspensory  apparatus  of  the  lens  by  providing  supplementary  points  of 
attachment  to  the  zonular  fibres  upon  which  falls  the  strain  incidental  to 
the  functional  activity  of  the  zonula,  their  purpose  is  best  attained  by 
the  combined  effect  of  bands  which  brace  the  fibres  directly  and  those  which 
aid  in  more  firmly  fixing  their  points  of  attachment  and  support.  The 
accessory  fibres  may,  therefore,  be  divided  into  two  groups, — (a)  those  which 
pass  from  the  ciliary  body  to  the  zonular  fibres,  and  (6)  those  which  extend 
from  one  point  to  another  within  the  ciliary  region. 

The  first  group  of  accessory  fibres  includes  the  numerous  short  bundles 
of  fibrillse  which  unite  the  orbiculo-capsular  fibres  with  the  adjacent  surface 
of  the  ciliary  area,  whether  they  be  strands  which  pass  to  the  long  fibres 
from  the  orbiculus  ciliaris  or  from  the  sides  and  bottoms  of  the  valleys. 
Since  these  supporting  bands  often  join  the  sides  of  the  zonular  fibres  with 
considerable  obliquity,  they  serve  to  brace  the  fibres  against  lateral  dis- 
placement as  well  as  to  afford  additional  security  of  attachment  in  the  line 
of  the  meridional  strain. 

The  second  group  of  accessory  fibres  embraces  the  bands  having  the 


THE    MICROSCOPICAL    ANATOMY    OF   THE    EYEBALL.  379 

support  of  the  ciliary  processes  as  their  particular  object.  Two  sets  of  such 
fibres  may  be  recognized, — (a)  the  arbiculo-ciliary  and  (6)  the  intra-ciliary. 
The  first  of  these  pass  from  the  orbicular  zone  to  the  ciliary  processes,  and 
hence  prevent  the  forward  displacement  of  these  structures ;  the  second 
extend  between  the  various  parts  of  the  processes  themselves,  and  insure 
additional  security  to  these  elevations  by  giving  greater  strength  to  their 
lateral  walls. 

It  is  evident  from  the  foregoing  description  of  the  zone  of  Zinn  that  the 
present  conceptions  concerning  this  structure  very  materially  differ  from 
the  older  views,  according  to  which  the  zonula  represented  membranous 
formations :  in  contrast  to  the  latter  view,  it  is  now  recognized  that  the 
zonula  consists  of  the  interlacement  of  the  several  sets  of  individual  fibres 
already  considered  in  detail,  and  that  the  membranous  suspensory  ligament 
of  the  lens  as  formerly  accepted  does  not  exist. 

Appreciation  of  these  details  must  necessarily  profoundly  influence  our 
conceptions  concerning  the  much-vexed  question  as  to  the  existence,  the 
character,  and  the  boundaries  of  the  canal  of  Petit. 

The  canal  of  Petit,  as  regarded  by  the  older  anatomists,  consisted  of  the 
annular  space,  triangular  in  section,  which  surrounded  the  margin  of  the 
lens,  and  was  bounded  by  the  last-named  structure  within  and  the  anterior 
and  posterior  Iamella3  of  the  zone  of  Zinn  in  front  and  behind.  The  cor- 
rectness of  the  older  classic  description  of  the  canal  of  Petit  was  conspicu- 
ously challenged  by  Merkel1  in  1870,  since  which  time  the  increasing 
exactness  of  our  knowledge  of  the  true  constitution  of  the  zonula  has  ren- 
dered the  existence  of  such  a  closed  perilenticular  annular  channel  less  and 
less  tenable.  A  striking  exception  to  the  general  tendency  of  belief  was 
presented  by  the  paper  of  Aeby,2  in  which  this  author  undertook  the  ener- 
getic defence  of  the  older  conception  of  the  canal.  Of  the  modified  views 
concerning  the  presence  of  a  canal  of  Petit,  that  of  Schwalbe3  is  note- 
worthy, since  this  authority,  while  denying  the  splitting  of  the  hyaloid, 
as  formerly  accepted,  still  holds  the  existence  of  a  closed  annular  space. 
According  to  Schwalbe,  the  anterior  wall  of  the  canal  of  Petit  is  formed 
by  the  zonula,  and  its  posterior  boundary  by  the  anterior  surface  of  the 
vitreous  body. 

The  more  recent  investigations  concerning  the  nature  of  the  zonula,  as 
already  described,  have  demonstrated  beyond  dispute  that  the  long-accepted 
membranous  zonula  does  not  exist,  the  component  fibres  in  no  sense  forming 
a  closed  canal.  In  the  light  of  our  present  knowledge,  therefore,  the  exist- 
ence of  a  canal  of  Petit  can  no  longer  be  maintained,  the  space  formerly 
apportioned  to  the  canal  being  in  reality  but  a  part  of  the  posterior  chamber. 
The  confines  of  the  latter  space,  as  emphasized  by  Topolanski,  are  the  lens, 

1  Merkel  :  Die  Zonula  ciliaris,  Gottingen,  1870. 

2  Aeby :  Der  Canalis  Petiti  und  die  Zonula  Zinnii  beim  Menschen  und  bei  Wirbel- 
thieren,  Archiv  f.  Ophthalmol.,  Bd.  xxvni.,  1882. 

8  Schwalbe  :  Lehrbuch  der  Anatomie  der  Sinnesorgane,  1887,  S.  145. 


380  THE   MICROSCOPICAL   ANATOMY    OF   THE    EYEBALL. 

the  iris,  the  ciliary  body,  including  the  orbiculus  ciliaris,  and  the  vitreous 
body,  covered  by  the  anterior  boundary  layer.  The  free  portions  of  the 
zonular  fibres  in  their  passage  to  the  lens  imperfectly  separate  the  posterior 
chamber  into  subdivisions,  one  lying  between  the  iris,  the  anterior  part  of 
the  ciliary  body,  and  the  anterior  bundles  of  zonular  fibres,  another  between 
the  anterior  and  posterior  zonular  fibres,  and  a  third  between  the  posterior 
fibres  and  the  vitreous  body.  While  such  subdivisions  are  apparent  in 
meridional  sections,  the  fact  must  not  be  overlooked  that  these  compart- 
ments are  freely  in  communication  with  one  another  through  the  interfas- 
cicular  clefts  of  the  zonula,  and  that  these  divisions  in  no  sense  represent 
isolated  portions  of  the  general  space  of  the  posterior  chamber,  and  are  all 
filled  with  the  aqueous  humor.  The  same  relations  apply  to  the  recessus 
camerce  posterioris  described  by  Kuhnt l  and  Schwalbe,  which  communicates 
with  the  general  space  occupied  by  the  contents  of  the  posterior  chamber. 

THE   AQUEOUS   HUMOR. 

The  aqueous  humor  and  its  chamber  belong  to  the  anterior  tract  of  the 
lymphatic  system  of  the  eyeball.  The  aqueous  space  is  subdivided  by  the 
iris,  the  free  edge  of  which  rests  upon  the  anterior  surface  of  the  lens,  into 
two  imperfectly  separated  compartments,  the  anterior  and  the  posterior 
chamber.  The  latter  space  is  especially  related  to  the  production  of  this 
lymphatic  fluid,  since,  as  already  described,  the  highly  vascular  ciliary 
body  is  the  particular  structure  interested  in  its  secretion. 

With  the  exception  of  a  few  leucocytes,  the  aqueous  fluid  is  without 
morphological  elements.  Its  chemical  composition,  as  given  by  Loh- 
meyer,2  includes, — 

Water 986.87 

Solids 13.13 

Proteids 1.22 

Extractives 4.21 

Inorganic  salts  (sodium  chloride,  6.89) , 7.70 

The  proteids,  according  to  Halliburton,3  are  similar  to  those  in  other 
forms  of  lymph, — namely,  fibrinogen,  serum-globulin,  and  serum-albu- 
min. The  reducing  substance  found  by  Kuhn 4  constantly  present  in  the 
aqueous  humor  of  the  rabbit  and  the  ox  resembles  sugar,  and  has  been 
usually  assumed  as  being  such ;  Gruenhagen,6  however,  denies  this,  on  the 
ground  that  the  substance  will  not  undergo  alcoholic  fermentation.  The 

1  Kuhnt :  Ueber  einige  Altersveranderungen  im  menschlichen  Auge,  Sitzungsberichte 
d.  ophthalmolog.  Gesellschaft  zu  Heidelberg,  1881. 

2  Lohmeyer:  Quoted  in  Gorup-Besanez's  Lehrbuch  d.  physiolog.  Chemie,  1878. 
8  Halliburton:  Text-Book  of  Chemical  Physiology  and  Pathology,  1891,  p.  350. 

*  Kuhn  :  Zur  Chemie  des  Humor  aqueus,  Archiv  f.  d.  gesammt.  Physiologie,  Bd.  XLI., 
1887. 

5  Gruenhagen :  Zur  Chemie  des  Humor  aqueus,  Archiv  f.  d.  gesammt.  Physiologie, 
Bd.  XLIII.,  1888. 


THE   MICROSCOPICAL   ANATOMY   OF   THE    EYEBALL.  381 

same  observer  reports  the  presence  of  minute  quantities  of  urea  and  sarco- 
lactic  acid. 

While  the  entire  quantity  of  aqueous  humor  is  affected  by  blood- 
pressure,  as  shown  by  Chabbas,1  the  fluid  usually  present  weighs  from  .23 
to  .32  gramme,  according  to  the  estimates  of  Jaeger 2  and  of  Rauber  • s 
the  specific  gravity  is  1.0053,  and  the  index  of  refraction  about  1.337. 
Compared  with  that  of  the  vitreous  body,  the  refractive  power  of  the 
aqueous  humor  shows  a  slight  increase,  being  below  that  of  the  cornea, 
and,  of  course,  distinctly  less  than  the  exponent  of  the  lens.  According 
to  the  experiments  of  Krause,4  Fleischer,5  Hirschberg,6  and  Aubert,7  the 
indices  of  refraction  of  the  eye-media  are,  approximately,  cornea  1.360, 
aqueous  humor  1.337,  crystalline  lens  1.425,  vitreous  body  1.336,  when 
the  refractive  index  of  distilled  water  is  1.334.  All  parts  of  the  posterior 
chamber,  including  the  intra-ciliary  recesses  and  the  intra-zonular  spaces, 
are  filled  with  the  aqueous  humor ;  the  latter  fluid  penetrates  into  the  ante- 
rior chamber  through  the  pupillary  opening  by  means  of  the  capillary  cleft 
which  usually  exists  between  the  anterior  surface  of  the  lens  and  the  edge 
of  the  iris.  The  important  rdle  continually  played  by  the  spaces  of  Fon- 
tana  and  the  adjacent  spongy  tissues  in  effecting  the  escape  of  the  aqueous 
fluid  from  the  anterior  chamber  into  the  annular  sinus  of  Schlemm  and 
the  associated  anterior  ciliary  veins  has  already  been  considered ;  it  is  here 
only  necessary  to  recall  the  existence  of  this  arrangement,  which  is  so 
essential  in  maintaining  the  equilibrium  of  intra-ocular  tension  by  per- 
mitting the  exit  of  the  lymph-fluid  which  collects  within  the  anterior 
segment  of  the  eyeball. 

1  Chabbas :  Ueber  die  Secretion  des  Humor  aqueus  in  Bezug  auf  die  Frage  nach  den 
Ursachen  der  Lymphbildung,  Archiv  f.  d.  gesammt.  Physiologie,  Bd.  xvi.,  1878. 

2  Jaeger:  Ueber  die  Einstellungen  des  dioptrischen  Apparatus  im  menschlichen  Auge, 
1861,  S.  14. 

3  Rauber :   Lehrbuch  des  Menschen,  4te  Aufl.,  Bd.  n.,  1894,  S.  725. 

*  Krause :  Die  Brechungsindices  der  durchsichtigen  Medien  des  menschlichen  Auges, 
1855. 

5  Fleischer :  Neue  Bestimmungen  der  Brechungsexponenten  der  durchsichtigen  flus- 
sigen  Medien  des  Auges,  1872. 

6  Hirschberg :  TJeber  Bestimmung   der  Brechungsindices   der  flussigen   Medien  des 
menschlichen  Auges,  Centralblatt  f.  d.  medicin.  Wissenschaften,  No.  13,  1874. 

7  Aubert :  Physiologische  Optik,  Graefe  u.  Saemisch's  Handbuch  d.  gesammten  Augen- 
heilkunde,  Bd.  n.,  1876. 


ANATOMY  OF  THE  INTRA-CRANIAL  POR- 
TION OF  THE  VISUAL  APPARATUS. 

BY  ALEX    HILL,  M.A.,  M.D., 

Master  of  Downing  College,  Cambridge,  England ;  late  Hunterian  Professor  at  the  Royal 
College  of  Surgeons  of  England,  Cambridge,  England. 


INTRODUCTION. 

IT  is  impossible  to  study  the  structure  of  the  brain  as  a  subject  by  itself. 
Only  when  considered  as  a  part  of  the  sciences  of  embryology,  comparative 
anatomy,  and  physiology  does  it  become,  intelligible.  The  cutting  and  stain- 
ing of  sections  in  series  have  been  brought  to  such  perfection  within  the 
last  few  years  that  those  who  are  not  experts  in  the  subject  are  tempted  to 
imagine  that  anatomists  have  at  their  disposal  methods  which  will  enable 
them  to  solve  all  questions  as  to  the  mutual  relations  of  the  fibres,  cells,  and 
plexus  of  which  the  central  nervous  system  is  composed.  Nothing  could 
be  further  from  the  truth,  for  the  facts  of  which  we  are  able  to  make  use  in 
our  attempts  to  picture  to  ourselves  the  machine  in  action  are  discovered 
from  a  study  of  its  development,  its  variations  in  animals  variously  endowed, 
its  reaction  to  stimulation,  and  its  degeneration  after  localized  disease,  rather 
than  through  any  strictly  anatomical  investigations.  It  is  impossible  to 
study  the  anatomy  of  the  central  nervous  system  apart  from  its  physiology, 
or  its  physiology  apart  from  its  pathology.  Neurology  is  a  study  to  which 
all  other  sciences  contribute ;  it  is  not  a  branch  of  any  other  science. 

If  it  be  true  that  even  the  structure  of  the  nervous  system  as  a  whole 
cannot  be  treated  as  a  separate  subject  standing  on  its  own  basis  of  ana- 
tomical observations,  no  argument  will  be  needed  to  prove  that  the  study  of 
a  part  of  the  system,  such  as  the  cerebral  mechanism  of  vision,  cannot  be 
detached  from  the  study  of  the  whole.  It  must  be  viewed  in  the  first  place 
in  its  relation  to  other  subjects. 

The  Ontology  of  the  Nervous  System. — The  earliest  changes  in  the  embryo 
are  associated  with  the  formation  of  the  nervous  system.  As  soon  as  the 
blastoderm  is  formed,  its  dorsal  layer,  or  epiblast,  begins  to  be  lifted  up  in 
two  ridges  which  border  a  longitudinal  groove.  The  ridges  continue  to 
grow  until  they  meet  in  the  mid-dorsal  line  and  thus  enclose  a  canal,  the 
canal  of  the  spinal  cord,  and  its  anterior  dilatations,  the  ventricles  of  the 
brain.  The  walls  of  this  canal  develop  into  the  central  nervous  system  ;  its 
cells  give  off  processes  which  run  towards  the  periphery  as  the  anterior 

383 


384  ANATOMY   OF   THE   INTRA-CRANIAL   PORTION 

roots  of  the  nerves.  The  posterior  roots  of  spinal  nerves  and  the  sensory 
cranial  nerves  are  not  developed  as  outgrowths  from  this  ueuro-epithelial 
tube,  but  as  ingrowths  from  a  series  of  rudiments  lying  on  either  side  of 
the  tube,  the  rudiments  of  spinal  ganglia  and  their  homologues  within  the 
cranium.  These  rudiments  are  also  of  epiblastic  origin,  and  therefore  the 
whole  nervous  system  is  developed  from  tissue  which  elsewhere  becomes  the 
skin. 

Hlstogeny. — The  neuro-epithelial  cells  which  enter  into  the  formation  of 
the  nervous  system  very  early  show  a  distinction  into  two  sets, — viz.,  (a)  the 
germ-cells,  which  retain  their  power  of  subdivision,  as  indicated  by  their 
abundant  protoplasm  and  large  nuclei  with  conspicuous  chromatin-skeins ; 
and  (6)  the  spongioblasts,  of  which  the  bases  rest  against  the  central  canal, 
while  their  bodies  are  elongated  until  they  not  only  traverse  the  whole 
thickness  of  the  spinal  cord,  but  also  give  off,  near  its  periphery,  lateral 
processes  which  enter  into  the  formation  of  a  reticulum,  the  velum  confine. 
The  germ-cells  (a)  retain  their  situation  close  beneath  the  lining  epithelium 
of  the  central  canal,  where  the  mitotic  figures  of  their  nuclei  make  them 
conspicuous  in  any  stained  section ;  the  cells  to  which  by  their  subdivision 
they  give  rise  take  up  their  positions  farther  outward  in  the  gray  matter,  as 
neuroblasts.  The  neuroblast  becomes  a  nerve-cell ;  its  undivided  process  a 
nerve.  The  process  may  run  out  of  the  cerebro-spinal  axis  as  a  peripheral 
nerve,  or  up  or  down  within  the  reticulum  of  the  velum  confine  as  a  longer 
or  shorter  intra-axial  commissural  fibre. 

The  Phytogeny  of  the  Central  Nervous  System. — Opinions  differ  as  to  the 
condition  among  existing  animals  which  we  may  look  upon  as  representing 
the  primitive  or  original  form  of  the  nerve-elements,  but  it  is  generally 
allowed  that  we  have  to  regard  nerve-fibres  as  filaments  which  came  into 
existence  for  the  purpose  of  uniting  contractile  cells  with  the  sensory  cells 
of  the  surface,  the  stimulation  of  which  rendered  contraction  desirable. 
Thus,  the  first  compound  animal  was  a  hollow  sphere  of  cells.  This,  being 
"  pitted  in,"  became  a  gastrula,  or  animal  with  a  stomach-cavity  lined  by 
endoderm  and  with  an  outer  wall  of  ectoderm.  The  ectodermal  or  epithelial 
cells  developed  contractile  processes,  and,  as  these  processes  increased  in  size 
and  importance  and  began  to  assume  the  properties  of  independent  muscle- 
cells,  constituting  a  third  layer  (mesoderm)  in  the  animal's  body  wall,  the 
conducting  strand  which  united  each  muscle-fibre  to  an  ectodermal  cell  was 
drawn  out  into  a  nerve-fibre.  At  this  stage  the  animal  was  only  capable  of 
answering  to  general  stimulation  of  its  surface  by  a  universal  contraction. 

The  origin  of  sense-organs  is  to  be  traced  to  the  fact  that  certain  spots 
on  the  surface  which,  owing  to  their  favorable  situation  or  to  chemical 
differentiations  which  gave  rise  to  pigment,  crystals,  or  other  substances,  were 
rendered  particularly  liable  to  stimulation,  became  the  organs  from  which 
information  of  danger  was  most  frequently  transmitted  to  the  muscle-fibres 
by  whose  contraction  the  danger  was  escaped.  These  sensitive  spots  were 
the  first  sense-organs. 


OF   THE   VISUAL   APPARATUS.  385 

So  long,  however,  as  the  sense-organs  retained  their  connection  with 
certain  groups  of  muscle-fibres  only,  their  usefulness  must  have  remained 
extremely  small.  The  next  step  in  advance,  and  one  which  seems  to  have 
occurred  with  great  rapidity,  was  the  introduction  of  a  distributive  mechan- 
ism at  the  base  of  each  sense-organ,  by  means  of  which  it  was  placed  in 
connection  with  various  parts  of  the  contractile  sheet  of  mesoderm.  There 
can  be  very  little  doubt  that  this  commencement  of  a  nervous  system  was 
effected  by  the  deposition  of  certain  of  the  cells  of  the  primitive  sense- 
organs,  from  their  posts  as  scouts,  and  their  utilization  for  the  purpose  of 
establishing  communications  between  the  cells  which  remained  on  the  sur- 
face and  the  contractile  cells.  It  is  possible  in  the  covered-eyed  Medusidse 
(the  larger  jelly-fishes)  to  find,  only  just  beneath  the  surface,  cells  inter- 
mediate in  form  between  sensory  cells  and  ordinary  nerve-cells. 

Formation  of  a  Central  Nervous  System. — The  efficiency  of  these  groups 
of  "  distributive  cells"  was  soon  increased  by  their  union,  by  means  of  com- 
missural  fibres,  into  a  central  nervous  system.  This  stage  in  the  develop- 
ment of  the  system  can  be  seen  in  the  naked-eyed  Medusidse.  The  margin 
of  the  swimming-bell  of  Sarsia  carries  two  such  nerve-rings,  the  lower  rich 
in  cells.  With  the  acquisition  of  this  ring  we  find  that  the  animal  becomes 
able  to  localize  any  spot  on  its  bell  which  may  be  injured ;  it  swings  its 
polypite  to  the  injured  spot  and  performs  other  simple  reflex  actions. 

For  the  purposes  of  the  present  article  it  is  most  important  to  remember 
that  the  sense-organs  are  segmental,  and  that  the  rudiments  of  the  central 
nervous  system  were  laid  down  about  the  bases  of  these  organs.  As  we 
ascend  the  animal  scale  we  find  centralization  carried  farther  and  still  farther, 
but  we  have  every  reason  for  believing  that  the  spinal  ganglia,  which  are 
formed  by  delamination  of  the  epiblast  and  not  from  the  neuro-epithelial 
tube,  are  the  remains  of  the  groups  of  cells  which  were  submerged  in  the 
mesoderm  beneath  the  primitive  sense-organs.  Save  these  large  bipolar 
cells,  which  give  off  one  process  towards  the  periphery  and  another  towards 
the  spinal  cord,  all  nervous  elements  have  been  withdrawn,  from  the  vicinity 
of  the  greater  number  of  the  sense-organs,  to  a  more  central  and  sheltered 
situation.  The  centralization  of  the  gray  matter  originally  laid  down  at 
the  base  of  the  ear,  the  nose,  and  the  eye  is  not,  however,  carried  to  any- 
thing like  the  same  extent  as  in  the  case  of  the  other  sense-organs.  The 
bipolar  cells  of  the  ganglion  spirale  are  still  to  be  found  in  the  vicinity  of 
the  cochlea.  The  olfactory  bulb  contains  granules,  plexus,  "  gelatinous  sub- 
stance," and  nerve-cells,  which  in  segments  farther  back  are  all,  save  the 
granules  (bipolar  cells),  withdrawn  into  the  cerebro-spiual  axis. 

The  retina  is  more  primitive  in  plan  of  structure  than  either  the  nose  or 
the  ear ;  for  its  several  layers,  beneath  its  epithelial  cells  and  their  nuclei  (outer 
nuclear  layer)  and  in  immediate  juxtaposition  with  them,  consist  of  granules 
(bipolar  cells),  plexus  (molecular  layer),  and  nerve-cells  arranged  in  regular 
strata.  Granules,  cells  of  ganglion,  spirale  and  cells  of  spinal  ganglia, 
molecular  layer  of  retina,  stratum  gelatinosum  of  olfactory  bulb,  and  sub- 
VOL.  I  —25 


386  ANATOMY   OF   THE   INTRA-CRANIAL    PORTION 

stantia  gelatinosa  Rolandi  of  the  spinal  cord  are,  as  we  believe  and  have 
taught  for  the  last  ten  years,1  strictly  homologous.  It  is  essential  that  this- 
fact  should  be  recognized  before  we  attempt  to  trace  the  connections  of  the 
optic  nerve  with  the  brain.  As  a  sense-organ  the  eye  probably  assumed  its 
permanent  and  immutable  form  before  the  formation  of  the  central  nervous- 
system  was  carried  very  far.  The  nervous  elements  at  its  base  have  never 
been  withdrawn  into  the  spinal  cord,  but  retain  their  local  situation  in  the 
retina.  Before  we  try  to  trace  the  course  of  the  optic  tract  within  the 
brain  we  must  disabuse  our  minds  of  the  idea  that  its  connections  will 
be  found  to  be  arranged  upon  the  same  plan  as  those  of  other  sensory 
nerves  or  posterior  roots.  All  other  sensory  roots  enter  into  intimate  rela- 
tions with  the  substantia  gelatinosa  Rolandi.  The  portion  of  this  substance 
which  belongs  to  the  optic  nerve  lies  in  the  retina.  After  the  posterior 
roots  have  undergone  arborization  in  the  central  gray  matter  of  the  spinal 
cord,  multipolar  cells  collect  the  impulses  which  the  roots  have  delivered 
and  transmit  them  by  their  axis-cylinder  processes  to  higher  regions  of 
the  cord  or  brain.  Such  collecting  cells,  also,  are  to  be  found  in  the  retina, 
and  there  is  no  analogy  in  other  nerves  from  which  we  can  judge  as  to  the 
nerve-tissue  in  which  we  ought  to  expect  that  the  axis-cylinder  processes- 
of  these  collecting  cells  will  end. 

The  cerebral  connections  of  the  retina  must  be  treated  as  a  problem  by 
itself.  We  must  be  careful  not  to  infer  that  the  connections  of  the  optic 
fibres  will  follow  any  plan  which  we  find  to  hold  good  for  other  sensory 
nerves. 

In  the  evolution  of  its  cerebral  mechanism  the  optic  nerve  stands  almost 
alone.  The  phylogeny  of  the  brain  is  far  from  being  understood,  but  there 
are  certain  conclusions  which  we  are  fully  justified  in  drawing  from  the 
study  of  its  typical  form  in  the  several  classes  of  vertebrates.  In  fishes 
the  cerebrum  is  very  small  and  the  rhinencephalon  distinct  from  the  rest 
of  this  organ.  The  optic  lobes  are  large  and  distinctly  cortical  in  plan  of 
formation,  and  we  find  in  addition  trigeminal  and  vagal  lobes  of  certain 
dimensions.  The  optic  lobes  are  just  as  large  in  birds  and  reptiles,  but  the 
other  lobes  have  almost  disappeared.  The  cortex  cerebri,  which  is  respon- 
sible for  the  immense  development  of  the  human  brain,  is  practically  absent 
in  fishes  and  birds  and  only  makes  its  appearance  in  an  unmistakable  form 
in  reptiles.  It  seems  impossible  to  avoid  the  conclusion  that  each  segment 
of  the  vertebrate  brain  was  at  one  time  complete  in  itself,  autonomous, 
carrying  on  all  the  traffic  brought  to  it  by  the  nerve  with  which  it  was 
especially  connected,  whether  it  were  the  olfactory,  optic,  trigeminal,  or 
vagus. 

In  fishes,  amphibians,  reptiles,  and  birds  the  chief  cerebral  connection 
of  the  optic  nerve  is  with  the  optic  lobe  (corpus  bigeminum).  The  large 

1  Hunterian  Lectures,  Royal  College  of  Surgeons,  1885-86 ;  British  Medical  Journal, 
March,  1885,  and  March,  1886. 


OF   THE   VISUAL   APPARATUS.  387 

size  and  complicated  structure  of  this  body  show  that  it  suffices  for  the 
elaboration  and  reflection  of  all  visual  impulses.  In  the  lower  mammals 
the  corpora  quadrigemina  are  still  of  large  size  relatively  to  the  rest  of  the 
brain.  Their  minute  structure  shows  that  they  carry  on  important  work. 
As  the  mammalian  scale  is  ascended  the  corpora  quadrigemina  become  pro- 
gressively smaller  and  poorer  in  gray  matter,  until  in  the  Primates  they 
form  but  a  very  small  proportion  of  the  whole  brain.  It  is  true  that  the 
proportion  between  their  weight  and  the  body-weight  is  not  greatly  changed, 
and  it  might  be  argued  that  they  still  retain  the  functions  which  they  first 
came  into  existence  to  perform ;  but  their  impoverishment  in  nerve-cells, 
considered  in  connection  with  the  diversion  to  the  fore-brain  of  a  great 
number  of  the  optic  fibres  which  are  distributed  to  the  mid-brain  in  lower 
animals,  indicates  in  an  unmistakable  manner,  as  we  think,  that  the  human 
cortex  cerebri  has  assumed  functions  originally  performed  by  other  parts  of 
the  brain. 

COURSE   OF   THE  OPTIC   NERVES. 

The  course  and  connections  of  the  optic  fibres  have  been  worked  out 
chiefly  in  the  human  brain,  and  especially  by  means  of  pathological  investi- 
gations. It  is  convenient,  therefore,  that  our  description  of  these  tracts  in 
their  minute  subdivisions  should  have  reference  to  man. 

Optic  Nerve. — This  nerve  has  a  length  of  about  five  centimetres,  of 
which  three  centimetres  lie  within  the  orbit,  one  centimetre  is  within  the 
optic  canal,  and  one  centimetre  is  intra-cranial.  Within  the  eyeball  its 
fibres  are  non-medullated.  They  acquire  their  myelin-sheaths  immediately 
after  traversing  the  sclerotic,  and  hence  the  nerve  appears  constricted  as  it 
leaves  the  eyeball.  Beyond  this  point  the  nerve  is  a  firm,  round,  white 
cord  five  millimetres  in  diameter,  its  cross-section  being  nine  square  milli- 
metres, of  which  area  four  square  millimetres  must  be  deducted  for  connective 
tissue.  Its  fibres  are  collected  into  fasciculi  very  much  after  the  manner 
of  an  ordinary  peripheral  nerve,  although  the  septa  of  connective  tissue, 
rich  in  nuclei,  which  enter  from  the  periphery  are  less  regular  in  thickness 
and  in  disposition  than  ordinary  endoneurium.  The  nerve  contains,  ac- 
cording to  Salzer's  enumeration,1  about  four  hundred  and  thirty-eight  thou- 
sand medullated  fibres,  of  extreme  tenuity.  Such  an  enumeration  must, 
however,  be  received  with  caution,  owing  to  the  obvious  difficulty  which  the 
counting  of  very  minute  fibres  presents ;  it  is  difficult,  perhaps  impossible, 
to  recognize  all  the  smaller  fibres,  and  certainly  impossible  to  count  them 
in  any  section. 

Krause 2  describes  the  measurable  medullated  fibres  as  varying  in  size 
from  0.001  to  0.014  of  a  millimetre,  the  most  numerous  being  those  of 
0.006  of  a  millimetre.  He  considers  that  Salzer  has  counted  these  fibres 

1  Sitzungsberichte  der  k.  Akad.  der  Wissensch.  zu  Wien,  Math,  naturw.  Klasse,  Bd. 
Ixxxi.,  Abth.  iii.  p.  7. 

2  Archiv  f.  Ophthalmologie,  xxvi.,  Abth.  I.  p.  102. 


388  ANATOMY   OF   THE   INTRA-CRANIAL    PORTION 

accurately,  but  has  omitted  from  his  reckoning  all  the  finest  fibres  (of  about 
0.0005  of  a  millimetre),  which  are  at  least  as  numerous  as  the  fibres  which 
he  counted,  bringing  the  total  number  up  to  about  one  million. 

Von  Gudden  divided  the  fibres  into  two  classes,  large  and  small ;  but 
he  did  not  doubt  that  both  were  afferent,  although  he  surmised  that  while 
the  large  fibres  carried  visual  impulses  the  small  might  very  probably  be 
devoted  to  the  mechanism  for  carrying  out  the  pupil-reflex.  Von  Mona- 
kow l  makes  a  further  use  of  the  difference  in  size,  for  he  conjectures  that 
the  two  classes  of  fibres  belong  to  two  distinct  systems, — (a)  the  large  fibres, 
which  originate,  he  thinks,  in  the  large  nerve-cells  of  the  retina  and  grow 
centripetally  into  the  reticulum  of  the  external  geniculate  body  and  the 
pulvinar  of  the  optic  thalamus,  and  (6)  the  fibres  of  small  calibre,  which 
originate  in  the  cells  of  the  superficial  gray  matter  of  the  corpora  quadri- 
gemina  and  grow  outward  to  terminate  in  the  inner  nuclear  layer  of  the 
retina. 

However  probable  such  a  division  into  smaller  centrifugal  and  larger 
centripetal  fibres  may  be,  it  would  be  premature  as  yet  to  regard  it  as  defi- 
nitely proved.  There  can  be  no  doubt  as  to  the  existence  of  larger  and 
smaller  fibres  (Fig.  1),  but  we  agree  with  Krause  in  thinking  that  the 
fibres  are  not  sufficiently  uniform  in  diameter  to  allow  of  arrangement  in 
two  classes.  We  estimate  that  there  are  on  the  average  about  eight  smaller 
fibres  of  various  categories  to  each  distinctly  large  fibre,  but  we  have  as  yet 
failed  to  follow  the  two  classes  to  their  destination  in  the  brain  or  to  corre- 
late the  difference  in  size  with  difference  in  function.  A  glance  at  the 
photograph  reproduced  in  Fig.  1  suggested  that  the  smaller  fibres  might 
be  ultimately  connected  with  rods,  the  larger  with  cones ;  but  this  is  not 
the  case,  for  the  distinction  in  size  is  as  well  marked  in  animals  which 
have  cones  only  (e.g.,  the  alligator  or  the  turkey)  as  in  mammals.  Nor  is 
Von  Monakow's  supposition  as  to  their  mode  of  growth  and  cerebral  desti- 
nation supported  by  an  examination  of  the  optic  tract  above  its  division 
into  external  and  internal  roots,  since  throughout  both  roots  of  the  tract 
the  large  fibres  are  scattered  about  among  the  small,  just  as  they  are  in  the 
optic  nerve.  The  only  definite  observations  upon  the  direction  of  growth 
of  the  fibres2  with  which  we  are  acquainted  show  us  the  fibres  growing 
inward  from  the  retina,  although  it  is  quite  possible  that  these  fibres  are 
the  first  to  grow  and  that  centrifugal  fibres  appear  later.  The  complete 
ascending  degeneration  which  follows  enucleation  of  the  eyeball  points  to 
the  centripetal  growth  of  all  the  fibres;  Von  Gudden,  Henschen,  and 
others,  however,  have  observed  descending  degeneration  as  a  consequence 
of  destruction,  experimental  or  pathological,  of  the  optic  tract.  It  is  of 
fundamental  importance  that  we  should  obtain  exact  information  as  to  the 
number,  size,  and  character  of  the  optic  fibres,  their  direction  of  growth, 

1  Archiv  fur  Psychiatric,  vol.  xx.  p.  780. 

*  Froriep,  Anat.  Anzeiger,  1891,  vi.  6,  p.  155. 


FIG.  1. 


-  ;•• 


Photomicrograph  of  a  transverse  section  of  the  optic  nerve  of  a  calf,  magnified  700  diameters. 
The  section  shows  trabeculae  of  connective  tissue,  connective-tissue  nuclei  (the  large  dots),  and.  much 
smaller  than  the  latter,  the  axis-cylinders  of  optic  nerve  fibres.  Of  the  nerve-fibres  a  certain  number 
are  larger  and  tend  to  lie  in  free  spaces,  but  the  majority  are  extremely  minute,  more  or  less  collected 
into  groups,  and  not  surrounded  by  free  spaces.  The  section  was  stained  in  carmine. 


Fio.  6. 


Mid-brain  of  ox,  showing  the  manner  in  which  the  optic  tract  grasps  the  back  of  the  thalamus  and 
surrounds  the  geniculate  bodies.  Behind  the  internal  geniculate  body  the  optic  tract  appears  to  give 
a  bundle  to  the  crus  cerebri. 


OF   THE   VISUAL   APPARATUS. 


389 


FIG.  2. 


and  their  retinal  and  cerebral  connections ;  but  the  answers  given  as  yet  to 
these  questions  are,  unfortunately,  little  more  than  speculations. 

Optic  Chiasm. — The  round  optic  nerves  meet  beneath  the  floor  of  the 
third  ventricle  to  form  the  transversely  disposed  and  somewhat  flattened 
optic  chiasm  or  commissure.  (Fig.  2,  6.)  The  chiasm  is  lodged  in  a  recess 
in  the  thin  wall  of  the  'tween-brain  in  front 
of  the  tuber  cinereum,  and  forms  a  part  of 
its  floor.  Owing  to  this  continuity  of  the 
floor  of  the  ventricle  and  the  optic  chiasm, 
advantage  is  taken  of  the  situation  of  the 
latter  by  certain  other  oommissural  fibres 
which  do  not  belong  to  the  optic  apparatus, 
— namely,  the  commissures  of  Meynert  and 
Van  Gudden.  Apart  from  such  accessory 
tracts  the  chiasm  consists  of  two  sets  of 
fibres, — the  crossing  fibres,  from  the  nasal 
sides  of  the  two  retinae,  which  occupy  its 
centre,  and  the  uncrossed  fibres,  from  the 
temporal  sides  of  the  retinae,  which  consti- 
tute its  lateral  portions.  The  fibres  which 
originate  in  the  macula  lutea  have  in  recent 
years  been  traced  with  such  precision  as 
almost  to  merit  separation  into  a  third  class 
or  system,  the  "  maculary  fascicle,"  although 
they  share  with  the  other  optic  fibres  the 
division  into  uncrossed  fibres  of  the  temporal 
side  and  crossed  fibres  of  the  nasal  side  of 
the  retina.  In  a  number  of  cases  of  central 

scotoma,1  for  the  most  part  of  toxic  origin,  this  maculary  fascicle  has  alone 
degenerated,  and,  although  it  cannot  be  distinguished  as  a  separate  bundle 
in  a  healthy  nerve,  it  is  found  when  degenerated  to  occupy  at  first  the  outer 
and  inferior  portion  of  the  optic  nerve,  gradually  withdrawing  to  its  centre, 
so  that  when  it  enters  the  chiasm  it  and  the  fascicle  of  the  opposite  side  lie 
symmetrically  in  the  two  foci  of  the  ellipse.  In  the  chiasm  it  divides  into 
a  nasal  portion,  which  decussates  with  its  fellow  in  the  centre  of  the  chiasm, 
and  a  lateral  uncrossed  portion,  which  continues  its  course  into  the  optic 
tract  of  the  same  side.  In  the  tract  the  two  bundles  apparently  reunite  to 
form  a  single  bundle  in  its  centre.  This  theory  of  the  double  constitution 
of  the  chiasm,  of  crossed  and  uncrossed  fibres,  now  almost  universally  held, 


The  base  of  the  brain.— 1,  infun- 
dibulum ;  2,  tuber  cinereum ;  3,  corpus 
albicans;  4,  cms  cerebri;  5,  pons 
varolii;  6,  optic  chiasm;  7,  oculo- 
motor nerve;  8,  fourth  nerve  (to  su- 
perior oblique  muscle  of  the  eyeball) ; 
10,  sixth  nerve  (to  external  rectus 
muscle  of  the  eyeball). 


1  Leber,  Archiv  fur  Ophthalmologie,  xv.  p.  67 ;  Samelsohn,  Archiv  fur  Ophthal- 
mologie,  xxviii.,  1,  p.  1;  Nettlcship,  Transactions  of  the  Ophthalmological  Society,  i. ; 
Vossius,  Archiv  fur  Ophthalmologie,  xxviii.,  3,  p.  201  ;  Bunge,  Gesichtsfeld  und  Faser- 
verlauf  in  optischen  Leitungsapparat,  Halle,  1884;  Uhthoff,  Archiv  fur  Ophthalmologie, 
xxxii.,  3,  p.  95;  Thomson,  Archiv  fur  Psychiatric,  xiii.  p.  352. 


390  ANATOMY   OF   THE    INTRA-CRANIAL    PORTION 

was  formulated  by  Newton  l  as  an  hypothesis  accounting  for  stereoscopic 
vision.  Since  it  is  not  accepted  by  all  anatomists,  however,  it  is  well  that 
we  should  examine  the  evidence  upon  which  this  theory  is  based,  and  the 
arguments  which  may  be  urged  against  it. 

Looking  at  the  subject  first  of  all  from  the  physiological  side,  it  will  be 
seen  that  the  division  of  each  retina  into  two  parts,  of  which  one  is  united 
with  the  brain  by  crossed  fibres  and  the  other  by  uncrossed  fibres,  allows  of 
the  connection  of  two  "  corresponding  retinal  points"  with  a  single  cortical 
area.  It  is  almost  impossible  to  conceive  of  a  cerebral  mechanism  of 
binocular  vision  arranged  upon  any  other  plan.  Human  beings  pay  very 
little  attention  to  any  objects  which  are  not  seen  by  both  eyes  at  once. 
Objects  situate  near  the  periphery  of  the  field  of  vision  which  are  brought 
to  a  focus  on  one  retina  only  are  perceived  by  most  people  merely  as  vague 
differences  of  light  and  shade.  In  all  fishes  and  birds  and  in  many  mem- 
bers of  each  of  the  other  classes  of  vertebrates  in  which  the  eyes  are  placed 
on  the  side  of  the  head,  we  must  suppose  that  the  conditions  of  sight  are 
quite  different.  In  them  each  field  of  vision  is  independent  of  the  other, 
and,  although  there  may  very  probably  be  a  part  of  each  retina  which  is 
more  sensitive  to  light  than  the  rest,  the  visual  fields  do  not  overlap,  but 
form  a  continuous  expanse,  each  object  giving  rise  to  an  independent  sen- 
sation. 

In  comparing  the  position  of  the  eyeballs  in  various  animals  we  must  not 
be  misled  by  the  direction  of  the  optic  nerve,  which,  in  order  that  the  blind 
spot  may  not  fall  in  the  optic  axis,  enters  the  retina  on  the  inner  side  of  the 
globe ;  nor  is  it  necessary  that  we  should  have  regard  only  to  the  axis  about 
which  the  refractive  media  of  the  eye  are  centred,  since  in  man,  as  in  most 
animals,  the  sensitive  screen  is  continued  farther  on  the  nasal  than  on  the 
temporal  side  of  the  globe ;  but  for  the  purposes  of  this  article — namely, 
the  tracing  of  retinal  fibres  to  the  brain — we  may  consider  the  position  of 
the  retinal  screen  as  a  whole.  In  a  man  the  axes  which  bisect  the  central 
points  of  the  retinre  would  meet  if  prolonged  backward  at  an  angle  of  less 
than  45°  (Fig.  3) ;  in  a  dog,  at  an  angle  of  about  90°  ;  in  a  horse,  at  an 
angle  of  about  135°  ;  in  a  rabbit  (Fig.  4)  they  lie  almost  in  the  same 

1  <l  Are  not  the  species  of  objects  seen  with  both  eyes  united  where  the  optic  nerves 
meet  before  they  come  into  the  brain,  the  fibres  on  the  right  side  of  both  nerves  uniting 
there,  and  after  union  going  thence  into  the  brain  in  the  nerve  which  is  on  the  right  side 
of  the  head,  and  the  fibres  on  the  left  side  of  both  nerves  uniting  in  the  same  place,  and 
after  union  going  into  the  brain  in  the  nerve  which  is  on  the  left  side  of  the  head,  and 
these  two  nerves  meeting  in  the  brain  in  such  a  manner  that  their  fibres  make  but  one 
entire  species  or  picture,  half  of  which  on  the  right  side  of  the  sensorium  comes  from  the 
right  side  of  both  eyes  through  the  right  side  of  both  optic  nerves  to  the  place  where  the 
nerves  meet,  and  from  thence  on  the  right  side  of  the  head  into  the  brain,  and  the  other  half 
on  the  left  side  of  the  sensorium  comes  in  like  manner  from  the  left  side  of  both  eyes  ?  For 
the  optic  nerves  of  such  animals  as  look  the  same  way  with  both  eyes  (as  of  men,  dogs, 
sheep,  oxen,  etc.)  meet  before  they  come  into  the  brain,  but  the  optic  nerves  of  such 
animals  as  do  not  look  the  same  way  with  both  eyes  (as  of  fishes  and  of  the  chameleon)  do 
not  meet,  if  I  am  rightly  informed." — Newton's  Optics,  QM.  15. 


OF  THE   VISUAL   APPARATUS. 


391 


straight  line ;  and  it  is  clear  that  the  more  nearly  the  axes  are  parallel  with 
one  another  the  more  extensively  do  the  fields  of  vision  overlap.     The  field 


FIG.  3. 


FIG.  4. 


Tracing  from  a  frozen  section  of  the  head  of  a  man, 
to  show  the  position  of  the  eyeballs. 


Tracing  from  a  frozen  section  of  the 
head  of  a  rabbit. 


of  vision  for  each  eye  in  man  (Fig.  5)  subtends  an  angle  of  about  1 35°  ; 
the  combined  field  of  vision  embraces,  in  the  horizontal  plane,  a  semicircle. 


FIG.  6. 


Uncrossed  tract  ••- 

Maculary  fascicle  •[  ""SSSJ'^ 
Crossed  tract ' 


f  Ant 

Corpora  quadrigemina< 

(.Post 


Parieto-occipital  fissure 

Cuneus 

Calcarine  fissure 


Optic  thalamuB. 


Pulvinar. 

External  geniculate  body. 
Internal  geniculate  body. 
Optic  radiations. 


Posterior  horn  of  lateral  ventricle. 


Hence  the  field  of  stereoscopic  vision,  or  region  in  which  the  visual  fields 
of  the  two  eyes  overlap,  subtends  an  angle  of  about  90°. 


392  ANATOMY   OF   THE   INTRA-CRANIAL   PORTION 

In  man,  therefore,  and  in  certain  monkeys  which  agree  with  him  in 
having  the  eyes  directed  forward,  each  retina  is  divided  by  a  vertical  line 
into  a  lateral  or  temporal  portion  (about  three-fourths  of  the  whole),  which 
is  concerned  with  binocular  vision,  and  a  mesial  or  nasal  portion  (about  one- 
fourth),  which  can  be  used  only  for  monocular  vision.  According  to  New- 
ton's theory  of  the  construction  of  the  chiasm,  all  the  fibres  from  the  nasal 
sides  of  the  retinae  cross  to  the  opposite  side  of  the  brain,  whereas  some  of 
the  fibres  from  the  temporal  sides  are  connected  with  the  brain  on  their  own 
side  of  the  body.  The  crossed  is  the  primitive  connection,  the  diversion  of 
certain  fibres  to  the  brain  on  the  same  side  being  a  secondary  adaptation 
which  keeps  pace  exactly  with  the  overlapping  of  the  fields  of  vision,  or 
development  of  stereoscopy.  The  size  of  the  uncrossed  tract,  as  traced  by 
observing  cases  of  natural  or  surgically  induced  degeneration  (and  we  may 
say  at  once  that  it  is  impossible  to  follow  the  tract  by  any  anatomical 
method,  whether  of  maceration,  teasing,  or  sections  in  the  coronal  or  hori- 
zontal planes),  should,  according  to  this  theory,  vary  directly  as  the  develop- 
ment of  binocular  vision  in  the  animal,  and  this  we  find  to  be  the  case.  In 
the  rabbit  it  is  so  small  that  it  was  for  a  long  time  overlooked ;  in  dogs  it 
is  much  larger ;  in  monkeys  it  is  larger  still. 

In  order,  however,  that  the  anatomical  facts  which  have  been  ascertained 
from  a  study  of  the  degeneration  of  fibres  through  the  chiasm  should  enable 
us  to  picture  to  ourselves  the  mechanism  of  binocular  vision,— to  under- 
stand what  we  may  term  the  mental  superposition  of  the  images  focussed 
on  corresponding  points  of  the  two  retina?, — the  division  of  the  retina  must 
not  simply  coincide  with  its  division  into  a  part  concerned  with  binocular 
and  a  part  concerned  with  monocular  vision,  but  the  part  concerned  with 
binocular  vision  must  be  further  divided  into  two.  It  can  be  proved  that 
the  connection  of  eye  and  brain  is  primitively  contra-lateral ;  it  is  assumed 
that  the  connection  with  the  same  side  is  adapted  to  binocular  vision.  It 
is  supposed  that,  whereas  all  that  part  of  the  nasal  side  of  the  retina  which 
can  be  used  only  in  monocular  vision  retains  its  primitive  crossed  connec- 
tion, the  superposition  of  corresponding  images  is  effected  by  the  division 
of  the  binocular  portion  of  the  retina  into  two  halves  about  a  vertical  line. 
Impulses  generated  by  the  impact  of  light  upon  the  temporal  side  of  this 
portion  of  the  retina  of  the  right  eye  are  carried  by  uncrossed  fibres  to  the 
right  side  of  the  brain,  to  which  side  of  the  brain  the  impulses  simulta- 
neously generated  in"  the  nasal  side  of  the  left  eye  are  also  carried  by  crossed 
fibres.  The  images  are  therefore  superposed,  as  it  were,  in  the  brain. 

According  to  this  view  of  the  cerebral  mechanism  of  vision,  the  chiasm 
contains  three  sets  of  fibres, — namely,  (a)  the  crossed  fibres  concerned  with 
monocular  vision,  (6j)  the  crossed  fibres  concerned  with  binocular  vision, 
and  (62)  the  uncrossed  fibres  concerned  with  binocular  vision. 

It  is,  we  believe,  impossible  to  recognize  these  three  sets  of  fibres  in  the 
optic  nerve  by  any  anatomical  difference  of  size  or  grouping.  The  obser- 
vation of  the  degeneration  which  results  in  various  animals  from  enucleation 


OF  THE   VISUAL   APPARATUS.  393 

of  the  eyeball  does,  however,  confirm  the  physiological  hypothesis,  for  the 
size  of  the  uncrossed  tract  keeps  pace  with  the  development  of  stereoscopic 
vision.  In  the  rabbit  the  bundle  of  fibres  which  degenerates  in  the  optic 
tract  of  the  same  side  is  so  small  as  to  be  easily  overlooked,  and  its  sup- 
posed absence  is  cited,  by  those  who  believe  in  the  total  decussation  of  the 
optic  nerves  in  the  chiasm,  as  a  proof  of  their  theory ;  but  it  must  be 
remembered  that  in  the  rabbit  binocular  vision  is  limited  to  very  small 
portions  of  the  retinae  on  their  temporal  sides,  and  therefore  the  uncrossed 
fibres  which  belong  to  half  only  of  the  stereoscopic  portion  of  each  retina 
are  but  a  small  fraction  of  the  total  number  of  optic  fibres.  Stereoscopy 
varies  greatly  in  development  in  different  breeds  of  dog.  It  is  not  sur- 
prising, therefore,  to  find  that  observers  differ  widely  in  their  estimate  of  the 
size  of  the  uncrossed  bundle  in  this  animal.  In  man  stereoscopic  vision 
reaches  a  high  state  of  development,  although,  owing  to  the  prominence  of 
the  bridge  of  the  nose,  it  probably  does  not  take  such  complete  possession 
of  the  retina  as  it  does  in  the  monkey,  and  we  have  now  a  numerous  series 
of  pathological  observations  which  show  that  in  man  the  direct  bundle  is 
of  considerable  size,  and  that  it  occupies  the  external  part  of  the  optic  nerve 
and  chiasm. 

Most  of  the  cases  which  have  been  recorded  of  disturbance  of  vision 
due  to  cerebral  disease  were  complicated  by  the  coexistence  of  cortical  lesion, 
but  certain  cases  in  which  lesion  was  limited  to  the  optic  nerves  and  tracts 
have  been  recorded.  One  such  case  in  which  disease  affected  the  optic 
nerve  of  one  side  and  the  optic  tract  of  the  opposite  side  would  appear  to 
be  absolutely  conclusive,  since  the  patient  retimed  his  sight  only  for  the 
nasal  half  of  the  eye  with  the  sound  optic  nerve,  the  limit  of  the  field  of 
vision  passing  accurately  through  the  fixation  point.1 

The  view  that  the  optic  nerve  is  divided  in  the  chiasm  into  crossed  and 
uncrossed  portions  is,  moreover,  supported  by  Von  Gudden's  and  Ganser's 
observations  of  the  degeneration  which  follows  enucleation  of  the  eyeball  in 
the  adult  animal,  as  well  as  by  the  observations  of  the  former  with  regard 
to  the  arrest  of  development  which  results  when  the  eyeball  is  removed  at 
birth. 

It  does  not  necessarily  follow,  however,  that  the  object  of  this  partial 
crossing  is  to  render  stereoscopic  vision  possible  by  the  "  superposition  of 
images  in  the  brain"  after  the  manner  we  have  described  at  some  length. 
Indeed,  certain  clinicians,  for  the  sake  of  explaining  cases  in  which  disease 
of  one  occipital  lobe  has  seemed  to  produce  crossed  amblyopia,  have  pro- 
pounded schemes  showing  the  direct  bundle  as  crossing  to  the  opposite 
hemisphere  in  the  corpora  quadrigemina ;  they  regard  the  right  optic  nerve 
as  connected  solely  with  the  left  hemisphere  of  the  brain,  its  fibres  reaching 
this  hemisphere  in  two  groups,  one  of  which  crosses  in  the  chiasm,  the  other 
in  the  corpora  quadrigemina.  Anatomy  is,  on  the  whole,  opposed  to  such 

^ettleship,  Transactions  of  the  Ophthalmological  Society,  October,  1883. 


394  ANATOMY   OF   THE   INTRA-CRANIAL   PORTION 

an  arrangement,  although  it  cannot  be  asserted  that  it  is  impossible.  The 
ultimate  settlement  of  this  question  must  rest  with  the  pathologists.  Look- 
ing at  the  matter,  however,  from  the  point  of  view  of  a  comparative  anato- 
mist, we  think  that  we  may  urge  very  strongly  that  the  evidence  before  us 
supports  the  simple  and^  practical  view  that  the  primitive  crossed  connection 
of  the  optic  nerves  with  the  brain  has  been  disturbed  by  the  diversion  of 
certain  fibres  to  the  same  side  of  the  brain  in  number  proportional  to  the 
area  of  retina  used  in  binocular  vision,  and  for  the  purpose  of  facilitating 
stereoscopy. 

ORIGIN   OP   THE   OPTIC   NERVES. 

The  anatomy  of  a  large  nerve  is  the  anatomy  of  the  separate  fibres 
of  which  it  is  composed.  It  is  now  almost  universally  allowed  that  the 
axis-cylinder  of  a  nerve-fibre  is  the  process  of  a  nerve-cell.  To  the  cell 
from  which  it  grew  it  looks  for  its  nutrition.  Cell  and  fibre  must  there- 
fore be  regarded  as  a  nerve-element  or  "  neuron."  The  cell  gives  rise  to  a 
number  of  branching  processes  in  addition  to  its  unbranched  nerve-fibre, 
but  whether  these  "  protoplasmic"  processes  are  to  be  regarded  as  nervous, 
in  the  sense  that  they  distribute  nerve-currents,  or  whether  they  are  simply 
the  roots  by  which  the  cell  sucks  up  its  nutriment,  as  Golgi  and  his  follow- 
ers believe,  is  a  question  under  discussion.  At  its  distal  end  the  nerve-fibre 
breaks  up  into  a  brush,  the  twigs  of  which  are  very  generally,  if  not  inva- 
riably, connected  with  minute  bipolar  cells  or  "granules,"  but  whether 
these  granules  are  to  be  regarded  as  belonging  to  the  neuron  or  as  separate 
elements  is  at  present  uncertain.  Certain  it  is,  however,  that  the  nervous 
system  is  made  up  of  innumerable  neurons.  The  cell  of  the  neuron  may 
lie  within  the  central  nervous  system,  in  a  spinal  ganglion,  or,  as  in  the 
case  of  the  retina,  at  the  periphery,  in  immediate  contact  with  the  sensory 
epithelium.  Each  bipolar  cell  of  a  spinal  ganglion  sends  one  process  into 
the  gray  matter  of  the  spinal  cord  and  another  to  the  periphery.  Each 
multipolar  cell  of  the  anterior  horn  sends  a  process  to  a  voluntary  muscle, 
which  divides  into  several  branches,  each  branch  ending  on  a  muscle-fibre 
in  a  brush  of  granule-bearing  twigs.  The  other  neurons  of  the  central 
nervous  system  are  placed  with  their  cells  in  the  cerebro-spinal  axis,  their 
processes  branching  in  the  cortex  of  the  cerebellum  or  cerebrum,  their  cells 
in  the  cortex  and  their  processes  ending  in  the  axis,  or  with  both  cells  and 
processes  confined  to  the  one  or  the  other  of  these  fields. 

Are  the  fibres  of  the  optic  nerve  and  tract  processes  of  cells  which  lie 
in  the  brain  as  well  as  of  cells  which  lie  in  the  retina?  While  inclined  to 
answer  this  question  in  the  affirmative,  we  need,  as  already  hinted,  additional 
evidence. 

In  what  parts  of  the  brain  do  the  processes  of  the  retinal  cells  branch  ? 
The  cell-processes  may  be  divided  into  two  groups, — (A)  the  fibres  con- 
nected with  the  fore-brain  and  (B)  the  fibres  connected  with  the  mid-brain. 
It  may  be  that  all  the  processes  of  the  retinal  cells  end  in  the  fore-brain, 
while  the  fibres  connected  with  the  mid-brain  are  the  processes  of  cells 


OP   THE   VISUAL   APPARATUS.  395 

which  lie  in  that  region  (as  Von  Monakow  believes)  and  have  their  ter- 
minal arborization  in  the  retina.  Von  Monakow  thinks  that  the  mid-brain 
fibres  are  distinguished  from  the  fore-brain  fibres  by  their  smaller  size. 
On  the  other  hand,  fibres  have  been  observed,  it  is  said,  to  arise  in  the  cells 
of  the  external  geniculate  body  and  to  take  their  course  into  the  optic  tract,1 
a  mode  of  connection  which  would  certainly  lead  us  to  look  for  their  termi- 
nal arborizations  in  the  retina ;  whereas  the  direct  origin  of  fibres  in  the 
mid-brain  has  not,  as  yet,  been  seen.  Certain  anatomists  consider  the  dis- 
tinction into  large  fibres,  afferent  to  the  fore-brain  from  retinal  cells,  and 
small  fibres,  efferent  from  the  mid-brain  to  the  retina,  as  definitely  proved ; 
others  describe  and  figure  afferent  and  efferent  fibres  as  connecting  both 
fore-brain  and  mid-brain  with  the  retina.  Further  research  is,  however, 
greatly  needed,  and  it  is  well,  at  present,  to  suspend  judgment  on  this 
fundamental  point. 

Let  us  consider  Group  A  first.  They  may  be  divided  into  three,  per- 
haps four,  classes, — (a)  the  fibres  which  enter  the  external  geniculate  body, 
(6)  the  fibres  which  end  in  the  pulvinar,  (c)  the  fibres  which  pass  on  to  the 
thalaraus,  and  (d)  the  fibres,  if  any,  which  take  up  their  position  in  the 
back  of  the  internal  capsule  and  continue  their  course,  without  cell-inter- 
ruption, to  the  cortex  of  the  brain. 

Unfortunately,  our  information  with  regard  to  the  exact  mode  of  ter- 
mination of  all  these  groups  of  fibres  is  almost  nil. 

(a)  The  Corpus  Geniculatum  Laterale.' — This  is  a  dense  mass  of  gray 
matter  of  very  irregular  form,  indistinctly  split  into  strata  by  the  plates  of 
optic  fibres  which  traverse  it.  From  its  anterior  surface  tongues  of  gray 
matter  project  into  the  substance  of  the  thalamus,  from  which  they  are 
conspicuously  distinguished  by  their  denser  ground-substance.  The  cells 
of  the  external  geniculate  body  are  variable  in  size,  but  smaller  than  those 
of  the  thalamus  (10-20  /*,  Henle),  more  angular,  and  formed  of  a  denser 
protoplasm.  The  external  geniculate  body  shrinks  both  when  the  eyeball 
is  destroyed  and  when  the  occipital  lobe  is  removed  or  diseased,  but  how 
far  this  shrinking  is  due  to  atrophy  of  the  plates  of  optic  fibres  which 
traverse  the  body  and  how  far  it  is  really  due  to  an  alteration  in  the  essen- 
tial part  of  the  geniculate  body — its  gray  nucleus — is  a  matter  whicli  needs 
further  investigation. 

It  is  asserted  by  Von  Monakow2  that  the  ground-substance  of  this 
body  tends  to  disappear  after  removal  of  the  eyeball,  while  its  cells  are 
not  affected.  If  this  be  true,  the  geniculate  body  receives  the  ascending 
processes  of  retinal  cells,  and  does  not  give  origin  to  the  descending  fibres 
of  the  optic  nerve  as  Bernheimer  supposes. 

(6),  (c).  The  Optic  Thalamus. — No  adequate  description  of  the  anatomy 
of  this  great  mass  of  gray  matter  has  yet  been  written.  It  is  easy  to  point 

1  Bernheimer,  Sehnerven-wurzeln  des  Menschen,  Wiesbaden,  1891. 
*  Archiv  fur  Psychiatric,  vol.  xx.  p.  780. 


396  ANATOMY   OF   THE    INTRA-CRANIAL   PORTION 

out  that  its  cells  are  large  (about  40  //),  soft,  very  liable  to  break  down 
and  disappear  in  specimens  of  tissue  hardened  in  the  ordinary  way,  and 
loaded  with  yellow  pigment ;  its  ground-substance  is  somewhat  loose,  light 
in  color,  and  traversed  by  innumerable  minute  nerve-fibres ;  but  we  know 
very  little  as  to  the  connections  of  the  cells  and  fibres. 

The  thalamus  is  covered  by  a  sheet  of  white  fibres,  its  stratum  zonale, 
about  three-fourths  of  a  millimetre  thick.  In  part,  at  any  rate,  the  stratum 
zonale  consists  of  fibres  of  the  optic  tract  which  pass  over  the  external 
geniculate  body,  but  it  also  receives  fibres  from  the  pedunculus  conarii  as 
well  as  from  the  sagittal  medullary  strata  of  the  cerebral  hemisphere.  It 
is  impossible  to  unravel  this  tangle.  Nor  can  we  even  say  how  much  of 
the  stratum  zonale  or  of  the  thalamus  itself  belongs  to  the  cerebral  mech- 
anism of  vision  in  the  mole.  The  thalamus  is  still  of  considerable  size, 
although  smaller  than  in  animals  which  see.  In  whales  and  other  aquatic 
mammals  which  are  destitute  of  the  sense  of  smell  it  is  short  but  broad  and 
large.  While,  therefore,  it  probably  contains  the  primary  centres  of  the 
first  and  second  nerves, — the  structure  of  the  centres  being  modified  by  the 
presence  in  the  retina  and  olfactory  bulb  of  much  gray  matter,  included  in 
the  case  of  other  peripheral  nerves  within  the  cerebro-spinal  axis, — the 
mass  of  the  thalamus  may  have  functions  unconnected  with  either  sight  or 
smell. 

The  pulvinar  is  the  only  part  of  the  thalamus  which  has  been  shown 
to  atrophy  as  the  consequence  of  destruction  either  of  the  optic  nerve  or 
of  the  occipital  cortex.  Whatever,  therefore,  may  be  the  function  of  the 
remainder  of  the  thalamus,  we  are  justified  in  regarding  the  pulvinar,  as  well 
as  the  external  geniculate  body,  as  a  primary  centre  of  the  optic  nerve, 
and  we  are  probably  right  in  believing  that  the  fibres  of  the  tract  which 
are  distributed  to  these  nuclei  end  by  branching  in  their  ground-substance, 
while  fibres  for  the  cerebral  cortex  take  origin  in  their  cells. 

The  external  geniculate  body  and  the  pulvinar  contain  the  end-brushes 
of  the  retino-thalamic  neurons,  the  cells  of  the  thalamo-cortical  neurons. 
Von  Monakow '  has  ascertained  that  when  the  occipital  cortex  is  destroyed 
in  new-born  animals,  the  cells  of  the  external  geniculate  body  and  the  cells 
and  ground-substance  of  the  pulvinar  atrophy  instead  of  developing  as  the 
animal  grows.  Certain  qualifications  as  to  the  parts  of  these  gray  masses 
which  atrophy,  and  as  to  the  amount  of  the  atrophy  in  different  animals, 
have  to  be  introduced  into  this  statement,  but  the  fact  may  be  expressed  in 
general  terms  as  above,  and  the  conclusion  applied  to  the  human  brain.  It 
appears,  therefore,  that  the  large  cells  of  the  external  geniculate  body  and 
of  the  pulvinar  belong  exclusively  to  thalamo-cortical  neurons ;  other  dis- 
tributing cells  must  provide  for  the  varied  sight-reflexes  which  may  occur 
after  removal  of  the  occipital  cortex. 

(d)  As  the  result  of  dissections,  Gratiolet,  in  1854,  came  to  the  conclu- 

1  Von  Monakow,  Archiv  fur  Psychiatrie,  xx.  p.  723. 


OF   THE   VISUAL   APPARATUS.  397 

siou  that  he  could  trace,  in  the  monkey,  the  direct  continuation  of  a  portion 
of  the  optic  tract  into  the  back  of  the  corona  radiata.  Von  Gudden  in 
his  earlier  researches  (1875)  traced  a  degeneration,  consecutive  upon  re- 
moval of  the  occipito-parietal  lobe,  downward  into  the  optic  tract.  The 
existence  of  this  direct  occipital  tract  is  generally  regarded  as  proved ;  it  is 
figured  in  the  schemes  of  Obersteiner l  and  Testut ; 2  but  we  confess  that 
the  evidence  upon  which  it  is  accepted  is  somewhat  unsatisfactory.  The 
method  of  dissection  when  applied  to  the  brain  is  absolutely  untrustworthy, 
and,  although  degeneration  of  the  tract  may  follow  a  lesion  in  the  occipital 
cortex,  it  is  so  difficult  to  explain  the  presence  in  the  cortex  of  the  nutritive 
centres  of  the  fibres  of  the  optic  nerves — i.e.,  their  cells  of  origin — that 
we  look  for  some  other  explanation,  and  imagine  that  the  descending  degen- 
eration is  due  to  vascular  or  trophic  disturbance. 

The  evidence  before  us  seems  to  show  that  most  of  the  fibres  of  the 
optic  tract  which  are  distributed  to  the  fore-brain  end  by  branching  within 
the  external  geniculate  body  and  the  pulvinar,  or  reach  other  parts  of  the 
thalamus,  particularly  its  superficial  strata,  by  the  stratum  zonale.  A 
direct  connection  with  the  cortex  cerebri  is  possible,  but  the  observation  of 
a  degeneration  descending  into  the  optic  tract  does  not  seem  to  throw  light 
upon  the  question,  and  we  are  not  aware  that  any  one  has,  as  yet,  traced  a 
degeneration  ascending  directly  from  the  optic  nerve  into  the  corona  radiata, 
although  the  degeneration  of  the  posterior  fibres  of  the  corona  radiata 
after  euucleation  of  the  eyeball  was  observed  by  Panizza 3  so  long  ago  as 
1856  as  a  part  of,  and  presumably  consequent  upon,  the  extensive  degen- 
eration of  the  external  geniculate  body,  pulvinar,  and  anterior  quadri- 
geminal  body  which  follows  this  operation.  Even  the  degeneration  of  the 
corona  radiata  or  the  arrest  of  its  growth  after  enucleation  of  the  eyeball 
has  not,  however,  been  seen  by  all  the  observers  who  have  performed  this 
experiment.  It  was  not  observed  by  Von  Gudden,  Fiirster,  Ganser,  or 
Von  Monakow. 

(B)  The  Connection  of  the  Optic  Tract  with  the  Mid-Brain. — The  corpora 
bigemina  of  lower  vertebrates  are  the  chief  end-stations  of  the  optic  nerves. 
In  mammals  a  second  pair  of  swellings  make  their  appearance  behind  the 
bigeminal  bodies,  and  the  four  swellings  together  are  known  as  corpora 
quadrigemina. 

As  the  hemispheres  of  the  great  brain  increase  in  size  the  number  of 
optic  fibres  going  to  them  increases  pan  passu.  Nevertheless  in  all  mam- 
mals, man  included,  a  large  number,  probably  the  majority,  of  the  fibres 
still  pass  to  the  mid-brain. 

Fibres  from  both  the  mesial  and  the  lateral  roots  of  the  tract  pass  to  the 
anterior  tubercles  of  the  corpora  quadrigemina.  The  fibres  of  the  lateral 
root  sweep  over  the  external  geniculate  body  and  turn  backward  along  the 

1  Obersteiner  and  Hill,  Anatomy  of  Central  Nervous  Organs,  p.  280. 

2  Traite  d'Anatomie  humaine,  2d  ed.,  vol.  ii.  p.  621. 

3  Panizza,  quoted  by  Tartuferi. 


398  ANATOMY   OF   THE    INTRA-CRANIAL   PORTION 

anterior  brachium  of  the  corpus  quadrigeminum.  In  like  manner  a  certain 
number  of  the  fibres  of  the  mesial  root  pass  over  the  mesial  geniculate  body 
without  interruption  in  its  cells  and  reach  the  anterior  tubercle,  while  other 
fibres  reach  the  same  destination  by  passing  between  the  two  geniculate 
bodies.  These  last  are  described  by  J.  Stilling  as  constituting  a  "  middle 
root,"  but  it  is  hardly  possible  to  regard  either  of  these  several  bundles  as 
separate  tracts.  In  any  animal  in  which,  as  in  the  ox  (Fig.  6,  Plate),  the 
fibre-tracts  are  large,  relatively  to  the  brain,  the  optic  tract  spreads  into  a 
flat  ribbon,  the  anterior  portion  of  which  reaches  the  external  geniculate 
body  and  pulvinar,  while  the  posterior  portion  goes  to  the  anterior  quadri- 
geminal  tubercle. 

Still  a  third  set  of  fibres  has  been  described  by  Stilling,1  but  their  exist- 
ence is  so  doubtful  that  we  have  not  thought  it  worth  while  to  place  them 
in  a  separate  class.  It  is  supposed  that  a  portion  of  the  fibres  which  pass 
from  the  optic  tract  into  the  crus  cerebri  (of  which  the  majority  join  the 
sagittal  medullary  strata  as  the  direct  occipital  root)  turn  downward  in  the 
crus  as  a  radix  descendens,  which  may  be  traced,  it  is  said,  as  far  as  the 
decussation  of  the  pyramids.  Darkschewitsch 2  says,  however,  that  the 
fibres  of  this  tract  which  Stilling  regarded  as  a  descending  root  of  the  optic 
nerve  acquire  their  myelin-sheaths  before  the  optic  fibres,  of  which,  therefore, 
they  cannot  form  a  part. 

Anterior  Tubercles  of  the  Corpora  Quadrigemina. — Tartuferi 3  described 
these  bodies  as  presenting  a  series  of  strata  of  gray  and  white  matter, — viz., 

(1)  a  superficial  layer  of  very  fine  white  fibres  derived  from  the  optic  tract ; 

(2)  a  thin  sheet  of  gray  matter  containing  a  small  proportion  of  minute 
cells ;  (3)  a  mass  of  mixed  gray  and  white  matter  containing  small  cells 
and  numerous  fine  fibres  which  run  sagittally  ;  (4)  a  second  layer  of  mixed 
gray  and  white  matter  the  deeper  fibres  of  which  arch  over  the  aquseductus 
Sylvii.     Ganser,4  who  investigated  the  structure  of  these  bodies  with  a  view 
to  determining  the  seat  of  the  arrest  of  development  in  the  mole,  still  further 
divides  the  third  layer  into  three, — viz.,  (a)  its  superficial  layer,  which  con- 
tains numerous  fibres ;  (6)  its  middle  layer,  or  gray  nucleus ;  and  (c)  its 
deeper  layer,  consisting  of  fibres  chiefly. 

We  cannot  think  that  the  attempt  to  divide  the  extremely  obscure  tissue 
of  the  mammalian  corpora  quadrigemina  into  distinct  strata  is  justifiable, 
except  in  so  far  as  the  stratification  is  dependent  upon  the  general  direction 
of  the  fibres  which  enter  these  bodies.  The  fibres  of  the  optic  tract  spread 
inward  and  obliquely  backward  over  their  surface.  The  fibres  which  leave 
their  gray  nuclei  on  the  deep  aspect  tend  to  take  a  ventral  course,  but  are 

1  Bau  der  optischen  Centralorgane,  Kassel,  1882. 

*  Die  sogenannten  primaren  Opticuscentren,  Archiv  fur  Anat.  und  Physiol.,  Anat. 
Abth.,  1886.  See  also  other  papers  by  the  same  author. 

3  Anatomia  minuta  dell'  eminenze  trigemine  ant.,  Archivio  Italiano  per  le  mal.  nerv., 
1885. 

4  Gehirn  des  Maulwurfes,  Morphol.  Jahrbuch,  vii. 


OF   THE   VISUAL   APPARATUS.  399 

distinguished  with  difficulty  from  the  arching  fibres  of  the  fillet.  The 
fibres  which  connect  them  with  the  cortex  cerebri  appear  to  enter  them 
chiefly  on  their  mesial  side  between  the  optic  fibres  and  the  deep  fibres, 
which  may  be  supposed  to  connect  them  with  the  eye-muscle  nerves ;  the 
cerebral  fibres  take  a  more  directly  antero-posterior  course  than  either  of 
the  other  sets. 

The  gray  matter  of  the  corpora  quadrigemina  consists  of  a  somewhat 
open  ground-substance  in  which  are  scattered  very  minute  (15-25  A)  nerve- 
cells,  of  pyramidal  or  triangular  shape,  with  here  and  there  a  cell  of  large 
size.  It  is  distinctly  separated  from  the  gray  matter  which  surrounds  the 
aqueduct  by  the  complex  of  fibres  which  curve  upward  from  the  tegmental 
region,  and  in  great  part,  at  any  rate,  decussate  in  the  dorsal  region  of  the 
mid-brain.  In  this  book  it  would  be  out  of  place  to  describe  this  mass  of 
fibres,  which  is  chiefly  connected  with  the  fillet  behind  and  with  the  pos- 
terior commissure  in  front,  and  contains,  besides,  fibres  proper  to  the  mid- 
brain.  The  arching  of  the  fibres  in  this  region  appears  to  be  necessary  for 
their  return  to  the  dorsal  situation  which  they  occupied  in  the  cord,  and  from 
which  they  were  displaced  when  the  spinal  canal  opened  out  into  the  fourth 
ventricle  of  the  medulla  oblongata.  We  have  every  reason  for  thinking 
that  the  central  gray  matter  and  its  investing  columns  of  nerve-fibres  are 
homologous  with  the  gray  matter  and  white  columns  of  the  cord,  while 
the  gray  masses  of  the  corpora  quadrigemina  are  of  the  nature  of  a  cortex- 
formation. 

In  lower  animals  the  elements  of  which  the  corpora  bigemina  or  optic 
lobes  are  formed  resemble  those  found  in  the  cortex  cerebri  and  present  a 
somewhat  similar  stratification.  The  surface  is  covered  by  a  thick  coat  of 
optic  fibres,  to  which  succeeds  a  layer  of  dense  gray  matter  containing  very 
small  cells ;  then  follow  regular  strata  of  granules,  beneath  which  again  are 
seen  pyramids  increasing  in  size  the  farther  they  lie  from  the  surface,  and 
supported  on  the  medullary  layer. 

The  most  noticeable  feature  of  the  anterior  tubercles  of  the  corpora 
quadrigemina  in  man  and  other  mammals  is  the  immense  richness  of  the 
deeper  portion  of  their  gray  matter  in  nerve-fibres,  which  cross  one  another 
in  all  directions  and  are  easily  distinguished  from  the  fibres  of  the  medullary 
substance  by  their  great  tenuity.  We  are  inclined  to  think  that  the  anato- 
mist will  most  accurately  picture  to  himself  the  structure  of  these  bodies  if 
he  forgets  the  descriptions  which  have  been  given  of  their  stratification,  and 
remembers  only  that  they  are  covered  on  the  surface  by  nerve-fibres,  and 
that  the  gray  matter  nearest  to  the  surface  is  almost  free  from  tangential 
fibres,  while  such  fibres  are  extraordinarily  abundant  in  its  deeper  portions. 

The  Nuclei  of  the  Eye-Muscle  Nerves.— The  posterior  of  the  nerves  which 
supply  the  muscles  of  the  eyeball,  the  sixth,  or  nervus  abducens,  pierces 
the  cerebro-&.pinal  axis  in  the  groove  between  the  pens  Varolii  and  the  ante- 
rior pyramid  of  the  medulla  oblongata.  It  reaches  this  spot  as  separate, 
slightly  curving  bundles  of  fibres,  which  take,  except  for  this  curvature,  a 


400  ANATOMY    OF   THE   INTRA-CRANIAL,   PORTION 

direct  course  from  the  globular  nucleus,  which  they  leave  on  its  dorsal  and 
mesial  aspects.  The  nucleus  lies  beneath  the  floor  of  the  anterior  portion 
of  the  fourth  ventricle,  and  is  visible  from  the  surface,  in  the  brain  of  a 
young  child,  as  a  rounded  swelling  in  the  course  of  the  funiculus  teres.  Its 
cells  are  large  and  angular. 

The  fourth  or  trochlear  nerve  is  the  only  nerve  which  joins  the  axis  of 
the  brain  on  its  dorsal  side.  Its  slender  cord  curves  round  the  crus  cerebri 
in  the  gap  between  the  cerebrum  and  the  cerebellum  to  reach  the  valve  of 
Vieussens,  which  stretches  from  the  posterior  tubercles  of  the  corpora 
quadrigemina  over  the  front  of  the  fourth  ventricle.  On  the  valve  of 
Vieussens  this  nerve  decussates  with  its  fellow  of  the  opposite  side. 

The  third  or  oculo-motor  nerve  takes  its  exit  from  the  brain  by  large 
bundles  of  coarse  fibres  which  pierce  the  crus  along  the  inner  edge  of  the 
curious  plate  of  deeply  pigmented  cells,  the  substantia  nigra,  which  divides 
the  crus  into  tegment  and  crusta.  Most  of  the  fibres  come  from  the  gray 
matter  on  the  same  side  of  the  brain,  but  some  cross  to  the  opposite  side 
before  taking  exit. 

The  whole  of  the  gray  matter  which  surrounds  the  aqueduct  of  Sylvius 
and  bounds  the  back  of  the  third  ventricle  is  given  up  to  the  nuclei  of  the 
third  and  fourth  nerves,  with  the  exception  of  a  small  column  of  singularly 
characteristic,  round,  full-bodied,  almost  processless  cells  (diameter  60-80  /*), 
usually  credited  to  the  ascending  (or  descending l  ?)  root  of  the  fifth  nerve. 

This  latter  column  lies  on  the  same  level  as  the  aqueduct  and  a  little  to 
its  dorsal  side.  It  is  a  very  striking  object  in  sections  through  the  posterior 
tubercles  of  the  corpora  quadrigemina,  not  only  owing  to  its  crescentic 
shape  and  to  the  remarkable  form  of  the  cells,  which  Deiters 2  described  as 
unlike  that  of  any  other  cells  found  in  the  cerebro-spinal  axis,  but  also  on 
account  of  their  grouping.  Towards  the  front  of  the  column  they  are 
placed  so  close  together  as  almost  to  touch  one  another.  Deiters  compared 
them  to  cells  of  a  root-ganglion,  such  as  the  Gasserian,  but  Golgi,3  who  says 
that  they  are  "  absolutely  unipolar,"  believes  them  to  be  cells  of  origin  of 
the  fourth  nerve.  To  us  this  nucleus  appeal's  to  be  allied  with  Clarke's 
column  and  the  medullary  vagus-nucleus.  If  its  cells  give  origin  to  ordi- 
nary motor  fibres  of  the  fifth  or  fourth,  they  differ  in  a  singular  way  from 
those  of  other  motor  nuclei. 

The  nuclei  of  the  fourth  and  third  nerves  occupy  the  ventral  portion 

1  The  cells  are  described  by  Ferrier  (Brain,  vol.  xvii.  p.  21)  as  .undergoing  simple 
atrophy  after  section  of  the  motor  root  of  the  fifth  nerve,  to  which  he  thinks  that  they 
exclusively  belong.  This  observation,  however,  raises  the  whole  question  of  the  relation 
of  nerves  to  the  cells  in  which  they  take  origin.  Further  information  with  regard  to  the 
time  of  onset  and  nature  of  the  changes  which  occur  in  a  cell  after  severance  from  its 
axis-cylinder  process  is  sorely  needed,  and  we  do  not  feel  disposed,  until  statistical  evidence 
is  forthcoming,  to  attach  much  importance  to  appearances  suggestive  of  the  "  atrophy  of 
disuse." 

8  Untersuchungen  iiber  Gehirn  und  Kiickenmark,  Brunswick,  1865,  pp   91,  92. 

'Archives  Italiennes  de  Biologie,  xix.,  iii.  p.  454,  August,  1893. 


OF   THE    VISUAL,   APPARATUS.  401 

of  the  gray  matter,  the  fourth  being  posterior  and  less  well  defined  than 
the  third ;  for,  while  the  cells  of  the  nuclei  of  the  third  are  collected  into 
compact  groups,  those  which  are  generally  regarded  as  giving  origin  to  the 
fibres  of  the  fourth  are  more  loosely  scattered  throughout  the  central  gray 
matter. 

The  nucleus  of  the  fourth  nerve  lies  beneath  the  front  of  the  posterior 
quadrigeminal  body.  Its  cells  are  large  and  angular,  like  those  of  the 
nucleus  of  the  third,  which  will  shortly  be  described,  and  the  fibres  which 
arise  from  them  have,  owing  to  the  situation  of  the  nucleus  so  far  in  front 
of  their  place  of  exit,  a  somewhat  long  course  within  the  brain,  curving  at 
first  outward,  then  backward  in  close  connection  with  the  root  of  the  fifth 
nerve,  and  finally  dorsalward  and  inward  to  the  valve  of  Vieussens.  A 
round  group  of  very  minute  cells  which  immediately  succeeds  the  nucleus 
of  large  cells  is  by  Westphal  supposed  to  be  a  posterior  nucleus  of  the 
trochlear  nerve. 

The  several  groups  of  cells  which  £ogether  constitute  the  nucleus  of  the 
third  nerve  have  been  mapped  out  with  great  precision.  They  occupy  a 
very  considerable  portion  of  the  central  gray  matter,  and  are  also  placed 
outside  this  gray  matter  among  the  fibres  of  the  tegmental  region,  appear- 
ing as  the  most  conspicuous  objects  in  any  section  through  the  mid-brain 
from  the  level  of  the  groove  between  the  posterior  and  anterior  quadri- 
geminal bodies  for  a  distance  of  from  seven  to  ten  millimetres  farther  for- 
ward,— a  distance  which  carries  us  into  the  back  of  the  third  ventricle. 
The  largest  of  the  cells  are  as  large  as  any  of  their  homologues  in  the  ante- 
rior horn  of  the  spinal  cord  (about  100  M),  and,  owing  to  the  quantity  of 
fibres  which  break  up  the  ground -substance  surrounding  them,  they  arrest 
attention  even  more  forcibly. 

The  clumps  of  nerve-cells  may  be  divided  into  a  chief,  or  posterior,  and 
a  smaller,  anterior  group  ;  or,  more  naturally,  as  we  think,  into  (1)  the  large- 
celled  clumps  (cells  of  about  100  /z  in  diameter),  which  probably  supply  the 
extrinsic  muscles  of  the  eyeball  and  lie  in  the  ventral  and  lateral  part  of  the 
tegment,  and  (2)  the  clumps  of  smaller  cells  (about  50-60  M),  which  give 
rise  to  fibres  for  the  intrinsic  muscles  of  the  eyeball  and  lie  on  the  dorsal  and 
mesial  side  of  the  large  cells,  extending  also  farther  forward,  towards  the 
third  ventricle. 

1.  The  large  cells  are  grouped  in  the  following  nuclei,  from  behind  for- 
ward, on  either  side : 

(a)  The  posterior  ventral  nucleus. 

(6)  The  anterior  ventral  nucleus. 

These  two  nuclei  lie  one  behind  the  other  in  the  same  sagittal  line,  resting 
on  the  dorsal  surface  of  the  mesial  portion  of  the  posterior  longitudinal 
fasciculus. 

(c)  The  posterior  dorsal  nucleus. 

(d)  The  anterior  dorsal  nucleus. 

The  isolation  of  these  four  nuclei  is  hardly  sufficiently  distinct  to  justify 
VOL  i  _26 


402  ANATOMY   OF   THE    INTRA-CRANIAL   PORTION 

their  description  as  separate ;  they  might  be  considered  as  forming  together 
the  "  lateral  nucleus."  (Obersteiner.) 

(e)  In  the  middle  line  lies  an  unpaired  almond-shaped  "central" 
nucleus. 

2.  Small  cells,  (a)  A  large  roundish  nucleus  of  smaller  cells  lies  to 
the  dorsal  side  of  the  large-celled  clumps  and  nearer  the  middle  line  than 
any  of  the  latter,  save  the  central  nucleus.  It  is  the  posterior  of  the  small - 
celled  nuclei,  or  nucleus  of  Edinger  and  Westphal. 

(6)  The  mesial  anterior  small-celled  nucleus  is  found  at  the  level  of 
the  posterior  commissure  and  lies  close  against  the  raphe",  by  which  alone  it 
is  separated  from  its  fellow  of  the  opposite  side. 

(c)  The  lateral  anterior  small-celled  nucleus  lies  on  a  level  with  the 
aqueduct  of  Sylvius  in  the  back  of  the  third  ventricle. 

The  exact  correspondence  of  these  nuclei  with  the  physiological  centres 
for  the  muscles  of  the  eye  as  mapped  out  by  Hensen  and  Volckers  has 
not  been  proved,  but  the  anatomical  position  of  the  nuclei  coincides  so 
nearly  with  the  situation  of  these  centres  as  determined  by  experiment  that 
we  feel  justified  in  attaching  some  importance  to  the  coincidence,  and  in 
concluding  that  we  know  within  a  very  little  the  muscles  to  which  their 
fibres  are  distributed. 

In  searching  out  any  object  in  the  field  of  vision  and  bringing  the  eyes 
to  a  focus,  it  appears  that  an  animal  calls  into  action  the  several  members 
of  a  series  of  centres  which  occupy  the  hind-  and  mid-brain,  beginning 
first  with  those  which  are  situate  farthest  back.  By  the  centres  for  the 
muscles  of  the  head  and  neck  the  head  is  placed  in  the  right  position.  Next 
the  external  recti  muscles  are  set  in  action  by  impulses  which  proceed  from 
their  centres  in  the  front  of  the  floor  of  the  fourth  ventricle,  and  the  eyes, 
which  were  probably  at  the  time  converged  upon  the  sward  on  which  the 
animal  was  feeding,  are  immediately  rendered  parallel  to  each  other.  Then 
the  several  extrinsic  muscles  raise  the  lids  and  the  eyeballs  and  direct  the 
gaze  rapidly  over  the  field  until  the  suspected  object  is  in  view.  The  in- 
ternal recti  then  cause  them  to  converge  to  the  appropriate  degree.  Only 
now,  when  the  extrinsic  muscles  have  done  their  work,  do  the  nuclei  of 
the  nerves  to  the  intrinsic  muscles  play  their  part,  and  of  these  the  centre 
for  the  pupil  is  the  first  to  act.  Finally,  and  only  after  all  these  mechan- 
isms are  adjusted, — the  eyes  brought  to  bear  upon  the  object  and  converged 
at  the  right  angle  for  the  distance,  and  the  pupil  properly  regulated  so  that 
it  admits  no  more  light  than  may  act  upon  the  retina  without  risk  of  in- 
juring its  tissue, — is  the  light  focussed  upon  this  sensitive  screen. 

Adamiik  succeeded  in  evoking  movements  of  the  eyeballs  by  electric 
stimulation  of  the  cortex  of  the  corpora  quadrigemina.  Hensen  and 
Volckers,  applying  their  electrodes  to  different  spots  in  the  gray  matter 
which  surrounds  the  aqueduct  of  Sylvius  and  spreads  out  at  the  back  of 
the  third  ventricle,  called  forth  movements  in  the  sequence  which  we  have 
just  described  as  the  natural  sequence  of  events  preceding  the  direct  vision 


OF   THE   VISUAL   APPARATUS.  403 

of  an  object  to  which  our  attention  has  been  called  by  indirect  vision  or  by 
hearing.  Such  pathological  observations  as  have  been  made  as  yet  confirm 
this  location  of  centres,  although  not  unequivocally.  Kahler  and  Pick 
observed  that  when  the  levator  palpebrse,  rectus  superior,  and  obliquus 
inferior  muscles,  which  act  together  in  raising  the  eyes,  are  paralyzed,  the 
posterior  and  lateral  bundles  of  the  oculo-motor  nerve  are  found  to  have 
undergone  degeneration.  Starr,1  by  analyzing  twenty  cases  of  partial  oculo- 
motor paralysis,  was  able  to  make  a  map  of  the  centres  for  the  several 
muscles  which  is  not  improbably  correct.  Many  more  cases,  however, 
accurately  observed,  are  needed  before  the  several  groups  of  cells  can  be 
allocated  to  the  muscles  which  they  innervate  respectively,  with  any  cer- 
tainty whether  the  reflection  of  optic  impulses,  the  actual  junctions  between 
aiferent  and  efferent  fibres,  the  transference  by  "  physiological  contact"  of 
impulses  from  the  retina  into  motor  channels,  occur  in  the  immediate 
vicinity  of  these  cells  or  at  some  higher  level. 

The  sum  of  such  physiological  and  pathological  evidence  as  we  possess 
at  present  points  to  the  conclusion  that  the  mesial  small-celled  nucleus  (2,  6) 
supplies  the  ciliary  muscle,  and  the  lateral  small-celled  nucleus  (2,  c)  the 
sphincter  iridis ;  while  we  know  nothing  as  to  the  functions  of  the  nucleus 
of  Edinger  and  Westphal.  It  is  possible  that  the  nucleus  last  named  has 
nothing  to  do  with  ocular  movements.  The  fibres  for  the  rectus  internus 
probably  originate  in  the  cells  of  the  anterior  ventral  large-celled  nucleus 
(1,  6),  but  this  is  much  disputed.  For  the  purposes  of  conjugate  deviation 
a  connection  seems  to  be  required  between  the  nucleus  for  the  external 
rectus  of  one  side  and  the  nucleus  for  the  internal  rectus  of  the  other. 
Such  a  connection  has  been  described,  but  a  study  of  the  literature  of  the 
subject  seems  to  suggest  that  neurologists  have  been  occupied  in  designing 
a  mechanism  capable  of  doing  the  work  which  we  know  to  be  required  of 
the  mid-brain,  rather  than  in  unravelling  the  structure  of  this  region  as  it 
is  found  to  exist. 

The  Connection  of  Optic  Fibres  with  the  Nuclei  of  the  Eye-Muscle  Nerves. — 
The  mid-brain  contains  the  mechanism  for  the  reflection  to  muscles  appro- 
priate for  the  execution  of  instinctive  movements  of  impulses  which  travel 
up  the  optic  nerve,  and  especially  to  the  muscles  of  the  eyeballs.  In  lower 
animals  these  reflex  actions  are  probably  carried  out  solely  by  the  mid-brain, 
while  in  mammals  much  of  the  work  hitherto  done  by  the  mid-brain  is 
transferred  to,  or  strictly  supervised  by,  the  great  brain.  There  does  not 
appear  to  be  much  risk  in  generalizing  as  to  the  plan  of  disposition  of  the 
elements  of  the  apparatus  in  animals  with  large  optic  lobes  ;  in  fishes  and 
birds  the  fibres  of  the  optic  nerves  are  distributed  over  the  surface  of  the 
mid-brain  ;  they  sink  down  into  the  substance  of  the  optic  lobes,  in  which 
each  breaks  up  into  a  brush  of  fibrils  which  bear  "  granules."  The  larger 
cells  of  the  deep  strata  of  the  optic  lobes  belong  to  neurons  the  axis-cylinders 

1  Journal  of  Nervous  and  Mental  Diseases,  May,  1888. 


404  ANATOMY   OF   THE    INTRA-CRANIAL    PORTION 

of  which  end  in  the  gray  matter  surrounding  the  cells  of  the  eye-muscle 
nuclei.  These  cells,  again,  belong  to  neurons  of  which  the  axis-cylinders 
constitute  peripheral  nerves.  Afferent  optic  fibres  and  efferent  motor  nerves 
are  therefore  connected  through  the  intervention  of  a  single  set  of  neurons 
of  the  optic  lobes.  While  it  is  most  probable  that  a  similar  arrangement 
obtains  in  the  higher  mammals,  the  anatomist  looks  with  astonishment  at 
the  poverty  in  cells  of  their  corpora  quadrigemina.  Optic  fibres  spread 
over  their  surface,  and  from  beneath  them  fibres  seem  to  make  for  the  nuclei 
of  the  third  and  fourth  nerves ;  but  the  gray  matter  which  intervenes  be- 
tween the  two  sets  of  fibres  and  also  gives  origin  to  the  fibres  which  con- 
nect the  corpora  quadrigemina  with  the  great  brain  appears  insufficient  to 
provide  the  connecting  neurons. 

If  what  has  been  said  as  to  the  connections  in  the  brain  of  the  optic  and 
motor-oculi  nerves  be  summed  up,  it  will  be  recognized  that  the  funda- 
mental plan  of  the  nervous  mechanism  has  yet  to  be  worked  out. 

According  to  the  older  and  simpler  view  the  optic  nerve  only  contains 
afferent  fibres.  These  are  of  two  kinds, — for  the  conveyance  of  visual  im- 
pulses, and  impulses  determining  movement  of  the  eyeball  respectively. 
Impulses  of  sight,  properly  so  called,  are  carried  to  the  cortex  cerebri  via  the 
optic  thalamus ;  impulses  of  sight-adjustment  are  carried  to  the  mid-brain 
for  reflection  to  motor  nerves.  The  cortex  cerebri  and  mid-brain  are  united 
by  ascending  and  descending  fibres.  It  is  no  longer  possible  to  believe  in 
so  simple  a  scheme,  for  it  is  necessary  to  admit  that  the  cortex  cerebri,  as 
well  as  the  thalamus  and  mid-brain,  is  the  seat  of  visual  reflexes.  Such  a 
semi-diagrammatic  separation  of  afferent  tracts  is,  therefore,  unjustifiable. 
Further,  it  is  urged  on  various  grounds  that  efferent  fibres  are  distributed 
to  the  retina.  These  are  supposed  to  come  from  the  mid-brain,  and  to  be 
recognizable  in  the  optic  nerve,  owing  to  their  small  size.  Conclusions 
have  somewhat  outstripped  evidence  in  this  matter.  It  is  desirable  that 
judgment  should  be  suspended  until  it  has  been  proved  that  the  small 
fibres  are  alone  connected  with  the  mid-brain  ;  and,  further,  until  an  intelli- 
gible explanation  has  been  given  of  the  efferent  functions  of  these  small 
fibres  which  so  vastly  preponderate  in  the  optic  nerve. 

THE   VISUAL    AREA   OF   THE   CEREBRAL   CORTEX. 

The  cortical  territory  of  optic  fibres  lies  at  the  back  of  the  hemisphere. 
That  optic  fibres  are  distributed  to  the  occipital  lobe,  including  its  mesial 
surface  (the  cuneus), — the  term  lobe  being  used  in  a  general  sense  and  not 
limited  to  the  area  so  defined  in  descriptive  anatomy, — has  been  proved 
beyond  the  possibility  of  doubt.  This  is  the  sphere  of  the  brain  of  which 
the  action  is  determined  and  controlled  by  visual  impulses.  Its  exact 
delimitation  has,  however,  hardly  been  accomplished  as  yet. 

Anatomical  Evidence. — There  is  nothing  in  the  arrangement  of  the  con- 
volutions, or  of  the  fissures  by  which  they  are  bounded,  which  clearly 
marks  out  the  frontiers  of  the  visual  sphere.  The  writer  some  ten  years 


OF   THE   VISUAL    APPARATUS. 


405 


ago  studied  all  the  brains  of  different  animals  preserved  in  tho  museums 
which  were  then  accessible  to  him,  and  particularly  the  rich  collection  in 
the  museum  of  the  Royal  College  of  Surgeons  of  England  made  by  the 
late  Sir  Richard  Owen,  in  the  hope  that  he  might  be  able  to  devise  some 
plan  of  mapping  out  the  cortex  by  means  of  its  fissures  into  areas  related 
to  the  several  nerves  or  groups  of  nerves  of  sense.  He  hoped  to  be  able  to 
establish  a  relation  in  size  between  the  cross-section  of,  or  number  of  fibres 
in,  each  of  the  cranial  sensory  nerves  and  the  area  of  the  cortex  to  which 
its  impulses  are  distributed.  Although  the  attempt  to  obtain  exact  nu- 
merical data  was  a  complete  failure,  owing  to  the  number  of  "  variables" 
involved  in  the  calculation,  the  result,  if  expressed  in  general  terms,  is 
sufficiently  striking.  It  is  easy  to  show  by  photographs  and  tracings  that 
animals  in  which  the  sense  of  smell  is  acute  have  large  temporal  lobes ; 
acute  hearing  is  accompanied  by  fulness  of  the  brain  about  the  end  of  the 
fissure  of  Sylvius ;  a  large  fifth  nerve  goes  with  great  development  of  the 
region  which  lies  below  and  behind  the  motor  area;  while  the  occipital 
region  is  strongly  developed  in  animals  which  depend  mainly  on  the  sense 
of  sight. 


FIG.  7. 


D(t. 


•racrntral 


lobud 


corpus  c 


fasci 
cfen/ala\ 


f.  of  Rolando 
intraparietal  fissure 

qf  Sylviut 
•  parallel  fissure 


Diagrammatic  representations  of  the  occipital  region  of  the  right  cerebral  hemispheres :  A,  from 
inner ;  B,  from  outer  aspect.    (One-third  natural  size.) 

Since  these  observations  were  published,  the  division  of  the  brain  into 
territories  occupied  by  the  secondary  connections  of  the  sensory  nerves  has 
been  placed  beyond  doubt.  The  nerve  of  sight  carries  its  commerce  to  the 
occipital  region,  or,  at  any  rate,  its  chief  relations  are  with  this  area,  for  it 
is  not  impossible  that  it  may  be  connected  also  in  a  less  concentrated  degree 
with  other  parts  of  the  cortex. 

Topography  of  the  Occipital  Region. — We  purposely  avoid  the  word 
lobe,  since  there  is  no  reason  to  attach  any  morphological  significance  to  the 
lines  adopted  as  the  landmarks  between  the  occipital  and  parietal  or  occip- 
ital and  temporal  lobes  of  human  anatomy.  The  occipital  region  of  the 
great  brain  is  prolonged  backward  over  the  cerebellum  as  a  three-sided 
pyramid.  The  fissures  on  its  outer  surface  (Fig.  7,  B)  are  irregular  and 
inconstant.  Those  on  its  under  surface  are  more  constant,  but  they  are 


406  ANATOMY   OF   THE    INTRA-CRANIAL    PORTION 

common  to  the  occipital  and  temporal  lobes,  and  the  extent  to  which  they 
are  prolonged  backward  is  subject  to  considerable  variation.  Only  on  its 
mesial  aspect  (Fig.  7,  A)  are  the  fissures  deep  and  regular,  but  on  this  side 
they  are  so  remarkable  as  to  suggest  the  idea  that  they  are  landmarks  of 
great  importance.  The  deep  parieto-occipital  fissure,  which  appears  also  on 
the  outer  surface  of  the  brain,  is  prolonged  forward  until  it  almost  reaches 
the  portal  margin  of  the  cortex, — i.e.,  until  it  nearly  cuts  the  gyms  forni- 
catus  in  two.  It  is  joined  by  the  calcarine  fissure,  a  "  total"  fissure  which 
causes  a  swelling  (the  hippocampus  minor)  in  the  third  ventricle.  A  well- 
defined  lobule,  the  cuneus,  is  thus  marked  off  by  the  parieto-occipital  fissure 
above  and  in  front  and  the  calcarine  fissure  below.  The  collateral  or  in- 
ferior occipito-temporal  fissure,  which  is  also  a  total  fissure  (giving  rise  to 
the  eminentia  collaterals  in  the  descending  horn  of  the  ventricle),  cuts  deeply 
into  the  occipital  and  temporal  lobes.  Between  the  calcarine  and  collateral 
fissures  lies  the  gyrus  lingualis,  or  gyms  occipito-temporalis  medialis. 
Below  the  collateral  fissure,  and  forming  the  transition  between  the  inner 
and  under  aspects  of  the  occipital  region,  comes  the  fusiform  or  inferior 
occipito-temporal  convolution,  which  in  turn  is  bounded  on  the  outer  side 
by  the  inferior  temporal  fissure.  The  outer  surface  of  the  occipital  region 
is  in  some  cases  devoid  of  convolutions.  The  small  fissures  which  groove 
it  are  never  deep,  and  the  nearest  fissure  of  importance  which  looks  as  if 
it  might  be  a  boundary  line  is  the  turned-up  end  of  the  parallel  or  superior 
temporal  fissure.  Around  this  fissure  hooks  the  angular  gyrus,  the  pos- 
terior half  of  which  appears,  therefore,  to  belong  to  the  occipital  region. 

The  value  of  the  fissures  as  boundary  lines  is  a  question  which  would 
carry  us  far  beyond  the  legitimate  scope  of  this  article.  Their  constancy, 
both  ontogenetic  and  phylogenetic,  proves  beyond  question  that  they  have 
the  highest  morphological  value.  They  are  not  accidental  furrows  pro- 
duced by  skull-pressure  or  for  the  accommodation  of  arteries,  but  they 
mark  out  parts  of  the  cortex  which  have  as  much  claim  to  be  regarded  as 
separate  organs  as  have  the  fingers  or  the  toes.  When,  however,  we  en- 
deavor to  settle  their  territorial  significance  we  find  our  judgment  divided 
between  evidence  of  different,  apparently  of  antagonistic,  bearing.  Certain 
fissures  (e.g.,  the  parieto-occipital  and  the  fissure  of  Sylvius)  seem  to  sepa- 
rate organs  of  different  function,  while  others  (e.g.,  the  fissure  of  Rolando 
and  [?]  the  calcarine  fissure)  appear  to  lie  in  the  centre  of  the  lobe  to  which 
they  belong,  to  represent,  as  it  were,  its  most  concentrated  function.  It  is 
possible  that  we  include  under  the  common  term  two  kinds  of  groove  of 
entirely  different  origin  and  meaning,  one  kind  of  fissure  being  the  valley 
which  separates  two  bulging  lobes,  the  other  the  depression  which  appears 
in  the  centre  of  a  lobe  when  sufficient  surface  for  its  cortex  cannot  be 
provided  otherwise.  Beyond  pointing  out  the  apparent  importance  as 
landmarks  of  the  parieto-occipital  and  parallel  fissures,  anatomy  can  do 
little  to  settle  the  vexed  question  of  the  boundary  of  the  optic  region  of 
the  cortex. 


OF   THE   VISUAL   APPARATUS.  407 

Pathological  Evidence. — The  observation  of  the  degenerations  which  re- 
sult from  disease  or  artificial  destruction  of  the  eyeballs,  and  conversely  the 
determination  of  the  situation  in  the  brain  of  all  the  lesions  which  have 
given  rise  to  blindness,  have  probably  contributed  in  a  larger  degree  than 
any  other  kind  of  research  to  the  localization  of  the  visual  area.  Atrophy 
consequent  upon  disease  or  destruction  of  the  eye  would  supply  the  more 
important  class  of  evidence  if  the  changes  in  the  brain  were  sufficiently 
distinct  for  recognition.  As  stated  already,  however,  certain  observers 
have  been  unable  to  detect  any  alteration  in  the  great  brain  after  enuclea- 
tion  of  the  eyeball,  even  when  the  operation  was  performed  on  new-born 
animals.  Atrophy  of  the  occipital  lobes  consequent  upon  early  disease  of 
the  eye  has  been  described,  but  the  boundaries  of  the  atrophied  area  cannot 
be  marked  out. 

A  very  large  number  of  cases  of  cortical  hemiopia  or  amblyopia  have 
been  observed.  Such  cases  have  been  described  or  collected  by  Luciani  and 
Tamburini,1  Nothnagel,2  Angelucci,3  Bellouard,4  Mauthner,5  Exner,6  Wil- 
brand,7  Haab,8  Starr,9  Seppilli,10  Philipsen,11  Seguin,12  Bouveret,13  Chauffard,14 
Dejerine,15  Von  Monakow,16  Henschen,17  and  others.  Henschen  collects 
and  analyzes  one  hundred  and  seventy-one  cases.  In  a  recent  memoir 
Vialet 18  also  has  given  a  very  careful  analysis  of  a  large  number  of  selected 
cases,  adding  others  which  he  had  himself  observed. 

This  anatomico-pathological  research  offers  very  peculiar  difficulties, 
and  still  further  statistics  are  necessary  before  the  evidence  from  cortical 
lesions  can  be  said  to  be  decisive.  In  comparatively  few  of  the  cases  was 
the  cortical  lesion  clearly  circumscribed,  and  even  when  this  condition  is 
fulfilled  it  has  still  to  be  asked,  What  other  functional  disturbance  was 
produced  as  well  as  blindness  ?  How  far  did  the  lesion  extend  beyond  the 
visual  area?  And,  again,  Was  the  blindness  due  to  the  focal  lesion,  or  did 


1  Studi  clinici  sui  centri  sensori  corticali,  Milan,  1879;  also  Luciani,  in  Brain,  1884. 

2  Topische  Diagnostik  des  Gehirnkrankheiten,  Berlin,  1879. 

3  Kaccoglitore  med.,  Forli,  1880,  etc.,  transl.  in  Rec.  d'Ophthalm.,  Paris,  1889. 
*  De  1'Hemianopsie,  Paris,  1880. 

5  Gehirn  und  Auge,  Wiesbaden,  1881. 

8  Untersuchungen  liber  die  Localisation,  1881. 

7  Hemianopsie,  Berlin,  1881. 

8  Klinische  Monatsblatter. 

9  American  Journal  of  the  Medical  Sciences,  1884. 

10  Luciani  and  Seppilli,  1885,  German  trans,  by  Fraenkel  (Functions  Localisation), 
Leipsic,  1886. 

11  Bibliothek  for  Laege,  1885. 

12  Journal  of  Nervous  and  Mental  Diseases,  1886,  and  Archives  de  Neurologic,  1886. 
»  Lyon  Medical,  1887. 

14  Revue  de  Medecine,  1888. 

15  Archives  de  Physiol.,  Paris,  1890,  p.  177. 

16  Loc.  cit. 

"  Pathologie  des  Gehirns,  Upsala,  1892. 

18  Les  Centres  cerebraux  de  la  Vision,  Paris,  1893. 


408  ANATOMY   OF   THE   INTRA-CRANIAL    PORTION 

the  disease  give  rise  to  pressure  or  secondary  degenerations  and  thus  cause 
blindness  by  an  indirect  action  ? 

Henschen l  concludes  that  the  visual  area  is  limited  to  that  portion  of 
the  cortex  which  is  sunk  within  the  calcarine  fissure.  Within  this  fissure 
and  for  a  short  distance  on  the  adjacent  convolutions  the  cortex  is  distin- 
guished by  the  presence  of  Vicq  d'Azyr's  (or  Genuari's)  band,  a  sharply 
defined  white  band  due  to  the  presence  of  a  sheet  of  tangential  fibres.  Hen- 
schen's  conclusion  is  chiefly  based  on  a  case  described  by  himself  and  Nor- 
denson  in  which  the  lesion  was  limited  to  this  area,  together  with  cases  of 
disease  of  the  eye  and  blindness  in  which  histological  study  showed  atrophy 
of  nerve-cells  in  this  area.  He  is  further  of  opinion  that  it  is  possible  to 
ascertain  the  projection  of  the  retina  on  the  cortex  ;  the  dorsal  quadrant  of 
the  retina  being  represented  in  the  upper  lip  of  the  fissure,  the  ventral 
quadrant  in  the  lower  lip,  the  macula  lutea  in  the  front  of  its  floor,  and  the 
peripheral  end  of  the  horizontal  meridian  at  its  back.  Since  loss  of  one 
eye  entails  histological  changes  in  the  cortex  on  both  sides  of  the  brain,  he 
concludes  that  each  cortical  visual  area  belongs  to  corresponding  parts  of 
both  eyes. 

Von  Monakow  concluded,  from  the  cases  which  he  first  observed,  that 
the  visual  area  is  localized  on  the  inner  surface  only  of  the  occipital  lobe, 
"  the  domain  of  the  calcarine  fissure."  A  subsequent  observation  has  in- 
duced him  to  extend  the  area  so  that  it  includes  the  external  surface  of  the 
occipital  lobe  and  even  the  angular  gyrus. 

Certain  other  pathologists  have  concluded,  on  the  strength  of  cases  which 
have  come  under  their  notice,  that  the  lingual  and  fusiform  convolutions 
also  form  part  of  the  visual  sphere. 

Experimental  Evidence. — Since  of  necessity  this  evidence  relates  to  the 
localization  of  the  visual  area  in  animals  and  not  in  man,  we  have  placed  it 
last.  Had  it  been  applicable  without  qualification  to  the  problem  before  us, 
we  should  have  regarded  it  as  more  satisfactory  than  clinical  evidence,  inas- 
much as  it  can  be  more  exactly  controlled. 

.  Experiments  upon  the  brain  take  two  forms,  which  are  complementary 
one  to  the  other.  In  the  first  place,  removal  of  portions  of  the  cortex 
abrogates  certain  of  the  animal's  sensory  endowments.  In  the  second  place, 
electrical  stimulation  of  spots  on  the  cortex  which  lie  outside  the  "  motor 
area"  induces  movements  which  have  the  appearance  of  being  adaptive 
movements  such  as  commonly  result  from  the  provocation  of  sense-presen- 
tations. 

The  results  of  each  kind  of  experiment  must  be  interpreted  with  full 
knowledge  of  the  risk  of  the  introduction  of  error  from  causes  which  are 
only  just  beginning  to  be  understood.  The  necessary  reservations  may  be 
briefly  summed  up  as  follows  : 

(1)  Localization  of  function  in  the  cortex  may  be  described  as  rapidly 

1  Henschen,  loc.  cit.,  p.  358. 


OF   THE    VISUAL    APPARATUS.  409 

progressive.  It  cannot  be  defined  in  the  rabbit's  brain  ;  in  the  dog  it  is  ill 
defined ;  in  the  monkey  it  is  fast  becoming  precise.  In  man  alone  is  it 
permanent.  In  the  lower  animals  the  sensori-motor  functions  are  settlers 
in  the  cortex.  In  man  they  have  acquired  the  freehold,  the  value  of  their 
title  increasing  as  they  become  specialized.  It  is  the  skilled  trades  only 
which  have  an  absolute  right  to  the  areas  they  occupy.  The  very  existence 
of  so  highly  skilled  a  function  as  that  of  speech  depends  upon  the  integrity 
of  the  mechanism  by  which  it  is  carried  out,  and  which  has  been  built  up 
by  a  laborious  process  of  training.  The  passage  of  impulses  determines 
the  growth  of  tissue.  The  development  of  tissue  facilitates  the  passage  of 
impulses.  This  function  of  speech  is  taken  as  an  illustration  of  the  most 
elaborate  and  human  of  the  motor  functions  of  the  brain, — a  function 
which,  on  account  of  its  intricacy,  has  fixed  its  seat  immovably  in  the 
cortex,  taking  complete  possession  of  a  spot  convenient  for  its  commerce ; 
even  limiting  its  residence  to  one  side  of  the  brain,  despite  the  symmetrical 
situation  of  the  muscles  of  the  mouth  and  larynx  by  which  it  is  expressed. 
The  consideration  of  this  extreme  case  puts  us  on  our  guard  in  applying 
the  results  of  experiments  upon  animals  to  the  elucidation  of  the  functions 
of  the  human  brain. 

(2)  The  lower  the  animal  in  the  scale  of  existence  the  more  elaborate 
are  the  reflex   actions   carried  out  by  the  subcerebral  mechanisms.     The 
cerebrum  of  a  rabbit,  still  better  that  of  a  pigeon  or  a  frog,  may  be  removed 
completely,  and  yet  the  animal  will  respond  to  stimulation  of  its  retina  in  a 
manner  which  often  seems  purposeful.     It  is  quite  possible  that  the  dogs 
from  which  Goltz  removed  the  whole  occipital  cortex  may  have  been  de- 
prived of  conscious  vision, — "  conscious"  is  perhaps  too  definite  a  term,  while 
"  psychical  vision"  is  incapable  of  definition, — although  the  impulses  which 
travelled  up  the  optic  nerve  clearly  found  their  way,  through  the  parts  of 
the  brain  which  were  left,  to  the  motor  nerves  of  the  muscles  by  which  the 
animal  executed  appropriate  movements. 

(3)  The  central  nervous  system  does  not  consist  of  isolated  organs,  but 
is  a  sympathetic  and  subtly  interdependent  whole.     The  force  of  the  knee- 
jerk  is  modified,  as  Lombard  and  Bowditch  have  shown,  by  impulses  passing 
through  parts  of  the  cerebro- spinal  axis  far  distant  from  its  reflex  centre. 
It  is  easy,  therefore,  to  understand  that  any  operative  interference  with  the 
brain  produces  an  action,  presumably  irritative  in  nature,  by  which  cerebral 
mechanisms,  not  immediately  affected  by  the  operation,  are  thrown  out  of 
gear.     Until  it  has  been  ascertained  that  the  phenomena  observed  after  the 
operation  were  not  the  effects  of  shock,  they  must  not  be  held  as  indicating 
that  the  part  removed  was  responsible  for  the  functions  which  seem  to  be 
lost.     It  is  generally  supposed  that  the  effects  of  shock  wear  off,  and  that 
after  an  interval,  very  differently  estimated  by  different  observers,  the  loss  of 
function  proves  the  destruction  of  the  apparatus  by  which  it  was  performed. 

(4)  Inflammatory  swelling,  secondary  degenerations,  etc.,  may  affect  parts 
of  the  brain  supposed  to  have  been  left  intact 


410  ANATOMY   OF   THE   INTRA-CRANIAL   PORTION 

(5)  The  production  of  movements  by  electric  stimulation  of  the  occipital 
cortex  has  not  as  yet  been  brought  into  line  with  the  production  of  move- 
ments by  stimulating  the  motor  area.  A  stronger  stimulus  is  required.  The 
movements  follow  after  a  longer  latent  period  and  are  irregular  and  uncer- 
tain. They  are  easily  antagonized  by  stimulation  of  the  motor  area.  Their 
character  is  not  much  affected  by  slicing  away  the  cortex.  In  short,  they 
are  hardly  sufficiently  definite  to  be  taken  as  proving  the  existence  of  sight- 
movement  reflexes,  although  they  confirm  the  conclusion  which  is  based  upon 
other  evidence  that  the  occipital  cortex  contains  the  visual  area. 

After  this  introduction  a  brief  summary  of  the  results  of  experiments 
upon  animals  will  suffice.  Observations  upon  dogs  indicate  that  the  cortex 
of  the  occipital  region  is  concerned  with  vision  ;  they  throw  no  light,  how- 
ever, upon  the  problem  before  us,  —  viz.,  the  exact  topography  of  the  visual 
area  in  man.  Upon  evidence  which  is  now  generally  regarded  as  insuf- 
ficient, Munk  l  concluded  that  he  could  map  out  the  visual  sphere  in  such  a 
way  as  to  represent  the  projection  of  the  retina  upon  the  brain.  If  subse- 
quent observers  find  in  the  dog  or  other  animal  any  similar  association  of 
the  several  portions  of  the  retina  with  the  several  parts  of  the  visual  sphere, 
it  will  lead  us  to  look  for  a  similar  allocation  in  man. 

Numerous  ablation-experiments  have  been  performed  upon  the  brain- 
cortex  of  monkeys.  All  observers  agree  in  asserting  that  enduring  visual 
disturbance  is  produced  only  when  the  back  of  the  brain  is  operated  upon, 
but  the  delimitation  of  the  visual  sphere  and  the  exact  effect  of  its  removal 
upon  vision  are  variously  described. 

Extent  of  the  Visual  Area.  —  Munk,2  Horsley,3  Schafer,4  and  Sanger 
Brown  regard  it  as  limited  to  the  occipital  lobe.  Luciani8  considers  that 
the  parietal  and  temporal  lobes  are  also  connected  with  vision,  although  the 
most  exclusively  visual  centre  is  situate  in  the  occipital  lobe. 

Ferrier6  looks  upon  the  angular  gyrus  as  more  important  than  the 
occipital  lobe,  inasmuch  as  it  contains,  he  thinks,  the  special  region  of  clear 
or  central  vision  of  the  opposite  eye,  and  perhaps  to  some  extent  also  of 
the  eye  on  the  same  side,  while  only  the  correlated  halves  of  the  peripheral 
portions  of  the  retinae  are  represented  in  the  rest  of  the  occipito-angular 
region. 

Effect  upon  the  Field  of  Vision  of  Removal  of  the  Visual  Area  of  the 
Cortex.  —  All  observers  agree  that  unilateral  decortication  produces  hemiopia 
towards  the  opposite  side  of  the  field  of  vision.  There  is-great  difference 
of  opinion  as  to  the  amount  of  amblyopia  which  accompanies  the  hemiopia, 


Physiologic  der  Grosshirnrinde,  1881. 
1  Op.  cit. 

8  Horsley  and  Schafer,   Philosophical  Transactions  of  the  Koyal  Society,  1888,  vol. 
clxxix.,  B,  p.  1. 

*  Schafer  and  Sanger  Brown,  ibid.,  1888,  vol.  clxxix.,  B,  p.  303. 

6'Brain,  1884. 

•The  Functions  of  the  Brain,  2d  ed.,  p.  288. 


OF  THE   VISUAL    APPARATUS.  411 

and  as  to  whether  it  is  only  the  opposite  eye  which  is  rendered  everywhere 
less  sensitive  or  whether  vision  is  also  obscured  on  the  otherwise  unaffected 
half  of  the  retina  of  the  same  side.  The  extreme  difficulty  of  testing 
vision  in  animals  renders  a  discussion  of  the  various  opinions  on  this  sub- 
ject undesirable. 

The  Curtailment  of  the  AnimaVs  Endowments. — There  are  three  possible 
views  as  to  the  effect  of  the  operation  upon  the  animal's  faculties,  and  each 
view  has  its  advocates. 

(1)  The  removal  of  the  cortex  may  abolish  visual  sensations  of  every 
description.  The  impulses  projected  up  the  optic  nerve  from  the  stimulated 
retina  may  come  to  an  end  in  the  injured  tissue, — be  lost  in  the  lesion, — 
producing  no  effect  upon  consciousness  and  no  transformation  of  energy 
within  or  without  the  body. 

This  is  certainly  not  the  case  in  lower  animals.  The  further  we  descend 
the  animal  scale  the  more  open  do  we  find  the  subcortical  reflex-paths  to  be. 
In  the  lower  animals  sensory  impulses  find  their  way  into  motor  paths 
without  the  intervention  of  the  great  brain.  Longet  showed  that  rabbits 
and  rats  deprived  of  the  cerebral  hemispheres  (the  optic  thalami  being  left 
intact)  execute  certain  movements  in  response  to  light.  Vulpian  *  repeated 
these  experiments,  and  suggested  that  we  should  speak  of  these  mutilated 
animals  as  retaining  "  sensations"  of  sight  although  their  "  perceptions"  are 
abolished.  There  is  reason  to  think  that  in  the  monkey  (and  still  more  in 
man)  even  such  primitive  reflex  actions  are  so  far  subject  to  the  control  of 
the  great  brain  that  they  cease  to  be  possible  after  its  removal.  Schaferand 
Sanger  Brown  tested  with  all  the  thoroughness  possible  a  monkey  from  which 
the  occipital  lobes  had  been  removed  nine  months  previously,  for  the  pur- 
pose of  determining  whether  the  animal  had  any  power  whatever  of  dis- 
criminating between  light  and  darkness.  Although  in  this  instance  they 
were  seeking  for  evidence  of  a  survival  of  sight-perceptions,  the  tests  they 
used  would  in  the  rat  or  rabbit  have  caused  some  reflex  response.  This 
monkey,  however,  bore  light  flashed  into  its  eyes  without  making  the 
slightest  movement,  although  its  pupils  reacted  normally.  Ferrier  describes 
experiments  of  a  similar  kind,  and  we  seem  to  be  justified  in  concluding 
that,  with  the  exception  of  the  pupil-reflex,  the  occipital  cortex  is  in  the 
Primates  the  only  seat  of  the  reflection  of  visual  impulses  into  motor  chan- 
nels j  for  there  can  be  but  little  doubt  that  the  small  size  of  the  quadri- 
geminal  tubercles  and  the  retrograded  structure  of  their  cortical  tissue  indi- 
cate that  their  office  is  less  important  in  higher  than  it  is  in  lower  animals. 
The  large  optic  lobes  of  reptiles  and  birds  are  covered  with  gray  matter 
formed  upon  what  may  be  regarded  as  essentially  the  cortex  type,  and 
therefore  fitted,  we  are  justified  in  supposing,  for  the  discharge  of  duties 
which  in  the  Primates  have  been  assumed  by  the  cerebral  hemispheres. 

If  this  be  true,  removal  of  the  cortex  in  the  monkey  abolishes  not  only 

1  Physiologic  du  Systeme  nerreux,  p.  709. 


412  ANATOMY   OF   THE   INTRA-CKANIAL   POETION 

perception  but  also  sensation.  It  is  probable,  however,  that  when  a  part 
of  one  side  of  the  brain  is  removed  the  sensory  impulses  which  are  accus- 
tomed to  produce  their  effects  on  this  part  may  be  received  by  the  opposite 
side,  or  they  may  even  find  attention  from  other  and  intact  parts  of  the 
same  hemisphere,  or,  at  any  rate,  from  the  remaining  part  of  their  own 
sensory  zone. 

(2)  It  may  be  that  removal  of  the  visual  area  abolishes  consciousness 
for  sensations  of  sight.     The  blindness  is  absolute  if  this  be  the  case,  since 
impulses,  so  long  as  this  condition  lasts,  do  not  give  rise  to  perceptions, 
although  they  may  provoke  reflex  actions.     Many  physiologists  prefer  to 
avoid  the  psychological  question,  but  Ferrier,  among  others,  takes  this  view. 

(3)  Munk  introduced  the  conception  that,  since  the  cortex  is  the  store- 
house of  visual  memories,  the  blindness  which  follows  its  removal  is  due 
not  to  the  abolition  of  current  perceptions,  but  to  the  extinction  of  those 
which  were  stored  away  in  the  cortex.     A  new  presentation  of  sense  con- 
veys henceforth  no  meaning  to  the  animal,  since  it  has  no  experience  with 
which  to  compare  it.     It  is  in  Munk's  terminology  "  psychically  blind." 
The  proof  of  this  theory  will  depend  upon  the  possibility  of  re-educating 
an  animal  from  which  the  existing  visual  sphere  has  been  totally  removed. 

Duration  of  the  Effects  of  Decortication. — Ferrier  holds  that  when  an 
operation  is  complete  and  successful,  the  whole  of  the  visual  areas  on  both 
sides  of  the  brain  (including  the  angular  gyri)  being  removed,  without  in- 
flammatory sequelae,  the  blindness  is  complete  and  permanent.  Schafer 
and  Sanger  Brown  obtained  a  similar  result  in  one  case  withbut  destruction 
of  the  angular  gyri.  Luciani,  on  the  other  hand,  asserts  that  even  after 
the  most  extensive  extirpation  of  the  occipito-temporal  area  absolute  blind- 
ness does  not  persist  beyond  a  few  days,  when  its  place  is  taken  by  com- 
plete "  psychical"  blindness,  which  in  turn  becomes  incomplete,  although  it 
remains  permanently.  The  animal  again  makes  use  of  the  sense  of  sight 
in  searching  for  its  food,  although  it  never  learns  to  distinguish  by  this 
sense  alone  between  widely  dissimilar  bodies,  such,  for  example,  as  meat 
and  sugar. 

Summary. — The  evidence  before  us  leaves  no  doubt  as  to  the  connection 
with  vision  of  the  posterior  part  of  the  cerebral  hemisphere,  but  the  con- 
flicting statements  of  those  who  have  investigated  the  subject  leave  us  in 
uncertainty  both  as  to  the  topography  of  the  visual  sphere  and  also  as  to 
the  nature  of  the  processes  which  are  carried  on  within  it.  The  balance  of 
evidence  is  in  favor  of  complete  limitation  of  vision  to  this  sphere  in  mon- 
keys and  man,  rather  than  the  concentration  in  this  region  of  a  function 
shared  in  a  lower  degree  by  other  parts  of  the  cortex.  There  is,  too,  posi- 
tive evidence  that  when  the  whole  area  has  been  removed  vision  is  lost, 
other  parts  of  the  cortex  being  unable  to  acquire  the  faculty ;  whereas  if 
but  a  small  part  of  the  visual  sphere  be  left  intact  the  animal  gradually 
learns  to  work  so  well  with  the  part  that  is  left  that  its  vision  becomes 
almost  as  good  as  before. 


OF   THE    VISUAL    APPARATUS.  413 

Speculations  as  to  the  nature  of  the  activities  of  the  cortex-centres 
appear  to  us  to  be  barren  in  the  present  state  of  physiological  psychology. 
"Sensation,"  "perception,"  "sense-judgment,"  are  most  valuable  terms 
when  used  .to  classify  subjective  observations.  Their  use  in  physiology  is 
allegorical.  Until  we  know  something  of  the  nature  of  the  changes  effected 
in  the  central  nervous  system  by  the  receipt  of  afferent  impulses,  it  is  use- 
less to  speculate  as  to  whether  a  particular  portion  of  nerve-tissue  may 
contain  -the  memories  of  sensory  impressions  and  yet  not  be  the  tissue  in 
which  current  sensations  become  conscious,  or  whether  it  may  be  the  tissue 
in  which  sensations  light  up  consciousness,  but  not  the  tissue  in  which  the 
motor  response  to  unfelt  sensations  originates. 

It  appears  to  us  sufficient  to  say  that  in  the  highest  mammals  removal 
of  the  occipital  cortex  produces  blindness,  whereas  in  lower  mammals  vision 
is  only  partially  destroyed  by  the  same  operation. 

CONNECTIONS   OF   THE   OCCIPITAL   CORTEX   WITH   THE   LOWER   VISUAL 

CENTRES. 

The  white  matter  of  the  occipital  lobe  forms  a  dense  cone,  hollowed  out 
by  the  posterior  horn  of  the  lateral  ventricle.  On  the  outer  side  of  the 
ventricle  the  wall  is  everywhere  very  thick ;  on  its  inner  and  under  sides 
the  calcarine  and  collateral  fissures  cut  so  deeply  into  the  wall  as  to  leave 
but  a  thin  shell  of  white  matter  between  the  ventricle  and  the  cortex. 
Many  attempts  have  been  made  to  disentangle  the  several  tracts  of  fibres 
which  together  make  up  this  dense  medullary  mass,  but  without  any  par- 
ticularly useful  results.  The  following  groups  of  fibres  enter  into  its 
constitution. 

(1)  Short  fibres,  or  fibrse  proprise,  which  unite  together  different  parts 
of  the  same  or  adjoining  convolutions.    These  intergyral  commissures  form 
a  sheet  immediately  beneath  the  cortex. 

(2)  Long  intra-hemispheral  commissures.     These  lie  below  the  fibrse 
propriae,  and  constitute  many  tracts  of  varying  length,  breadth,  and  dis- 
tinctness.    None  of  these  tracts  is  more  easily  distinguished  by  dissection 
than  the  inferior  longitudinal  fasciculus,  or  more  easily  followed  with  the 
naked  eye  or  with  the  microscope  through  a  series  of  sections  stained  after 
Weigert's  method  in  hsematoxylin  and  decolorized  with  potassic  ferrid- 
cyanide  or  potassium  permanganate.     It  runs  as  a  broad  band  from  the 
anterior  end  of  the  temporal  lobe  to  the  occipital  pole.      Sagittal  fibres 
beneath  the  fibrse  propriae  on  the  outer  surface  of  the  lobe  constitute  a  dis- 
tinct sheet  or  bundle  (fasciculus  occipitalis  perpendicularis)  from  the  neigh- 
borhood of  the  gyrus  angularis  to  the  gyrus  fusiformis.     Other  shorter 
tracts  or  sheets  of  fibres  have  been  described  as  proper  to  this  region,  but, 
when  it  is  realized  that  practically  every  part  of  the  cortex  of  the  great 
brain  is  connected  by  "  association  fibres"  with  every  other  part,  it  will  be 
doubted  whether  it  is  desirable  to  give  a  name  to  each  associating  tract 
which  is  found  to  be  somewhat  distinctly  segregated  from  the  general  mass. 


414  ANATOMY   OF   THE    INTRA-CRANIAL    PORTION 

i 

If  ever  our  knowledge  of  the  functions  of  the  cortex  is  more  detailed,  it 
will  be  necessary  to  look  out  for  evidence  of  unusually  intimate  union  be- 
tween this  convolution  and  that.  At  present  we  may  be  content  to  mention, 
without  describing,  the  stratum  calcarinum,  which  unites  the  lips  of  the 
calcarine  fissure,  the  stratum  proprium  cunei,  for  the  most  part  longitudi- 
nal, the  stratum  cunei  transversum,  and  the  fasciculus  transversus  lobuli 
lingualis.  The  names  given  to  these  several  tracts  indicate  their  general 
disposition. 

(3)  Interhemispheral  fibres  of  the  corpus  callosum.     The  fibres  of  this 
vast  commissural  system  are  largely  responsible  for  the  thickness  of  the 
outer  wall  and  floor  of  the  posterior  horn.     Instead  of  crossing  directly 
from  occipital  lobe  to  occipital  lobe,  the  posterior  fibres  of  the  corpus  callo- 
sum are  gathered  up  into  a  thick  beam  (the  splenium).     From  this,  after 
crossing  in  the  roof  of  the  third  ventricle,  they  sweep  backward  on  either 
side  in  the  forceps  posterior,  keeping  close  to  the  lining  epithelium  of  the 
posterior  horn,  for  which   they  form  an   imperfect  sheath,  the  tapetum. 
From  this  sheath  fibres  stream  to  every  part  of  the  occipital  cortex ;  those 
which  reach  its  inner  wall  curve  round  the  posterior  horn. 

(4)  In  addition  to  the  "  association  fibres,"  connected  at  both  ends  with 
the  cortex,  the  medullary  substance  of  the  occipital  lobe  is  made  up  of 
"projection  fibres,"  or  fibres  of  the  corona  radiata.     It  is  customary  to 
name  these,  in  a  vague  way,  the  "  optic  radiations  of  Gratiolet,"  although, 
as  we  have  already  shown,  Gratiolet's  description  applies  not  to  the  mass 
of  fibres,  but  to  those  which  he  supposed  he  could  trace  into  the  optic  tract. 

Between  the  tapetal  fibres  which  sheathe  the  ventricle  and  the  com- 
missural fibres  which  line  the  cortex,  the  medullary  mass  is  made  up  of  in- 
terwoven radiating  and  tangential  fibres.  Owing  to  this  intermixture  of 
elements  crossing  one  another  at  various  angles,  rather  than  to  any  pecu- 
liarity in  calibre  or  reaction  to  staining  agents,  the  central  mass  appears, 
especially  when  treated  after  Weigert's  method,  much  lighter  than  the  com- 
pact bundles  of  fibres  by  which  it  is  surrounded.  This  must  not  be  re- 
garded, however,  as  a  character  distinguishing  the  tracts  of  which  it  is 
composed. 

What  is  the  destination  of  these  tracts?  Our  answer  is  based  upon 
presumptive  evidence  rather  than  observation,  (a)  They  unite  the  cortex 
cerebri  with  the  cortex  cerebelli,  especially  its  upper  or  anterior  surface. 
(6)  They  certainly  unite  the  cortex  with  the  optic  thalamus,  both  directly 
and  through  its  stratum  zonale,  and  also  with  the  external  geniculate  and 
anterior  quadrigeminal  bodies.  To  these  fibres  the  name  "  optic  radiations" 
would  be  most  appropriately  applied,  (c)  They  also  unite  the  cortex  with 
the  back  of  the  internal  capsule  and  therefore  with  the  sensory  gray  matter 
of  the  cerebro-spinal  axis,  (d)  Possibly  they  contain  fibres  of  the  optic 
tract  which  are  not  interrupted  in  the  basal  gray  matter.  The  fibres  which 
are  added  to  the  back  of  the  internal  capsule  from  the  pulvinar  and  from 
the  anterior  quadrigeminal  and  external  geniculate  bodies  form  a  mass, 


OF   THE    VISUAL    APPARATUS.  415 

shaped  like  a  cornucopia,  with  its  broad  end  directed  backward  and  down- 
ward. Its  more  condensed  handle,  which  curves  at  first  forward  and  out- 
ward, then  upward  and  backward,  is  sometimes  termed  the  "  sagittal  tract 
of  Wernicke,"  or,  with  even  less  precision,  the  "  intra-cerebral  optic  tract" 
In  addition  to  these,  which  are  chiefly  ascending  tracts,  the  results  of 
physiological  experiment  lead  us  to  look  for  (e)  fibres  descending  to  the 
nuclei  of  the  eye-muscle  nerves  and  perhaps  to  other  motor  nuclei  farther 
down  the  axis. 


CONGENITAL  MALFORMATIONS  AND 
ABNORMALITIES  OF  THE  HUMAN  EYE. 

BY  WILLIAM   LANG,  F.K.C.S.,  ENO., 

Surgeon  to  the  Royal  London  Ophthalmic  Hospital ;  Ophthalmic  Surgeon  to  and  Lecturer 
on  Ophthalmology  at  the  Middlesex  Hospital,  London,  England, 

AND 

E.  TREACHER   COLLINS,  F.R.C  S.,  ENQ., 
Curator  and  Librarian  to  the  Royal  London  Ophthalmic  Hospital,  London,  England. 


DEVELOPMENT  OF  THE  EYEBALL  AND  ITS  APPENDAGES. 

BEFORE  describing  the  various  congenital  abnormalities  to  which  the 
eye  and  its  appendages  are  liable,  it  is  well  to  give  a  brief  outline  of  their 
mode  of  normal  development. 

The  first  stage  in  the  formation  of  the  eye  commences  exceedingly  early 
in  foetal  life.  By  an  outgrowth  of  the  wall  of  the  anterior  cerebral  vesicles 
is  produced  the  primary  optic  vesicle,  the  connection  of  which  with  the  brain 
becomes  gradually  more  and  more  constricted,  forming  a  stalk,  in  which 
afterwards  is  developed  the  optic  nerve.  The  cavity  of  the  primary  optic 
vesicle  communicates  at  first  with  the  cavities  of  the  cerebral  vesicles  through 
this  stalk.  The  next  stage  consists  in  an  involution  of  the  primary  optic 
vesicle.  By  the  involution  of  its  anterior  surface  a  cup  is  produced,  the 
secondary  optic  vesicle,  into  which  passes  a  downgrowth  from  the  superficial 
epiblast ;  this  downgrowth  subsequently  becomes  cut  off  from  the  rest  of 
the  surface  epiblast,  and  forms  the  lens  vesicle.  The  lower  surface  of  the 
primary  optic  vesicle  also  becomes  involuted  by  an  upgrowing  process  of 
mesoblast,  which  afterwards  develops  into  the  vitreous  humor.  This  in- 
volution below  extends  backward  for  a  short  distance  into  the  stalk  of  the 
primary  optic  vesicle.  The  cup  of  the  secondary  optic  vesicle  has  two  layers, 
which  are  continuous  with  each  other  at  its  margin.  At  first  the  cup  is 
imperfect  below,  due  to  the  involution  which  has  taken  place  there.  This 
gap  constitutes  what  is  known  as  the  foatal  ocular  fissure.  It  is  at  first  wide, 
but  gradually  becomes  narrower,  and  is  at  last  closed  altogether,  thus  shut- 
ting off  the  mesoblast  in  the  interior  of  the  eye  from  that  external  to  it. 
The  outer  of  the  two  layers  of  the  secondary  optic  vesicle  remains  as  a  single 
row  of  cells,  which  later  become  pigmented  and  form  the  pigmented  layer 
of  the  retina ;  the  inner  becomes  thickened,  being  subsequently  differentiated 
into  the  other  layers  of  the  retina. 

The  lens  vesicle  is,  as  we  have  said,  formed  by  a  downgrowing  process 

of  cuticular  epiblast,  which  gradually  becomes  shut  off  from  the  surface 

epiblast :  this  shutting  off  is  effected  by  the  intrusion  of  mesoblast  between 

them.     From  the  anterior  portion  of  this  mesoblast  is  developed  the  sub- 

VOL.  I.— 27  417 


418      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF   THE  EYE. 

stantia  propria  of  the  cornea,  and  from  the  posterior  part  the  anterior  fibro- 
vascular  sheath  of  the  lens. 

In  the  mesoblastic  tissue  which  grows  up  into  the  interior  of  the  secondary 
optic  vesicle  a  blood-vessel  forms,  the  hindermost  part  of  which  remains 
permanently  as  the  central  artery  of  the  retina ;  the  anterior  part  persists 
for  a  short  time  as  the  central  artery  of  the  vitreous.  This  artery  breaks 
up  at  the  posterior  pole  of  the  lens  into  branches,  which  are  distributed  over 
its  posterior  surface  in  what  is  called  the  posterior  fibre-vascular  sheath ; 
prolongations  from  this  are  continued  forward  round  the  sides  of  the  lens 
to  join  the  anterior  fibro-vascular  sheath ;  in  this  way  the  whole  lens  is 
encircled  by  blood-vessels.  Meanwhile  the  lens  vesicle  has  undergone  con- 
siderable changes.  The  cells  on  the  posterior  wall  have  lengthened  out,  and 
in  so  doing  have  filled  the  whole  of  the  cavity  of  the  vesicle.  The  cells  on 
the  anterior  wall  have  remained  unchanged,  or  become  somewhat  flattened. 
The  cells  in  the  centre  of  the  posterior  layer  elongate  the  most ;  at  the  sides 
there  is  a  gradual  transition  to  the  ones  which  have  undergone  no  change. 
It  is  in  this  transitional  zone  that  fresh  lens-fibres  are  subsequently  laid  on. 
The  cells  of  the  anterior  layer  proliferate,  multiply,  and  shift  round  on  the 
inner  surface  of  the  capsule  towards  the  transitional  zone,  where  they  lengthen 
out  into  fibres ;  as  these  lie  behind  the  fibres  formed  from  the  original  pos- 
terior layer  of  cells,  the  latter  come  to  constitute  the  nucleus  of  the  lens. 
As  the  tension  in  the  capsule  increases,  the  fibres  get  more  and  more  flattened 
out,  and  their  nuclei  disappear. 

The  capsule  of  the  lens  very  early  makes  its  appearance.  There  are  two- 
views  as  to  its  mode  of  origin  :  one,  that  it  is  a  cuticular  deposit  from  the 
epithelial  cells  of  the  lens  itself,  and  the  other,  that  it  is  derived  from  the 
mesoblastic  fibro-vascular  sheath ;  the  former  is  the  more  probable. 

The  suspensory  ligament  is  formed  from  adhesions  which  take  place 
between  the  fibro-vascular  sheath  of  the  lens  and  the  ciliary  processes  while 
these  structures  are  in  contact, — i.e.,  about  the  third  month.  As  the  eyeball 
grows,  the  ciliary  processes  become  separated  from  the  sides  of  the  lens,  and 
the  adhesions,  which  are  at  first  cellular,  become  lengthened  out  into  fibres. 

The  mesoblastic  tissue  which  surrounds  the  outer  layer  of  the  secondary 
optic  vesicle  is  at  first  a  mass  of  round  cells,  and  no  diiferentiation  can  be 
made  out  between  the  sclerotic  and  choroid  coats ;  later  the  inner  part  is 
vascularized,  and  the  tissue  between  the  two  coats  becomes  spaced  out ;  pig- 
ment does  not  form  in  the  choroid  until  the  seventh  month,  sometimes  later. 
The  ciliary  processes  develop  about  the  third  month  by  folds  forming  in  the 
two  layers  of  the  anterior  portion  of  the  secondary  optic  vesicle,  into  which 
mesoblastic  tissue  extends  and  becomes  vascular.  The  iris  is  produced  by 
an  extension  inward  of  mesoblast  from  the  anterior  portion  of  the  ciliary 
body,  together  with  the  two  layers  of  the  secondary  optic  vesicles  on  its 
posterior  surface.  The  posterior  or  inner  of  these  two  layers  is  at  first, 
like  the  corresponding  layer  on  the  inner  surface  of  the  ciliary  body,  un- 
pigmented ;  but  later  pigment  develops  in  it.  The  iris  grows  inward 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.       41 9 

beneath  the  anterior  fibro- vascular  sheath,  and  has  to  insinuate  itself  tatween 
it  and  the  cornea  on  the  one  side  and  the  lens  on  the  other ;  as  it  does  so  it 
pushes  the  prolongation  from  the  posterior  fibre-vascular  sheath  in  front 
of  it.  Its  stroma  is  unpigraented  until  after  birth.  The  central  portion  of 
the  anterior  fibro-vascular  sheath  left  stretching  across  the  pupil  forms  the 
pupillary  membrane,  which  disappears  before  birth.  The  periphery  of  the 
anterior  fibro-vascular  sheath,  beneath  which  the  iris  grows,  becomes  incor- 
porated with  that  structure  and  forms  its  anterior  layer.  As  the  eyeball 
increases  in  size,  the  central  artery  of  the  vitreous  becomes  stretched,  the 
circulation  of  the  blood  through  it  becomes  arrested,  and  both  it  and  the 
posterior  fibro-vascular  sheath  disappear. 

The  upper  and  lower  eyelids  commence  early  as  protrusions  of  the  in- 
tegument a  short  distance  from  the  corneal  margin  ;  these  continue  to  grow, 
and  ultimately  their  margins  meet  and  unite  in  front  of  the  globe.  The 
cavity  which  is  thus  enclosed  forms  the  conjunctival  sac.  The  Meibomian 
glands  and  the  follicles  for  the  cilia  commence  to  form  during  the  adhesion 
of  the  lids,  by  the  downgrowth  into  the  mesoblastic  connective  tissue  of  the 
lids  of  solid  columns  from  the  rete  Malpighii.  A  short  time  before  birth 
the  epithelial  connection  along  the  margin  of  the  lids  gives  way,  and  they 
again  become  separate. 

The  lacrymal  gland  is  developed,  about  the  third  month,  by  the  down- 
growth  of  a  solid  mass  of  epithelium  from  the  upper  and  outer  portion  of 
the  conjunctival  sac;  branches  subsequently  jut  out  from  the  central  mass, 
and  thickenings  form  on  these ;  then  the  central  cells  of  each  branch  undergo 
a  fatty  degeneration,  and  so  become  tubular. 

The  lacrymal  ducts  are  developed  in  the  groove  which  is  early  formed 
between  the  maxillary  processes  externally  and  the  outer  nasal  processes 
internally  ;  whether  from  an  adhesion  of  the  edges  of  the  groove  or  from  an 
epithelial  column,  formed  by  proliferation  of  the  epithelium  at  the  bottom 
of  it,  which  subsequently  becomes  tubular,  is  as  yet  undecided. 

CONGENITAL  ABNORMALITIES  OF  THE  EYEBALL. 

In  describing  the  numerous  congenital  abnormalities  of  the  eyeball  and 
its  appendages,  we  propose  first  to  speak  of  the  more  gross  defects,  in  which 
the  whole  globe  is  involved  ;  then,  having  dealt  with  those  of  the  appendages, 
to  proceed  to  the  malformations  met  with  in  each  individual  structure  of  the 
eye. 

ANOPHTHALMOS. 

The  term  is  applied  to  cases  in  which  clinically  no  eyeball  can  be  seen 
or  felt  (Fig.  1).  Cases  of  this  sort  are  rarely  met  with.  Judging  from 
the  cases  published,  it  would  appear  that  it  is  more  frequent  to  find  the 
two  eyes  absent  than  only  one.  When  only  one  eye  is  absent,  the  other 
not  uncommonly  presents  some  congenital  defect.  As  a  rule,  patients  who 
present  this  abnormality  are  born  of  healthy  parents,  and  they  themselves 


420      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 

are  healthy  and  well  formed,  being  free  from  any  malformations ;  though 
occasionally  harelip,  supernumerary  digits,  and  other  defects  have  been  met 
with.  The  two  sexes  seem  equally  liable  to  this  malformation.  The  lids 
are  usually  well  formed,  though  small ;  sometimes  they  are  adherent  at 
their  margins.  The  orbits,  too,  are  often  smaller  than  normal.  The  lac- 
rymal  gland  is  usually  present,  but  the  lacrymal  puncta  and  canaliculi  may 
be  absent  in  one  or  both  lids.  Dissection  has  shown  that  in  some  of  these 
cases  a  small  rudimentary  globe  exists  far  back  at  the  apex  of  the  orbit,  and 
that,  strictly  speaking,  they  are  not  cases  of  auophthalmos,  but  high  degrees 
of  microphthalmos.  Ten  cases  are  recorded,  however,1  in  which  not  a  trace 
of  an  eyeball  or  of  anything  representing  it  could  be  found  in  the  orbit  on 
dissection,  and  in  which  the  optic  nerve  did  not  enter  the  orbit.  In  one  it 
ended  in  the  shape  of  a  cone  at  the  optic  foramen,  in  another  in  a  fibrous 
filament,  and  in  five  the  chiasma  was  absent.  What  had  happened,  then, 
in  these  cases  was  this :  no  primary  optic  vesicle  had  budded  out  from  the 
anterior  primary  encephalic  vesicle,  or,  having  budded  out,  it  had  failed  to 
form  a  secondary  optic  vesicle.  In  one  case  the  olfactory  nerve  was  absent 
on  one  side,  and  in  another  case  on  both  sides,  together  with  one  of  the 
cerebral  hemispheres ;  these  structures  are  also  expansions  of  the  anterior 
primary  encephalic  vesicle.  So  it  would  appear  that  some  disturbance 
affecting  the  development  of  the  anterior  primary  encephalic  vesicle  is,  at 
any  rate,  one  cause  of  anophthalmos. 

MICROPHTHALMOS. 

Microphthalmic  eyes  may  be  divided  into  two  classes  : 

1.  Those  in  which  there  is  no  apparent  congenital  defect  in  the  eye 
except  the  smallness  of  the  globe. 

2.  Those  in  which,  in  addition  to  the  eye  being  unusually  small,  there 
is  some  other  abnormality,  resulting  from  imperfect  closure  of  the  feetal 
fissure. 

No  very  definite  line  can  be  drawn  between  highly  hypermetropic  eyes 
and  those  of  the  first  class.  Sometimes,  when  the  central  artery  of  the 
vitreous  remains  persistent  and  patent  (as  is  described  under  the  heading 
of  Abnormalities  of  the  Vitreous),  the  eyeball  fails  to  develop  to  its  normal 
size,  though  there  has  been  perfect  union  of  the  foetal  ocular  cleft. 

The  second  class  can  be  best  treated  under  two  heads, — viz.,  those  where 
the  defect  in  the  closure  of  the  ocular  fissure  is  slight,  and  in  which  the  eye- 
ball retains  nearly  its  normal  shape,  and  those  where  the  accompanying  abnor- 
mality is  very  gross,  the  eye  being  usually  exceedingly  small,  while  connected 
with  it  ave  one  or  more  cysts.  (Figs.  2  and  3.)  The  subject  of  coloboma  will 
be  dealt  with  under  the  heading  of  Abnormalities  of  the  Choroid.  Here  some 
description  may  be  given  of  the  rare  cases  in  which  cysts  are  met  with  in  con- 
nection with  rudimentary  eyes.  These  patients  are  brought  with  cystic  swell- 

1  Royal  Lond.  Ophth.  Hosp.  Rep.,  vol.  xi.  p.  429. 


Fio.  1. 


Child,  aged  nine  weeks,  with  congenital  anophthalmos.    (Royal  Lond.  Ophth.  Hosp.  Reports,  vol.  xi.) 

Fia.  4. 


Portion  of  the  eye  represented  in  Fiji.  2  included  in  the  square,  highly  magnified.  It  shows  the 
neck  of  the  cyst,  with  retinal  tissue  passing  through  it  from  the  eyeball  into  the  cyst.  (Royal  Lond. 
Ophth.  Hosp.  Reports,  vol.  xii.) 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.      421 

ings,  sometimes  of  considerable  size,  situated  in  the  lower  part  of  the  orbit  or 
lower  lid  ;  they  are  often  very  thin-walled  and  translucent.  The  skin  of  the 
lower  lid  stretched  over  them  presents  sometimes  a  bluish  hue.  The  cyst 
may  be  so  large  as  to  fill  the  orbit  and  to  all  appearances  replace  the  eye- 
ball ;  the  small  eyeball  itself  is  usually  situated  far  back  near  the  apex  of 
the  orbit,  and  its  presence  or  absence  cannot  be  determined  by  clinical  exam- 
ination alone.  It  is  this  which  has  led  to  some  of  these  cases  being  described 


Diagrammatic  representation  of  a  microph- 
thalmic  eye  with  a  cyst  attached. — Co,  cornea ;  L, 
lens;  /,  iris;  R,  retina  much  folded;  Ch  and  P, 
choroid  and  pigment  epithelium;  S,  sclerotic; 
O.n,  optic  nerve  ;  Cy,  cyst  lined  by  atrophied  ret- 
ina. The  part  marked  off  by  straight  lines  is 
shown  magnified  in  Fig.  4.  (Royal  Lond.  Ophth. 
Hosp.  Rep.,  vol.  xii.) 


Diagrammatic  representation  of  microphthal- 
mic  eye  with  two  cysts  attached. — Co,  cornea ;  L, 
lens  displaced  and  shrunken ;  1,  iris ;  S,  sclerotic ; 
Ch,  choroid ;  B,  retina  much  folded ;  O.n,  optic 
nerve ;  Cy,  cysts  lined  by  retina.  (Trans.  Ophth. 
Soc.,  vol.  xiii.) 


under  the  head  of  anophthalmos.  Dr.  Monacho1  exhibited  at  the  Catalonia 
Academy  and  Laboratory  of  Medical  Sciences  a  little  girl,  aged  thirteen 
months,  the  subject  of  this  abnormality,  in  whom  the  cysts  became  tense 
during  crying,  and  who  was  observed  to  press  them  frequently  with  her 
hands,  and  then  to  smile,  a  phosphene  or  subjective  sensation  of  light  being 
probably  thus  produced. 

Dissection  of  these  cysts  reveals  that  they  are  connected,  usually  by  a 
thin  neck,  with  the  small  ill-developed  eyeballs  at  their  lower  and  pos- 
terior part,  and  that  they  are.  not  separately  formed  cysts  which  have  by 
their  development  checked  the  growth  of  the  eye,  as  was  supposed  by  some 
observers. 

Considerable  interest  attaches  to  the  composition  of  the  walls  of  these 
cysts.  All  of  them  seem  to  have  two  coats, — an  outer  one  of  fibrous  tissue 
continuous  with  the  sclerotic,  and  an  inner  of  more  or  less  highly  developed 
retina.  (Fig.  4.)  In  a  case  examined  by  the  authors,2  the  inner  coat  con- 
sisted only  of  bodies  like  those  met  with  in  the  granular  layers  of  the  retina, 


1  Lancet,  May  26,  1888. 

2  Royal  Lond.  Ophth.  Hosp.  Rep.,  vol.  xii.  p.  287. 


422      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 

arranged  in  separate  patches.  In  a  case  examined  by  Gallemaerts,1  and  in 
another  examined  by  one  of  the  authors  (E.  T.  C.),2  in  which  there  were 
two  cysts,  it  consisted  of  a  continuous  layer  of  branching  cells  and  granular 
bodies.  Rindfleisch 3  found  the  inner  wall  of  a  cyst  composed  of  retina 
with  its  layers  well  developed,  pigment  epithelium,  and  an  elastic  lamina. 
The  inner  surface  of  the  retina  was  directed  towards  the  interior  of  the  cyst. 
De  Lapersonne  and  Czermak 4  also  found  fairly  well  developed  retina  lining 
the  interior  of  cysts  connected  with  microphthalmic  eyes,  but  in  their 
specimens  the  outer  surface  of  the  retina  (that  with  rudimentary  rods  and 
cones  on  it)  was  directed  towards  the  interior  of  the  cyst.  The  following 
changes  have  been  met  with  in  the  eyeballs  to  which  these  cysts  are  attached  : 
opacity  and  vascularity  of  a  small  ill-developed  cornea ;  coloboma  of  the  iris ; 
opacity  of,  calcareous  deposit  in,  and  displacement  of,  the  lens ;  absence 
or  only  partial  development  of  the  vitreous ;  a  folded,  rucked  condition  of 
the  retina,  with  imperfect  differentiation  of  its  layers ;  colloid  bodies  among 
the  pigment  epithelial  cells  on  the  inner  surface  of  the  choroid ;  an  absence 
of  the  choroid  in  the  region  of  the  attachment  of  the  cyst ;  nodules  of  car- 
tilage in  the  fibrous  tissue  of  the  sclerotic,  and  a  break  in  the  continuity  of 
this  latter  structure  where  it  is  continuous  with  the  outer  wall  of  the  cyst. 

The  following  theories  have  been  put  forward  to  account  for  the  origin 
of  these  cysts : 

Arlt 5  considers  them  to  be  due  to  increased  intra-ocular  pressure  with 
stretching  of  the  lower  wall  of  the  globe,  weakened  by  the  absence  of  choroid 
and  partial  defect  of  retina  and  sclerotic.  Hess 6  accepts  this  explanation 
for  some  of  his  cases.  Kundrat7  describes  the  cystic  formations  connected 
with  the  lower  wall  of  the  eye  as  due  to  a  projection  of  retinal  tissue  through 
the  fetal  fissure  into  the  mesoblastic  tissue  beneath  the  globe,  this  following 
on  some  defect  in  the  development  of  the  middle  cerebral  vesicle.  Rind- 
fleisch,8 who  examined  a  microphthalmic  eye,  with  cyst  attached,  from  a  six 
to  seven  months'  foetus,  which  had  hydrocephalus  and  the  orbital  roof  pressed 
down  so  that  it  was  convex  below,  thought  that  the  alteration  in  the  orbital 
roof  compressed  the  globe,  so  bringing  about  a  reopening  and  widening  of 
the  choroidal  cleft  and  an  intrusion  of  the  retinal  tissue  into  the  surrounding 
mesoblast.  De  Lapersonne,  to  account  for  the  position  of  the  retina  men- 
tioned above,  suggests  that  it  had  become  secondarily  detached  and  convo- 
luted ;  that  one  of  these  folds  facing  the  ocular  cleft  had  become  pushed  into 
it,  perhaps  by  a  fluid  analogous  to  that  contained  in  retinal  cysts,  and,  yield- 


1  Kyste  congenital  de  la  Paupiere  avec  Microphthalmos.     Bruxelles,  1893. 

2  Trans.  Ophth.  Soc.,  vol.  xiii.  p.  114. 

3  Archiv  fur  Oph.,  Bd.  xxxvii.,  Abth.  3,  S.  192. 

4  Archives  d'Oph.,  t.  xi.  p.  207. 

5  Anzeiger  der  k.  k.  Gesellschaft  der  Aerzte.     Wien,  1885,  No.  17. 

6  Arohiv  fur  Ophth.,  Bd.  xxxvi.,  Abth.  1,  S.  135. 

7  Wiener  Medizin.  Blatter,  Nos.  51,  52  (1885),  and  53  (1886). 

8  Ibid. 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.      423 

ing  to  this  pressure,  was  invaginated  like  a  glove-finger  and  forced  outward 
into  the  cellular  tissue  of  the  orbit. 

The  retina  in  the  human  foetal  eye  is  normally  in  a  folded  condition, 
and  it  seems  possible  to  imagine  that  if  there  was  some  delay  in  the 
closure  of  the  ocular  cleft  and  in  the  development  of  the  vitreous,  the 
mere  continued  growth  of  the  retina  would  tend  to  make  it  protrude 
through  the  cleft. 

BUPHTHALMOS,    HYDROPHTHALMOS   CONGENITUS,  OR  CONGENITAL 

GLAUCOMA. 

The  condition  of  the  eye  which  has  received  the  above  names  is  best 
spoken  of  as  congenital  glaucoma ;  for  the  peculiar  appearances  which  are 
produced  are  all  the  result  of  increased  tension  in  an  eye  the  cornea  and 
sclerotic  of  which  are  thin  and  extensile,  and  which  have  not  become  tough 
and  inelastic  as  they  do  in  later  life.  The  cornea  in  this  disease  is  con- 
siderably enlarged ;  occasionally  there  are  nebulae  in  its  centre,  and  some- 
times the  opaque  white  tissue  of  the  sclerotic  is  prolonged  into  it  for  a  short 
distance  at  the  margins.  The  sclerotic  is  much  thinned,  and  the  pigmented 
tissue  of  the  ciliary  body  shows  through,  giving  it  a  dull  bluish  appearance. 
The  curvature  of  the  cornea  and  sclerotic  is  altered,  so  that  the  front  of  the 
globe  has  a  more  globular  shape  than  normal ;  the  whole  appearance  is 
very  much  that  of  an  ox's  eye ;  hence  the  name  buphthalmos.  The  anterior 
chamber  is  generally  deep,  but  in  a  few  cases  it  is  absent  altogether,  the 
entire  anterior  surface  of  the  iris  being  in  contact  with  the  cornea.  The  iris 
is  much  stretched,  and  often  tremulous ;  the  lens  in  some  cases  is  mobile, 
swaying  backward  and  forward  on  movement  of  the  globe.  The  tension,  as 
a  rule,  is  found  to  be  increased ;  occasionally  cases  are  met  with  in  which 
the  tension  is  increased  and  the  globe  enlarged,  but  in  which  after  a  time 
spontaneous  arrest  of  the  disease  seems  to  occur,  the  tension  for  the  remainder 
of  life  being  normal  and  the  vision  not  further  deteriorating.  The  refrac- 
tion is  usually  myopic,  but  the  amount  of  myopia  is  not  such  as  might  have 
been  expected  from  the  increased  lengthening  of  the  globe.  The  optic  nerve 
is  always  deeply  cupped. 

It  is  easy  to  understand  that  increase  of  tension  will  cause  the  elastic 
coats  of  an  infant's  eye  to  expand  more  than  those  of  an  adult ;  but  what 
does  at  first  seem  difficult  to  explain  is,  why  the  anterior  chamber  should 
be  deep,  and  why  some  of  the  cases  go  on  to  spontaneous  cure ;  for  in  pri- 
mary glaucoma  of  adults  the  anterior  chamber  is,  as  a  rule,  shallow,  and  no 
amelioration  of  the  symptoms  occurs,  if  treatment  is  not  resorted  to. 

The  primary  block  to  the  circulation  of  fluids  in  the  eye  in  primary 
glaucoma  is  now  generally  believed  to  be  at  the  circumlental  space,  an 
obstruction  being  thus  occasioned  to  their  passage  forward  from  the  vitreous 
into  the  posterior  and  anterior  chambers ;  consequently  the  iris  and  lens 
are  pressed  forward  and  the  anterior  chamber  is  made  shallow.  If  the 
primary  block  was  situated,  not  at  the  circumlental  space,  but  at  the  angle 


424      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 

of  the  anterior  chamber,  then  fluid  would  collect  in  the  anterior  chamber, 
and  instead  of  its  being  made  shallow  it  would  be  deepened. 

In  the  article  on  congenital  anterior  synechia  it  is  shown  that,  through 
a  failure  in  the  separation  of  the  anterior  fibre-vascular  sheath  from  the 
back  of  the  cornea,  a  congenital  adhesion  of  the  iris  to  the  cornea  may 
result.  Such  an  adhesion  situated  at  the  periphery  of  the  chamber,  in  the 
region  of  the  ligamentum  pectinatum,  would  prevent  the  passage  of  fluid 
into  the  spaces  of  Fontana.  In  several  buphthalmic  eyes  examined  micro- 
scopically by  one  of  the  authors,  an  adhesion  of  the  periphery  of  the  iris  to 
the  cornea  was  found.  It  is  by  some  supposed  that  it  is  not  possible  to  have 
a  deep  anterior  chamber  and  at  the  same  time  to  have  its  angle  closed  by 
an  adhesion  of  the  root  of  the  iris  to  the  cornea ;  but  in  these  eyes  this  is 
constantly  found  to  be  the  case,  an  abrupt  bend  occurring  in  the  iris  at  the 
point  where  the  adhesion  ceases.  It  can  easily  be  understood  how,  as  the 
globe  enlarged  and  the  anterior  chamber  deepened,  any  adhesions  between 
the  periphery  of  the  iris  and  the  cornea  would  become  stretched,  and  might 
possibly  give  way,  or  at  any  rate  be  so  attenuated  as  to  cease  to  cause  any 
obstruction  to  the  exit  of  the  aqueous  humor  ;  in  such  cases  a  spontaneous 
cure  would  occur.  It  is,  then,  probable  that  congenital  glaucoma  is  due  to 
the  iris  failing  to  become  separated  from  the  back  of  the  cornea  at  the 
periphery  of  the  anterior  chamber,  and  that  the  adhesions  which  are  thus 
left  may  in  some  cases,  as  the  eye  enlarges,  become  much  stretched  and  even 
give  way,  and  a  normal  tension  be  established. 

MULTIPLE   EYES   AND  CYCLOPIA. 

Such  strange  malformations  as  the  occurrence  of  more  than  two  eyes,  or 
the  presence  of  only  one  central  eye  (cyclopia),  are  met  with  in  monsters, 
and  may  be  studied  in  teratological  specimens.  Parts  ending  in  free  ex- 
tremities at  times  bifurcate ;  such  a  condition  in  the  case  of  the  digits  leads 
to  supernumerary  fingers  and  toes.  This  tendency  to  bifurcation  may,  how- 
ever, affect  the  trunk ;  when  it  affects  the  head  end  it  is  spoken  of  as  ante- 
rior dichotomy ;  when  the  opposite  end,  as  posterior  dichotomy.  There  may 
be  various  degrees  of  anterior  dichotomy.  Thus,  one  body  may  have  two 
completely  separate  heads,  and  then,  of  course,  there  would  be  four  eyes. 
Or  the  two  heads  may  be  partially  joined,  when,  if  the  two  median  orbits 
are  quite  distinct,  there  will  still  be  four  eyes.  If  the  two  median  orbits 
are  fused  together,  the  two  median  eyes  may  be  joined  to  a  various  extent : 
sometimes  the  globes  are  united  only  posteriorly,  and  the  two  cornese  are 
quite  distinct ;  in  others  one  large  globe  is  formed,  the  eyelids  of  the  median 
eyes  or  eye  become  united,  and,  having  no  inner  commissures,  bound  a  single 
fissure.  Posterior  dichotomy  of  the  axis  may  extend  forward  into  the  basi- 
sphenoid  region  and  an  accessory  face  be  produced.  In  the  Royal  College 
of  Surgeons  Museum  there  are  two  specimens  of  foetal  pigs  showing  this 
condition.  In  one,  the  two  eyes  of  the  accessory  face  are  fused  together  as 
far  as  the  edges  of  the  irides  ;  in  the  other,  the  eyes  of  the  accessory  face  are 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OP  THE  EYE.      425 

distinct,  but  so  obliquely  placed  that  the  fissures  of  the  eyelids  are  almost  ver- 
tical. There  is  also  a  female  human  foetus  similarly  affected,  in  which  there 
are  four  eyes,  but  the  inner  ones  are  contained  in  a  single  orbit.  The  presence 
of  only  one  central  eye  (cyclopia)  is  due  to  a  deficiency  of  the  axis  in  the 
facial  region.  The  two  orbits  and  the  two  eyes  forming  the  two  sides 
become  fused  into  one.  The  extent  of  the  fusion  of  the  two  eyes  into  one 
varies  considerably ;  a  frontal  proboscis  is  always  present. 

CRYPTOPHTHALMOS. 

This  is  the  name  which  has  been  given  to  a  very  rare  condition,  in  which 
there  is  complete  absence  of  the  eyelids  and  palpebral  fissure,  and  where  the 
skin  passes  continuously  from  the  brow  over  the  surface  of  the  globe  (which 
can  be  seen  moving  beneath  it)  to  the  surface  of  the  cheek.  The  case  of  a 
child  exhibiting  such  a  malformation  has  been  published  by  Zehender,1  with 
an  anatomical  description  of  the  condition  by  Manz.  He  found  that  all  the 
appendages  belonging  to  the  lids  were  absent, — viz.,  eyelashes,  lacrymal 
gland,  and  lacrymal  ducts ;  only  the  muscle  existed.  On  the  left  side  there 
was  a  well-developed  orbicularis  ;  on  the  right  only  part  of  it  could  be  found. 
The  skin  extending  over  the  eyes  was  connected  with  its  surface  by  a  sub- 
cutaneous cellular  material,  and  not  limited  by  a  closed  cavity. 

Van  Duyse 2  has  described  a  similar  case, — a  child  three  weeks  old,  who 
had,  in  addition,  other  abnormalities ;  among  them  imperfect  development 
of  the  parietal  bone,  and  meningo-encephalocele. 

CONGENITAL  ABNORMALITIES  IN  THE  OCULAR 
APPENDAGES. 

EYEBROWS   AND   ORBIT. 

The  eyebrows  may  vary  in  shape,  size,  and  color.  Cases  have  been 
observed  of  complete  absence  of  the  eyebrows  and  eyelashes.  Patches  of 
white  hair  in  a  dark  brow — piebald  eyebrows — are  sometimes  seen.  In 
albinos  the  eyebrows  and  eyelashes  are  quite  white ;  the  latter  thin,  long, 
and  silky. 

Dermoid  cysts  in  connection  with  the  eyebrows  or  the  lids  are  by  no 
means  uncommon  ;  they  have  been  met  with  deeply  situated  in  the  orbit,  in 
which  case  it  is  very  difficult  to  diagnose  them  from  other  orbital  growths, 
and  also  to  remove  them  completely.  These  dermoid  cysts  occur  in  the 
course  of  the  foetal  orbito-nasal  fissure,  and  are  most  frequent  at  the  outer 
angle  of  the  brow.  They  lie  beneath  the  orbicularis  muscle  and  in  contact 
with  the  pericranium  of  the  frontal  bone,  which  may  be  hollowed  out  under 
them.  To  the  touch  they  feel  firm,  smooth,  and  rounded,  and  they  roll 
beneath  the  finger  on  the  bone.  They  vary  in  size,  but  are  not  often  much 
larger  than  a  cherry.  Dermoid  cysts  at  the  inner  angle  of  the  orbit  are 

1  Proceedings  of  Fourth  International  Ophthalmological  Congress,  p  86. 

2  Annales  d'Oculistique,  1889,  t.  x.  p.  69. 


426      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 

more  rarely  met  with ;  a  connection  may  exist  between  them  and  the  dura 
mater, — a  circumstance  which  it  is  well  to  bear  in  mind  in  order  that  they 
may  not  be  confused  with  meningoceles. 

Power  1  recently  recorded  the  case  of  an  infant  with  rnicrocephalus,  in 
which  the  orbits  were  extremely  small,  measuring  only  two  and  one-half 
centimetres  from  the  inferior  border  to  the  foramen  opticus  ;  their  floors 
were  horizontal  and  their  roofs  almost  vertical.  The  eyes  naturally  were 
exceedingly  proptosed. 

Epicanthus  is  a  name  which  was  first  applied  by  von  Ammon  to  a  con- 
dition in  which  there  is  an  excess  of  skin  between  the  two  eyes  about  the 
root  of  the  nose,  so  that  crescentic  folds  of  it  overlap  the  inner  canthi  and 
part  of  the  palpebral  aperture.  It  is  generally  due.  to  some  defect  in  the 
development  of  the  bridge  of  the  nose.  In  the  Mongolian  race,  who  have 
no  bridges  to  their  noses,  slight  epicanthus  appears  to  be  the  normal  con- 
dition. Among  European  children,  before  the  bridge  of  the  nose  develops, 
a  tendency  to  it  is  frequently  seen,  which  disappears  as  they  grow  older. 
One  of  the  most  frequent  causes  of  defect  of  development  of  the  bridge 
of  the  nose  is  congenital  syphilis  :  hence,  as  might  have  been  expected,  a 
history  of  this  affection  can  frequently  be  obtained  in  cases  of  epicanthus. 

Epicanthus  is  sometimes  associated  with  congenital  ptosis.  In  severe 
cases  the  fold  of  skin  may  extend  as  far  over  the  eye  as  the  inner  margin 
of  the  cornea ;  the  patient  then  appears  very  much  as  though  he  had  an 
internal  squint.  Frequently  a  mother  brings  her  child  complaining  that  it 
squints,  when  really  it  has  epicanthus. 

CONGENITAL   ABNORMALITIES   OF   THE   LACRYMAL   APPARATUS. 

Malformations  of  the  Canaliculi. — These  are  the  commonest  forms  of 
abnormalities  met  with  in  connection  with  the  lacrymal  apparatus.  The 
canal iculi  are  sometimes  represented  by  a  groove  the  edges  of  which  have 
failed  to  become  united  in  the  ordinary  way,  or  the  edges  of  the  groove  may 
have  united  in  only  a  part  of  their  extent ;  there  are  then  two  openings  into 
one  canaliculus.  Occasionally  one  or  more  of  the  canaliculi  may  be  com- 
pletely absent,  or  one  or  more  of  the  puncta  may  not  be  patent.  Steffan 2 
has  described  a  patient  who  had  a  second  punctum  a  line  below  and  to  the 
outer  side  of  the  normal  one ;  it  was  uncertain  whether  it  opened  into  the 
sac  or  into  the  other  canaliculus. 

In  several  cases  of  anophthalmos,  defects  of  the  canaliculi  have  been 
met  with. 

Fistula  of  the  Lacrymal  Sac. — Defects  in  the  closure  of  the  groove 
which  ultimately  develop  into  the  lacrymal  sac  and  duct  are  exceedingly 
rare.  Cases  of  it  have  been  recorded  by  G.  Beer,  Scarpa,3  Hartridge,4 

1  Trans.  Ophth.  Soc.,  vol.  xiv.  p.  212. 

2  Klin.  Monatsblatt,  1866,  S.  45. 

3  Traite  complet  d'Ophtbalmologie,  vol.  iv.  p.  1103. 

4  Trans.  Ophth.  Soc.,  vol.  xii.  p.  172. 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.       427 

Agnew,1  and  Casey  Wood;2  in  all  of  which  the  opening  into  the  lacrymal 
sac  was  exceedingly  small,  and  symmetrical  on  the  two  sides. 

Stricture  of  the  Lacrymal  Duct. — Cases  of  this  are  sometimes  met  with 
as  a  congenital  defect. 

Malformations  of  the  Caruncle. — The  caruncle  may  be  the  seat  of  two 
forms  of  congenital  growth  :  both  are  very  rare.  1.  There  may  be  small 
dermoid  tumors,  of  which  an  excellent  example  is  pictured  by  Demours,3 
with  hairs  growing  from  it.  Microscopically,  these  growths  are  seen  to  be 
precisely  similar  to  the  dermoids  met  with  so  much  more  frequently  at  the 
sclero-corneal  margin.  2.  There  may  be  small  vascular  growths  of  a 
bright-red  color,  microscopical  sections  of  which  show  numerous  thin-walled 
blood-vessels  cut  in  various  directions,  with  a  small  quantity  of  loose 
fibrous  tissue  between  them. 

Abnormalities  of  the  Lacrymal  Gland. — Morton  4  has  recorded  the  case 
of  a  girl,  aged  six,  who  never  shed  any  tears  with  her  right  eye,  though 
they  flowed  copiously  from  the  left  whenever  she  cried. 

The  secretion  of  the  lacrymal  gland  has  also  been  noticed  to  be  absent 
in  some  cases  of  anophthalmos.  It  must  be  borne  in  mind  that  absence  of 
the  lacrymal  secretion  does  not  necessarily  imply  absence  of  the  gland. 
No  child  sheds  tears  during  the  first  few  days  of  life;  and  it  is  quite 
possible  that,  though  the  gland  were  present,  its  function  might  never  be 
established. 

EYELIDS. 

Both  eyelids  may  be  congenitally  absent ;  the  conjunctival  sac  then  fails 
to  develop,  and  the  front  of  the  eye  remains  covered  with  skin.     This  con- 
dition has  already  been  spoken 
of  under  Cryptophthalmos. 

Coloboma  of  the  Eyelid. — A 
large  portion  of  one  of  the  eye- 
lids may  be  congenitally  absent, 
or  the  lid  may  be  divided  into 
two  parts  by  a  fissure  of  greater 
or  less  extent :  such  cases  are 
spoken  of  as  colobomata  of  the 
eyelid. 

Dor6  and  Nicolin  collected 
forty-six  recorded  cases  of  this 

Double  congenital  colobomaof  the  right  upper  lid  abnormality,  and  twelve  of  ob- 

in  a  boy  aged  twelve  years.  *  '          •      - 

lique  fissures  of  the  face,  accom- 
panied by  partial  coloboma  of  the  lid.    Of  the  forty-six  cases,  twenty-seven 

1  Trans.  Am.  Ophth.  Soc..  1874,  p.  209. 

2  Arch,  of  Ophth  ,  vol.  xxii.  p.  25. 

3  Maladies  des  Yeux,  1818,  pi.  Ixiv.,  fig.  1. 

4  Trans.  Ophth.  Soc.,  vol.  iv.  p.  350. 

5  Kevue  Generale  d'Ophth.,  1888,  p.  529. 


428      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 


FIG.  6. 


had  only  one  lid  affected,  two  had  the  same  eyelids  of  the  same  eye,  sixteen 
had  one  lid  of  each  eye,  and 
one  had  all  four  lids  involved. 
The  gap  in  the  lid  is  usually 
situated  to  the  inner  side  of 
the  middle  line,  and  when  a 
portion  of  the  lid  is  absent  it 
is  usually  the  inner  part  of  the 
upper  one.  The  defect  may 
extend  from  the  palpebral  to 
the  orbital  margin  (Figs.  5 
and  6),  or  only  be  a  small  in- 
dentation of  the  free  border  of 
the  lid.  (Fig.  7.)  The  shape 
of  the  gap  left  is  either  trian- 
gular or  quadrate ;  when  the  former,  the  base  of  the  triangle  is  at  the  free 
margin  of  the  lid. 

This  abnormality  is  often  found  associated  with  other  congenital  defects, 

FIG.  7. 


Congenital  coloboma  of  the  left  upper  lid  in  a  boy  aged 
two  years. 


Small  congenital  coloboma  of  the  right  upper  lid  in  a  girl. 

such  as  dermoid  growths  of  the  cornea,  subconjunctival  fatty  growths,  hare- 
lip, cleft  palate,  and  supernumerary  auricles. 

Nicolin  and  Dor  have  offered  the  most  probable  explanation  of  its  cause. 
They  consider  it  to  be  due  to  an  imperfect  closure  of  the  oblique  facial 
fissure,  the  persistence  of  the  upper  end  of  which  causes  the  lid  to  develop 
in  two  parts. 

Anchyloblepharon,  or  adhesion  of  the  lids,  might  have  been  expected  to 
be  a  more  common  congenital  anomaly  than  it  is,  seeing  that  the  edges  of 
the  lids  are  united  during  several  months  of  foetal  life.  This  adhesion, 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.      429 

however,  is  only  a  cementing  together  of  the  epithelium.  In  anchylobleph- 
aron  the  adhesion  is  composed  of  vascularized  tissue ;  it  is  probable  that  it 
results  from  an  inflammation  of  the  margins  of  the  lids  while  they  are  in 
contact.  It  has  been  met  with  in  several  of  the  cases  recorded  under  the 
name  anophthalmos. 

Symblepharon,  or  adhesion  of  the  eyelid  to  the  globe  of  the  eye,  is 
exceedingly  rare  as  a  congenital  affection. 

Lagophthalmos  and  Congenital  Shortness  of  the  Lid. — Incomplete  closure 
of  the  palpebral  fissure  when  the  lids  are  shut  is  spoken  of  as  lagophthalmos. 
When  this  condition  is  congenital,  it  is  due  to  an  abnormal  shortness  of  the 
upper  lid.  The  lids  can  be  brought  together  by  a  strong  contraction  of  the 
orbicularis  muscle,  but  on  gentle  closure  a  gap  is  left ;  they  remain  also 
separated  during  sleep,  but  the  cornea  is  not  exposed,  for  on  shutting  the 
lids  the  eyeball  rolls  upward. 

Many  varieties  exist  in  the  size  and  direction  of  the  palpebral  aperture, 
constituting  racial  peculiarities  which  need  not  be  treated  of  here.  An  ab- 
normal smallness  of  the  palpebral  aperture — blepharophimosis — is  common 
in  cases  of  microphthalmos  and  anophthalmos. 

Congenital  Growths  of  the  Eyelids. — The  lids  may  be  the  seat  of  con- 
genital growths,  such  as  moles,  mevi,  and  cysts.  The  nsevi  may  be  either 
lymphatic  or  vascular.  Both  forms  tend  to  increase  in  size  after  birth. 
Lymphatic  nsevi  are  rare ;  at  times  they  are  very  large,  extending  into  the 
orbit  and  involving  the  conjunctiva. 

The  vascular  nsevi  may  be  superficial  or  deep,  and  telangiectatic  or 
cavernous.  A  further  description  of  them  is  not  necessary  in  this  article. 

Dermoid  cysts  of  the  eyelids,  and  cysts  in  connection  with  microph- 
thalmic  eyes,  which  extend  into  the  lower  lid,  have  already  been  treated  of. 

Ectropion  of  the  eyelid  is  an  exceedingly  rare  congenital  affection.  It 
has  been  met  with  in  a  few  cases  associated  with  other  congenital  ab- 
normalities affecting  the  eye,  such  as  microphthalmos  (von*  Ainmon)  and 
buphthalmos  (Marcus  Gunn). 

Congenital  entropion  and  trichiasis  are  more  frequently  met  with  ;  several 
examples  of  the  latter,  affecting  the  lower  lid,  are  recorded  in  a  recent  paper 
by  Sydney  Stephenson.1  They  have  been  attributed  to  inflammation  and 
to  defective  development  of  the  tarsus. 

CONGENITAL   DEFECTS  OF   THE   MOVEMENTS  OF   THE   EYE  AND  EYELIDS. 

Ptosis,  or  drooping  of  the  upper  lid,  is  a  common  congenital  affection ; 
it  is  almost  always  bilateral,  and  is  met  with  in  very  various  degrees.  In 
the  most  complete  cases  patients  present  a  very  characteristic  appearance. 
Besides  the  drooping  of  the  lids  and  the  obliteration  of  the  palpebral  folds, 
the  forehead  is  much  wrinkled,  due  to  the  contraction  of  the  anterior  portion 
of  the  occipito-frontalis  muscle,  which  tends  slightly  to  raise  the  lids.  The 

1  Trans.  Ophth.  Soc.,  vol.  xiv.  p.  13. 


430      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OP  THE  EYE. 

head  is  also  thrown  back  and  the  eyes  are  rotated  down,  by  which  means, 
when  the  patient  is  looking  straight  forward,  a  portion  of  the  pupil  is 
brought  opposite  the  palpebral  aperture. 

Of  the  ocular  muscles,  the  internal  recti  are  the  most  frequently  affected. 
The  superior,  inferior,  and  external  recti  and  the  superior  oblique  have  also 
been  found  at  fault.  Often  two  or  more  muscles  are  found  affected  on  the 
same  side.  Congenital  ptosis  and  congenital  defect  of  the  superior  rectus 
not  uncommonly  go  together.  The  subjects  of  these  anomalies  do  not,  how- 
ever, usually  present  any  malformations  in  other  parts  of  the  body.  In 
many  of  them  it  has  been  observed  that  the  optic  axes  were  not  parallel, 
but  in  none  was  diplopia  a  symptom. 

The  condition  is  not  infrequently  hereditary.  It  is  generally  due  to  a 
developmental  defect  of  the  muscles  ;  in  a  few  exceptional  cases  in  which 
there  were  gross  changes  in  the  nervous  system  it  was  attributed  to  an 
anomaly  of  the  nerves  supplying  the  muscles. 

The  muscles  may  be  either  completely  absent,  badly  developed,  too 
short,  or  inserted  at  an  unusual  position  on  the  globe.  (Henck l  and  Law- 
ford.2)  It  is  not  always  possible  to  diagnose  the  precise  nature  of  the 
defect  from  the  movements  of  the  globe. 

Nystagmus,  or  an  oscillatory  movement  of  the  two  eyes,  is  commonly 
met  with  in  cases  in  which  there  is  a  congenital  defect  of  the  sight.  It  is 
very  common  in  connection  with  congenital  cataract  and  albinism.  The 
movements  of  the  eyes  vary  very  much  in  direction  and  in  rapidity  in 
different  cases.  By  far  the  most  usual  direction  for  the  motion  to  occur  in 
is  from  side  to  side ;  it  may,  however,  be  vertical  from  above  downward, 
or  rotatory ;  sometimes  rotation  is  combined  with  a  lateral  motion.  The 
rapidity  with  which  the  eyes  move  may  be  so  great  that  the  outline  of  the 
cornea?  can  barely  be  distinguished,  or  it  may  be  a  slow,  twitching,  and  easily 
overlooked  motion.  In  congenital  nystagmus,  though  the  eyes  are  moving, 
the  patient  never  complains  that  the  object  to  which  his  vision  is  directed 
does  not  appear  stationary. 

CONGENITAL   ABNORMALITIES   OF   THE   CONJUNCTIVA. 

The  conjunctiva  may  be  the  seat  of  certain  congenital  growths.  The 
most  frequent  of  these  are  the  dermoid  tumors,  which  may  be  partly  corneal 
and  partly  conjunctival ;  they  are  dealt  with  at  length  under  the  abnor- 
malities of  the  former  structure.  Congenital  growths  very  similar  to  these 
dermoids  in  structure,  and  sometimes  associated  with  them,  are  met  with  in 
the  outer  angle  of  the  upper  conjunctival  cul-de-sac  ;  they  are  spoken  of  as 
subconjunctival  fatty  growths.  When  small,  they  remain  concealed  beneath 
the  upper  lid  ;  when  large,  they  project  into  the  palpebral  fissure.  Micro- 
scopically, the  epithelium  over  them  is  found  thick  and  laminated,  and  the 

1  Centralblatt  fur  Augenheilk.,  1881,  S.  335. 

2  Trans.  Ophth.  Soc.,  vol.  viii.  p.  262. 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OP  THE  EYE.      431 

mass  itself  is  seen  to  be  composed  of  fibrous  and  fatty  tissue.  Pigmented 
patches  similar  to  the  moles  of  the  skin  are  occasionally  met  with  in  the  con- 
junctiva ;  most  often  in  persons  who  have  multiple  moles  of  the  face.  They 
must  be  distinguished  from  the  congenital  pigmentation  of  the  sclerotic 
which  is  so  frequent  in  some  animals  that  it  may  be  considered  the  normal 
condition,  and  in  man  is  not  an  uncommon  abnormality  about  the  seat  of 
the  anterior  perforating  arteries. 

The  other  forms  of  congenital  growths  met  with  in  connection  with  the 
conjunctiva  are  naevi,  vascular  and  lymphatic,  and  tumors  composed  of 
well-developed  bone-tissue,  which  belong  to  the  class  of  teratomata ;  they 
are  usually  situated  beneath  the  conjunctiva,  between  the  outer  margin  of 
the  cornea  and  the  external  canthus. 

CONGENITAL  ABNORMALITIES  OF  THE  CORNEA. 

The  cornea  may  be  congenitally  defective  as  regards  its  transparency,  its 
size,  and  its  shape.  It  may  also  be  the  seat  of  a  congenital  fleshy  growth. 

OPACITIES   OF   THE   CORNEA. 

A  congenital  opacity  of  the  cornea  may  be  complete  or  partial.  In 
many  of  the  cases  of  complete  opacity,  as  in  some  recorded  by  Farar,1  the 
cornea  is  enlarged,  and  not  only  the  cornea,  but  the  whole  globe,  consti- 
tuting the  condition  termed  congenital  hydrophthalmos  or  buphthalmos, 
which  has  already  been  described.  Complete  opacity  of  the  cornea  is  also 
met  with  in  connection  with  microphthalmos.  In  two  cases  of  this  sort 
which  we  examined  microscopically  we  found  that  the  anterior  elastic 
lamina  was  absent,  and  that  the  anterior  layers  of  the  cornea  did  not  present 
their  usual  laminated  arrangement,  but  crossed  each  other  in  an  irregular 
way,  whilst  coursing  among  them  were  numerous  blood-vessels. 

Partial  congenital  opacities  of  the  cornea  present  various  degrees  of  in- 
tensity, sometimes  being  densely  white,  at  other  times  only  a  faint  haze. 
They  may  be  situated  in  any  part.  Two  forms  which  present  special  char- 
acteristics have  had  names  applied  to  them.  One  of  these  is  dense  white  in 
color  and  situated  at  the  corneal  margin,  looking  as  though  a  portion  of 
the  sclerotic  had  been  prolonged  inward  into  the  cornea :  Keiser  termed  it 
sclerophthalmos.  The  other  is  a  ring  of  opacity  situated  in  the  periphery 
of  the  cornea,  closely  resembling  an  arcus  senilis ;  the  ring,  however,  is 
usually  more  complete  than  in  the  latter.  The  condition  looks  very  like 
microcornea,  with  which  it  is  sometimes  associated.  It  may  be  distinguished 
from  it  by  the  presence  of  a  diaphanous  ring  between  the  margin  of  the 
cornea  and  the  opacity.  Wilde 2  speaks  of  it  as  arcus  juvenilis  ;  Sybil,  as 
macula  arcuata. 


1  Medical  Communications,  vol.  ii.  p.  463,  1790. 

2  Malformations  and  Congenital  Diseases  of  the  Organs  of  Sight,  1862. 


432      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 
VARIATIONS   IN   SIZE   OF  THE   CORNEA. 

The  cornea  is  enlarged  (macrocornea)  in  buphthalmic  eyes ;  it  is  fre- 
quently abnormally  small  (microcoruea)  in  microphthalmic  ones.  It  also 
sometimes  has  less  than  its  normal  diameter  in  eyes  otherwise  perfectly 
healthy. 

CONICAL   CORNEA. 

The  cornea  may  be  altered  so  that,  instead  of  presenting  its  usual  cur- 
vature, it  becomes  dome-shaped  or  conical ;  the  apex  of  the  cone  may  be 
in  the  centre,  but  usually  it  is  a  little  below.  Dissection  of  such  eyes 
has  shown  that  the  substance  of  the  cornea  is  thinned  in  that  situation. 
(Middlemore,1  Hulke.2) 

Some  cases  of  conical  cornea  are  certainly  congenital,  though  in  many 
the  defect  does  not  appear  until  adult  life.  Tweedy,3  however,  holds  that 
in  these  cases  also  there  is  some  latent  embryological  defect  which  predis- 
poses the  cornea  to  alter  its  shape  in  the  way  it  does. 

DERMOID   GROWTHS   OF   THE   CORNEA. 

Fleshy  growths  of  the  eye  may  be  divided  into  two  classes :  those  which 
are  entirely  confined  to  the  globe,  and  those  in  which  a  cuticular  band  passes 
between  the  surface  of  the  globe  and  the  brow  or  some  other  part  of  the 
face. 

The  first  variety  are  situated  generally  at  the  lower  and  outer  margin  of 
the  cornea,  springing  partly  from  the  cornea  and  partly  from  the  sclerotic. 
They  are  most  often  single,  and  present  in  one  eye  only ;  they  may,  how- 
ever, be  symmetrically  placed  on  the  two  eyes,  or  one  eye  may  have  two 
tumors,  one  on  each  side  of  the  cornea.  Their  size  is  very  variable :  they 
are  sometimes  smaller  than  the  head  of  a  pin,  at  other  times  they  are  so  large 
as  to  protrude  between  the  lids  and  prevent  their  closure.  They  are  often 
small  at  birth  and  enlarge  about  puberty ;  it  is  at  this  period  of  life  that 
the  surgeon  is  most  often  consulted  respecting  them,  partly  because  their 
increase  in  size  renders  them  more  conspicuous,  and  partly  because  at  this 
time  hairs  frequently  commence  to  grow  from  them  which  give  rise  to  some 
conjunct! val  irritation.  Wardrop4  describes  a  case  in  which  upwards  of 
twelve  long  and  strong  hairs  grew  from  the  middle  of  the  tumor  and, 
passing  between  the  eyelids,  hung  over  the  cheek.  These  hairs  did  not 
commence  to  appear  until  the  patient  reached  his  sixteenth  year,  at  which 
time  also  his  beard  began  to  grow. 

Dermoid  tumors  on  the  eye  may  be  conical,  flat,  or  pedunculated  ;  they 
usually  have  an  oval  base,  the  long  axis  of  which  always  corresponds  with 
the  palpebral  aperture.  They  are  firm  in  consistence  and  of  a  yellowish- 

1  Treatise  on  Diseases  of  the  Eye  and  its  Appendages,  vol.  i.  p.  532. 

2  Royal  Lond.  Ophth.  Hosp.  Rep.,  vol.  ii.  p.  155. 

3  Trans.  Ophth.  Soc.,  vol.  xii.  p.  67. 

4  Morbid  Anatomy  of  the  Human  Eye,  1834. 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.      433 

pink  color ;  fine  blood-vessels  are  sometimes  visible  on  their  surface.  They 
have  been  met  with  in  dogs,  horses,  oxen,  and  sheep,  as  well  as  in  man.  A 
specimen  contained  in  the  Royal  College  of  Surgeons  of  England  shows 
one  in  a  sheep's  eye,  from  which  a  tuft  of  wool  is  sprouting. 

The  second  form  of  dermoid  growths  of  the  eye  is  much  less  common 
than  the  first ;  among  the  few  which  have  been  recorded  may  be  mentioned 
those  of  van  Duyse  *  and  Polaillon,2  in  which  a  fleshy  band  passed  from 
the  surface  of  the  cornea  downward  and  inward  to  the  skin  at  the  margin 
of  the  inner  commissure.  In  a  case  of  Manz's  there  was  a  coloboma  of  the 
lid  upward,  and  a  band  presenting  all  the  characters  of  skin  stretched  from 
the  brow  to  the  cornea.  Burns  3  described  a  foetus  in  which  two  cuticular 
bands  passed  from  the  centre  of  each  cornea  and  joined  into  a  larger  one, 
which  ended  in  a  broken  extremity.  Picque 4  found  that  out  of  ninety-four 
cases  of  dermoid  growths  of  the  eye  twenty-seven  were  complicated  with 
some  other  abnormality.  The  most  important  and  most  frequent  of  these  was 
coloboma  of  the  lid.  When  a  dermoid  tumor  exists  in  association  with  colo- 
boma of  the  lid,  it  is  found  that  it  corresponds  to  the  gap  in  the  latter,  so  that 
when  the  lids  are  closed  it  exactly  fills  it.  The  other  complications  that  have 
been  met  with  are  :  of  the  eye,  microphthalmia  and  coloboma  of  the  iris  and 
choroid  ;  of  other  parts  of  the  body,  preauricular  tumors,  macrostoma,  hare- 
lip, absence  of  the  external  auditory  meatus,  and  syndactylism.  Microscopi- 
cally, dermoid  growths  are  seen  to  be  composed  of  fibrous  tissue  often  mixed 
with  adipose  tissue ;  this  is  covered  by  laminated  epithelium,  the  surface 
cells  of  which  are  scaly  and  devoid  of  nuclei,  like  those  on  the  skin,  and 
unlike  those  on  the  surface  of  the  cornea  and  conjunctiva.  Hair-follicles, 
sweat-glands,  and  sebaceous  glands  are  met  with,  also  glands  similar  in  char- 
acter to  the  glands  of  Moll,  in  the  eyelid.  Blood-vessels  course  through  the 
growths,  and  nerves  have  been  demonstrated  in  them.  Several  explanations 
of  the  mode  of  formation  of  these  dermoid  growths  of  the  eye  have  been 
offered.  The  epibulbar  variety  is,  we  think,  best  explained  by  the  theory 
put  forward  by  Rhyba.5  He  pointed  out  that  on  all  parts  of  the  surface 
of  the  body  which  remain  exposed,  skin  is  formed,  and  that  when  a  part 
becomes  covered,  as  the  eye  is  by  the  lids,  a  mucous  membrane  is  developed. 
He  believes  that  when  a  dermoid  growth  occurs  in  consequence  of  imper- 
fect or  delayed  closure  of  the  lids,  a  portion  of  what  should  be  conjunctiva 
remains  exposed  and  assumes  the  character  of  skin.  The  cases  in  which 
cuticular  bands  are  formed  are  probably  due,  as  suggested  by  van  Duyse, 
to  adhesions  forming  between  the  inner  layer  of  the  amnion  and  the  surface 
of  the  embryo. 

1  Ann.  Soc.  M6d.  de  Gand,  1882,  p.  141. 

2  Bull,  de  la  Soc.  Anat.,  1874,  p.  12. 

3  Handb.  der  Prakt.  Chir.,  vol.  i.  p.  262,  1859. 

4  Anomalies  de  Developpement  et  Maladies  congenitales  du  Globe  de  I'QSil.     These, 
Paris,  1886,  pp.  356-420. 

5  Prager  Vierteljahrschrift,  vol.  x.  p.  3,  1853. 
VOL.  I.— 28 


434      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 

CONGENITAL  ABNORMALITIES  OF   THE  IRIS. 

The  iris  may  vary  as  to  its  color  and  present  congenital  defects  in  the 
condition  of  the  pupil,  in  the  persistence  of  a  portion  of  the  pupillary 
membrane,  in  being  tremulous  (iridodonesis),  and  in  a  deficiency  of  the 
whole  (irideremia)  or  part  of  its  structure  (coloboma). 

VARIATIONS   IN   THE   COLOR   OF   THE   IRIS. 

The  iris  presents  many  variations  with  regard  to  its  color.  Pigment 
is  located  in  it  in  two  parts :  (1)  in  the  epithelial  layers  on  its  posterior  sur- 
face ;  (2)  in  the  branching  cells  of  its  stroma,  chiefly  those  in  the  anterior 
part.  When  no  pigment  is  present  in  this  latter  position  the  iris  is  blue, 
when  only  a  slight  amount  it  is  green,  and  when  in  large  quantities  brown, 
or  even  almost  black  in  the  negro .  races.  Absence  of  pigment  from  the 
epithelial  layers  is  always  associated  with  absence  of  pigment  in  the  stroma 
and  in  the  other  parts  of  the  body  where  it  is  usually  present,  constituting 
the  condition  known  as  albinism.  Pigment  is  deposited  in  the  epithelial 
layers  very  early  in  foetal  life,  but  does  not  make  its  appearance  in  the 
mesoblastic  tissue  of  the  iris  until  after  birth.  The  iris  tissue  at  this  time 
is  very  thin,  and  for  these  two  reasons  all  babies'  eyes  are  for  the  first  few 
weeks  after  birth  of  the  same  grayish  hue.  Irregularities  may  occur  in  the 
deposition  of  pigment  in  the  iris,  sometimes  only  temporary,  at  other  times 
persisting  throughout  life.  A  sector  of  the  iris  may  remain  blue,  whilst 
the  rest  of  it  is  brown ;  or  patches  of  brown  may  be  scattered  about  in  a 
blue  iris,  forming  what  is  termed  a  piebald  iris.  In  some  individuals  the 
irides  of  the  two  sides  are  of  different  colors  (heterochromia).  W.  G.  Sym l 
has  shown  that  when  this  is  the  case  the  parents  of  the  individual  are 
usually  of  different  complexion.  His  cases  also  seem  to  show  a  greater 
liability  of  the  blue  over  the  brown  eye  to  disease. 

Little,  dark,  raised  patches  are  occasionally  met  with  in  the  iris  com- 
parable to  moles  of  the  skin.  Microscopically,  they  are  found  not  to  be 
simply  a  deeply  pigmented  patch,  but  to  consist  also  of  a  group  of  round 
and  branching  cells.  They  are  liable  to  be  the  starting-points  of  a  mela- 
notic  sarcoma,  but  this  is  a  very  rare  disease  of  the  iris. 

Congenital  darkly  pigmented  nodules  at  the  pupillary  margin  of  the 
iris  are  sometimes  met  with,  due  to  extension  forward  to  an  abnormal  ex- 
tent of  the  two  layers  of  uveal  pigment  which  line  its  posterior  surface. 
Such  an  extension  forward  of  the  pigment  epithelium  is  the  normal  con- 
dition in  the  horse's  eye.  The  condition  is  termed  ectropion  of  the  uveal 
pigment.  A  good  case  of  it  is  pictured  and  described  by  Wicherkiewicz.2 

Complete  or  nearly  complete  absence  of  pigment  from  the  eye  occurs, 
as  has  been  stated,  in  connection  with  absence  of  pigment  from  other  parts 

1  Ophth.  Review,  vol.  viii.  p.  202. 

3  Archiv  fur  Ophth.,  Bd.  xxxrii.,  Abth.  1,  S.  204. 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.       435 

of  the  body,  in  the  condition  known  as  general  or  universal  albinism,  and 
persons  so  affected  are  spoken  of  as  albinos.  In  them  the  skin  has  a 
peculiar  pinkish-white,  transparent  appearance ;  the  hair  may  be  perfectly 
white  or  of  a  faint  straw  color ;  sometimes  there  is  a  fine,  white,  downy  hair 
all  over  the  body.  The  eyelashes  are  long,  fine,  white,  and  silky.  The 
eyes  appear  pink,  a  red  reflex  being  seen  from  the  pupil  in  ordinary  day- 
light. This  is  due  to  the  non-absorption  of  light  by  the  retinal  epithelium 
on  account  of  the  absence  of  pigment  in  it,  and  to  light  entering  the  eye 
through  its  tunics.  A  pinkish  color  is  often  seen  through  the  iris  tissue, 
which  in  these  cases  is  generally  of  a  grayish  hue.  The  excess  of  luminous 
rays  entering  the  eye  leads  to  defect  of  vision  and  intolerance  of  light  by 
the  retina ;  consequently,  individuals  thus  affected  are  found  invariably  to 
screw  up  the  eyes  and  to  go  about,  especially  in  a  bright  light,  with  their 
lids  half  closed.  They  frequently  have  nystagmus,  or  oscillation  of  the 
eyes,  and  are  usually  amblyopic. 

Ophthalmoscopically,  owing  to  the  absence  of  the  pigment,  the  ramifi- 
cations of  the  choroidal  blood-vessels  are  well  seen. 

Little  is  known  as  to  the  cause  of  albinism.  It  is  sometimes  hereditary ; 
several  children  of  the  same  parents  may  be  affected,  or  it  may  affect  mem- 
bers of  different  generations.  It  is  also  found  more  frequently  in  some 
races  of  mankind  than  in  others ;  it  is  common  among  negroes.  Insalu- 
brious conditions  of  climate  and  hygiene  are  supposed  to  favor  its  develop- 
ment. It  is  met  with  very  frequently  in  such  parts  as  the  west  coast  of 
Africa. 

ABNORMALITIES   OF   THE   PUPIL. 

The  pupil  may  present  several  congenital  abnormalities.  It  may  be 
altered  in  its  position  (corectopia) ;  in  its  size,  being  abnormally  small 
(microcoria) ;  in  its  shape  (discoria) ;  and,  finally,  instead  of  one  pupil 
there  may  be  several  (polycoria). 

Corectopia. — It  is  by  no  means  infrequent  to  find  the  pupil  in  one  or 
both  eyes  slightly  excentric,  but  the  more  extreme  cases  of  corectopia  are 
only  rarely  met  with ;  these  are  often  associated  with  ectopia  of  the  lens. 
A  displaced  pupil  is  frequently  small,  inactive,  and  sometimes  not  regularly 
circular.  The  direction  in  which  it  is  displaced  is  usually  upward  and 
outward. 

Microcoria. — As  just  mentioned,  a  displaced  pupil  is  frequently  small ; 
other  cases  of  microcoria  are  the  result  of  fatal  iritis  and  the  formation  of 
posterior  synechise.  A  pupil  may  be  of  the  normal  size  and  yet  not  dilate 
well  on  the  application  of  a  mydriatic ;  such  a  condition  is  not  uncom- 
monly associated  with  congenital  cataract. 

Discoria,  or  abnormality  in  the  shape  of  the  pupil  without  any  absence 
of  the  iris  tissue,  may,  like  microcoria,  be  the  result  of  posterior  synechiae 
following  foetal  iritis.  It  may  also  occur  from  persistence  of  portions  of 
the  pupillary  membrane ;  sometimes  from  this  cause  the  margin  of  the 
pupil  is  toothed  and  the  pupil  itself  is  star-shaped.  (Fig.  9a.) 


436      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 


Polycoria,  or  the  condition  in  which  there  is  more  than  one  opening  in 
the  iris,  is  but  seldom  met  with.  No  well-authenticated  case  has  been  re- 
corded in  which  more  than  one  opening  surrounded  by  a  sphincter  muscle 
existed,  though  there  are  several  in  which  the  size  of  the  additional  openings 
was  affected  by  myotics  and  mydriatics,  due  to  the  contraction  or  dilatation 
of  the  sphincter  surrounding  the  normal  pupil  altering  the  condition  of  the 
adjacent  iris. 

Cases  of  polycoria  may  be  divided  into  the  four  following  distinct  classes : 

1.  Those  in  which  the  normal  pupil  is  divided  into  two  by  the  per- 
sistence of  a  band  of  the  pupillary  membrane.     A  good  example  of  this 
variety  has  been   figured  by  Wilde,  in  which  a  band  passing  vertically 
across  the  normal  opening  produced  a  figure-of-eight  pupil,  both  sections 
of  which  acted  to  light. 

2.  Cases  which  may  be  termed  "  coloboma  with  a  bridge," — that  is, 
cases  of  complete  or  partial  coloboma  of  the  iris  in  which  a  band  of  tissue, 
probably  a  portion  of  the  pupillary  membrane,  stretches  across  the  opening 
and  divides  it  into  two.     Examples  of  this  are  recorded  by  von  Ammon 
and  Saemisch. 

3.  A  variety  of  which  Mittendorf  *  has  described  two  cases,  father  and 
daughter.     One  of  them  had  in  one  eye  five  pupils, — the  central  normal 
one,  which  was  oval,  and  four  others,  situated  at  the  periphery  of  the  iris, 
conical  in  shape,  with  their  bases  at  the  margin  of  the  cornea.     The  other 
had  in  one  eye  a  central  pupil,  and  below  it  at  the  periphery  of  the  iris  a 
large  opening  divided  into  two  by  a  thin  vertical  band  of  tissue. 

They  might  be  termed  cases  of  congenital  iridodialysis.  Talko  has 
jecorded  one  somewhat  similar,  in  which  numerous  posterior  synechiae  were 
present. 

4.  Those  of  which  Fig.  8  represents  a  good  example.     In  it  there 
were  as  many  as  nine  openings  in  the  iris, — the  normal  pupil,  and  below 

and  to  one  side  of  it  eight 
smaller,  somewhat  triangular 
openings.  These  appeared 
to  be  situated  just  external 
to  the  outer  border  of  the 
sphincter  muscle,  and  looked 
like  gaps  left  between  the 
radiating  fibres  of  the  iris. 

The  child,  at  the  time  the 
drawing  was  made,  was  three 
and  a  half  years  old ;  the 
defect,  however,  had  been 
noticed  soon  after  birth ;  its 
other  eye  was  normal.  At  the  periphery  of  the  left  iris,  on  the  outer  side, 


Congenital  polycoria  and  anterior  eynechia  in  a  girl 
aged  three  and  a  half  years. 


1  Trans.  Am.  Ophth.  Soc.,  vol.  iii.  p.  735. 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.       437 

were  several  little  white  projections  from  its  anterior  surface ;  these  seemed 
to  be  attached  to  the  back  of  the  cornea. 

Cases  similar  in  character  to  this  have  been  recorded  by  Rumschewitsch, 
Baudry,  and  de  Schweinitz. 

PERSISTENT   PUPILLARY    MEMBRANE. 

The  formation  of  the  pupillary  membrane  from  the  anterior  fibro-vas- 
cular  sheath  of  the  lens  has  been  described  at  the  commencement  of  the 
article.  In  man  it  usually  completely  disappears  before  birth  ;  persistence 
of  a  portion  of  it  is  one  of  the  commonest  congenital  abnormalities  of 
the  eye. 

Franke x  found  remnants  of  it  in  thirty-two  patients  out  of  three  thou- 
sand five  hundred  and  eight,  or  0.9  per  cent.  ;  in  eighteen  only  filaments 

FIG.  9. 


FIG.  9a. 


Congenital  microphthalmos  and  cataract,  with  persistence  of  numerous  tags  of  pupillary  mem- 
brane, in  a  girl  aged  eight  months.  Fig.  9a  represents  the  iris  of  the  right  microphthalmic  eye 
magnified. 

were  present,  and  in  fourteen  a  membrane.  Three  cases  had  remnants  in 
both  eyes.  It  occurred  in  the  ratio  of  seven  cases  in  the  right  eye  to  five 
in  the  left,  and  in  the  female  sex  in  the  ratio  of  nineteen  to  thirteen  in  the 
male. 

Stephenson 2  gives  a  higher  percentage  than  this.  Out  of  a  total  of 
three  thousand  four  hundred  and  fourteen  eyes  he  detected  vestiges  of 
pupillary  membrane  no  less  than  sixty-eight  times,  or  in  1.7  per  cent,  of 
the  cases  examined.  In  thirteen  of  the  sixty-eight  cases  there  was  persist- 
ent pupillary  membrane  in  both  eyes.  Of  the  monocular  cases,  the  right 
eye  was  the  seat  of  the  anomaly  in  twenty-five  instances,  and  the  left  eye 
in  the  remaining  seventeen.  As  regards  sex,  the  percentage  among  nine- 
teen hundred  and  ninety-four  males  was  1.81,  and  among  fourteen  hundred 
and  twenty  females  2.25. 

When  persistent,  the  pupillary  membrane  varies  in  extent,  in  color, 
and  in  its  relation  to  surrounding  structures.  Fibres  of  the  pupillary 

1  Archiv  fiir  Ophth.,  Bd.  xxx.,  Abth.  4,  S.  289. 
1  Trans.  Ophth.  Soc.,  vol.  xiii.  p.  139. 


438      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 


membrane  are  distinguished  from  posterior  synechise  due  to  iritis  by 
arising  from  the  anterior  surface  of  the  iris,  from  the  corona  or  smaller 
circle. 

Cases  have  been  recorded  by  van  Duyse  in  which  fibres  arising  from 
the  small  circle  of  the  iris  have  converged  towards  the  centre  of  the  pupil 
and  there  united  into  a  true  membrane.  More  often  no  true  membranes 
exist,  only  fibres  being  present.  These  fibres  may  be  arranged  in  several 
different  ways. 

1.  Several  fibres  arising  at  different  points  in  the  circumference  of  the 
small  circle  of  the  iris  stretch  across  the  pupil  and  form  a  delicate  net- 
work in  front  of  it. 

2.  Fibres  run  tangentially  between  two  points  in  the  small  circle  of 
the  iris. 

3.  All  the  toothed  projections  of  the  small  circle  are  prolonged  inward 
and  project  beyond  the  pupillary  margin.     (Fig.  9.) 

4.  One  or  more  fibres  attached  to  the  small  circle  of  the  iris  float  free 
at  their  other  extremities.     (Fig.  10.) 

FIG.  11. 


FIG.  10. 


Section  of  the  fellow-eye  to  the  one  shown  in 
Fig.  11,  which  also  had  apparently  complete  ab- 
sence of  the  iris.  The  rudimentary  iris  is  shown 
with  a  tag  of  pupillary  membrane  proceeding  from 
its  free  extremity,  and  a  tag  of  adhesion  passing 
between  its  root  and  the  back  of  the  cornea.  (Trans. 
Ophth.  Soc.,  vol.  xiii.) 


The  front  of  an  eye  which  had  apparently 
complete  congenital  absence  of  the  iris ;  the 
cornea  and  sclerotic  have  been  removed  and 
a  rudimentary  iris  exposed.  Loops  of  persist- 
ent pupillary  membrane  pass  from  the  rudi- 
mentary iris  to  the  surface  of  the  lens.  There 
is  also  a  congenital  anterior  polar  cataract. 
(Trans.  Ophth.  Soc.,  vol.  xiii.) 


5.  A  loop  is  formed  by  two  fibres  in  front  of  the  pupil.     (Fig.  11.) 

6.  One  or  more  fibres  arising  from  the  iris  are  attached  to  the  capsule 
of  the  lens.     This  variety  is  sometimes  spoken  of  as  a  capsulo-pupillary 
membrane. 

7.  A  fibre  arising  from  the  iris  is  attached  to  the  posterior  surface  of 
the  cornea.     (Fig.  12.) 

This  last  arrangement  is  apparently  but  seldom  met  with.  In  some 
of  the  cases  recorded  there  has  been  a  history  of  ophthalmia  neonatorum, 
and  an  opacity  of  the  cornea  was  present  which  rendered  it  likely  that  the 
attachment  of  the  pupillary  membrane  was  the  result  of  inflammation.  In 
the  case  here  figured  there  was  not  the  least  sign  of  past  or  present  iuflam- 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.      439 

mation,  and  other  congenital  abnormalities  existed  in  the  eye :  so  that  there 
seems  little  doubt  that  it  is  a  true  case  of  congenital  non-separation  of  the 
pupillary  membrane  from  the  back  of  the  cornea. 

The  color  of  persistent  fibres  of  pupillary  membrane  may  be  gray,  the 
color  of  the  iris  in  which  they  occur,  or  partially  black.  This  black  pig- 
mentation is  most  frequently  seen  about  the  lenticular  extremity  of  a  cap- 
sulo-pupillary  membrane.  A  portion  of  the  anterior  fibro-vascular  sheath 
of  the  lens  or  the  pupillary  membrane  may  remain  persistent  on  the  sur- 


Fio.  12. 


Congenital  adhesion  of  iris  and  of  a  persistent  pupillary  membrane  to  the  back  of  the  cornea. 

face  of  the  lens  without  having  any  connection  with  the  iris  at  all ;  should 
it  occur  at  its  anterior  pole,  it  will  very  closely  resemble  an  anterior  polar 
cataract. 

Anterior  Synechia  of  Iris  and  Persistent  Pupillary  Membrane. — Cases  of 
adhesion  of  persistent  pupillary  membrane  to  the  back  of  the  cornea  have 
been  recorded  by  Beck,1  Samelsohn,2  Makrocki,3  and  Zinn.4  In  each  of 
the  cases  related  by  these  authors  there  seems  to  have  been  a  possibility  that 
the  adhesion  was  caused  through  intra-uterine  perforation  of  the  cornea. 
Seeing  that  the  anterior  fibro-vascular  sheath  of  the  lens,  which  afterwards 
becomes  the  anterior  layer  of  the  iris,  and  the  pupillary  membrane  are  de- 
veloped from  the  posterior  part  of  the  mesoblast  which  grows  in  to  sepa- 
rate the  lens  from  the  cuticular  epiblast,  the  anterior  part  of  which  forms 
the  substantia  propria  of  the  cornea,  it  might  reasonably  be  expected  that 
occasionally  the  anterior  fibro-vascular  sheath,  in  part  of  its  extent,  would 
fail  to  become  separated  from  the  cornea,  and  an  anterior  synechia  of  the 
pupillary  membrane  or  the  iris  result.  The  authors  of  this  article  have 
found  clinical  and  pathological  evidence  that  such  cases  do  occur.  In  the 

1  Ammon's  Zeitschrift,  Bd.  i.  Heft.  i. 

2  Centralblatt  fur  Augenheilk.,  1880,  S.  215. 

3  Archiv  fur  Augenheilk.,  Bd.  xiv.  S.  83. 

4  Klinische  Monatsblutt  fur  Augenheilk.,  Bd.  xxviii.  S.  290. 


440      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 

case  which  has  been  described  and  pictured  under  polycoria  (Fig.  8),  in  addi- 
tion to  the  numerous  openings  through  the  iris,  a  number  of  little  whitish 
elevations  of  the  iris  tissue  could  be  seen  coming  forward  to  the  back  of  the 
cornea.  Fig.  12  represents  the  cornea  and  iris  of  an  eye  in  which  the  cen- 
tral artery  of  the  vitreous  was  persistent  and  patent  and  ended  in  an  opaque 
membrane  behind  the  lens.  The  cornea  of  this  eye  was  quite  clear  and 
microscopically  appeared  perfectly  healthy,  but  adherent  to  its  posterior 
surface  were  a  large  piece  of  a  persistent  pupillary  membrane  and  part  of 
the  pupillary  border  of  the  iris.  In  Fig.  11,  representing  the  section  of 
the  portion  of  an  eye  in  which  the  iris  appeared  clinically  to  be  absent,  and 
in  which  a  rudimentary  one  was  found  to  be  present,  a  tag  of  adhesion  is 
seen  passing  from  the  anterior  surface  of  the  iris  to  the  back  of  the  cornea 
in  the  region  of  the  ligamentum  pectinatum. 

IRIDODONESIS,   OR   TREMULOUS   IRIS. 

Cases  in  which  the  iris  is  found  to  be  tremulous  from  birth  are  those  in 
which  there  is  ectopia  of  the  lens,  or  in  which  the  eye  is  buphthalmic.  In 
both  these  conditions  the  posterior  supporting  structures  of  the  iris  are 
in  abnormal  relation  to  it.  Occasionally  cases  are  met  with  in  which,  on 
movement  of  the  eye,  a  slight  oscillatory  motion  of  the  iris  can  be  detected. 

IRIDEREMIA. 

Cases  are  sometimes  met  with  of  congenital  defect  in  the  eye,  in  which 
the  iris  appears  to  be  entirely  absent.  On  looking  at  the  eye  in  the  part 
where  the  iris  should  be,  nothing  but  blackness,  like  that  of  the  pupil,  is 
to  be  seen.  Irideremia  or  aniridia  is  the  term  applied  to  such  cases.  In 
others  the  iris,  though  to  a  great  extent  absent,  is  seen  not  to  be  entirely 
so,  a  small  crescentic  piece  or  little  nodules  of  iris  tissue  being  found  at 
the  periphery  of  the  chamber ;  these  are  spoken  of  as  cases  of  partial  or 
incomplete  irideremia.  On  examination  of  a  case  of  irideremia  with  an 
ophthalmoscopic  mirror,  a  large  area  of  red  reflex  is  obtained,  broken, 
however,  at  its  periphery  by  a  circular  dark  line,  the  border  of  the  lens. 
Outside  the  margin  of  the  lens  a  fine  striation,  due  to  the  fibres  of  the 
suspensory  ligament,  can  be  made  out.  In  some  cases  the  cornea  has  an 
opacity  in  it,  in  some  the  lens  is  opaque ;  often  there  is  an  anterior  polar 
cataract.  In  cases  of  partial  irideremia  there  may  be  tags  of  persistent 
pupillary  membrane  present.  It  is  always  a  bilateral  affection,  the  eyes 
being  frequently  nystagmic.  The  sight  is  defective,  and  the  patient  acquires 
a  habit  of  screwing  up  his  lids,  endeavoring  to  shut  off  some  of  the  excess 
of  light  which  enters  the  eyes.  The  affection  is  frequently  hereditary. 

Microscopical  examinations  of  eyes  with  irideremia  have  been  made  by 
Pagenstecher l  and  G.  Rindfleisch.2  Fig.  12  represents  sections  from  a  case 

1  Klinik  fur  Augenheilk.,  Bd.  ix.  p.  425. 

a  Archiv  fur  Ophth.,  Bd.  xxxvii.,  Abth.  3,  S.  192. 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OP  THE  EYE.      441 

examined  by  one  of  the  authors  (E.  T.  C.),  in  which  clinically  no  iris  was 
seen,  but  in  which  pathological  examination  revealed  a  rudimentary  one. 

Several  theories  have  been  brought  forward  to  account  for  the  absence 
of  the  iris  in  these  cases.  It  was  suggested  that  the  iris  might  have 
become  absorbed  together  with  the  pupillary  membrane.  F.  Arnold  at- 
tributes its  absence  to  failure  in  the  formation  of  the  anterior  ciliary  ar- 
teries. In  the  case  the  microscopical  sections  of  which  are  figured,  these 
arteries  were  found  well  developed.  The  more  probable  explanation  of 
this  defect  is  that  suggested  by  Manz.  He  believes  the  iris  to  be  arrested 
m  its  development  by  an  unusually  strong  union  of  the  lens  to  the  cornea. 
Rindfleisch  attributes  this  abnormal  union  or  contact  of  lens  and  cornea  in 
his  case  to  intra-uterine  inflammation  of  the  choroid  extending  forward  and 
causing  perforation  near  the  sclero-corneal  margin,  with  escape  of  aqueous. 
It  is  highly  unlikely  that  an  affection  which  is  almost  invariably  bilateral 
could  be  caused  in  this  way.  It  is,  moreover,  unnecessary  to  have  a  per- 
foration for  the  lens  to  come  in  contact  with  the  cornea,  for  at  the  time 
the  iris  is  developing  these  structures  are  in  apposition,  the  anterior  fibro- 
vascular  sheath  alone  intervening. 

COLOBOMA   OF  THE   IRIS. 

Coloboma  of  the  iris  is  a  deficiency  in  the  tissue  of  the  irjs  by  which 
the  pupil  is  altered  in  shape.  It  is  one  of  the  most  commonly  met  with 
malformations  of  the  eye.  The  term  pseudo-coloboma  of  the  iris  is  applied 
to  cases  in  which  a  portion  only  of  its  thickness  is  wanting  in  one  position, 
the  deeper  layers  being  left  exposed. 

Coloboma  of  the  iris  is  nearly  always  met  with  in  its  lower  half,  and 
either  directly  downward,  or  downward  with  an  inclination  inward  or  out- 
ward. Exceptional  cases  of  the  defect  have,  however,  been  recorded  in 
which  it  has  occurred  in  other  directions.  It  has  been  seen  directed  upward 
by  von  Arnmon  and  Theobald  ; l  upward  and  outward  by  Fage,2  Theobald,1 
and  Frost;3  upward  and  inward  by  Fage2  and  Pollock;4  outward  (Figs. 
13  and  14)  by  Manz,  Nuel  and  Leplat,  Makrocki,6  and  Lang;5  inward  by 
Makrocki.6  Two  colobomata  of  the  iris  have  been  met  with  in  one  eye  by 
Manz  and  Rau. 

The  extent  and  shape  of  the  defect  are  even  more  inconstant  than  the 
direction.  It  may  consist  only  in  a  slight  notching  of  the  pupillary  border, 
or  the  whole  thickness  of  a  sector  of  the  iris  may  be  absent,  from  the  margin 
of  the  pupil  up  to  the  ciliary  body.  In  some  cases  the  gap  left  has  a  sort 
of  shoulder,  marking  the  limit  of  the  original  pupil.  The  edges  of  the  cleft 

1  Trans.  Am.  Ophth.  Soc.,  vol.  v.  p.  99. 

2  Gaz.  Hebdomadaire  des  Sciences  Medicales  de  Bordeaux. 

3  Trans.  Ophth.  Soc.,  vol.  xiii.  p.  144. 
*  Archives  of  Ophth.,  vol.  xii.  p.  410. 

5  Trans.  Ophth.  Soc.,  vol.  x.  p.  106. 

6  Archiv  fur  Augenheilk.,  Bd.  xiv.  S.  73. 


442      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 

may  be  parallel  to  one  another ;  then  the  gap,  together  with  the  normal  pupil, 
will  be  of  the  shape  of  a  key-hole.  In  other  cases  the  edges  of  the  gap  con- 
verge to  a  point  at  the  ciliary  margin  ;  the  gap  and  the  normal  pupil  then 
present  a  pear-shaped  opening.  A  rare  form  of  coloboma  is  that  in  which 
its  margins  diverge,  its  base  being  at  the  ciliary  border.  The  condition 
termed  coloboma  with  a  bridge  is  referred  to  under  polycoria.  In  coloboma 
of  the  iris,  though  a  sector  of  the  sphincter  muscle  is  absent,  the  pupil  is 

Fio.  13. 


FIG.  14. 


Congenital  coloboma  of  iris  and  lens  outward. 
(Trans.  Ophth.  Soc.,  vol.  x.) 


Microscopical  appearances  in  the  region  of  the 
coloboma  of  the  iris  in  the  eye  pictured  in  Fig. 
13.  (Trans.  Ophth.  Soc.,  vol.  xiii.) 


found  to  react  to  myotics  and  mydriatics  in  the  usual  way.  Alone,  it  gives 
rise  to  no  defect  in  vision.  The  other  defects  of  the  eye  with  which  it  is 
often  associated  are  :  coloboma  of  the  choroid  or  ciliary  body,  coloboma  or 
displacement  of  the  lens,  and  microphthalmos.  (Fig.  15.)  Patients  aifected 


FIG.  15. 


Microscopical  appearance  of  the  front  part  of  a  microphthalmic  eye  with  coloboma  of  the  iris.    (Royal 

Lond.  Ophth.  Hosp.  Rep.,  vol.  xii.) 

with  it  are  also  occasionally  found  to  have  other  congenital  defects,  such  as 
harelip,  cleft  palate,  or  coloboma  of  the  eyelid. 

Coloboma  of  the  iris  is  not  due,  as  some  have  supposed,  to  an  unclosed 
foetal  fissure  in  that  structure.  The  iris  is  not  developed  in  two  sectors,  and 
the  normal  foetal  iris  never  has  any  cleft ;  it  grows  as  a  prolongation  for- 
ward from  the  ciliary  body, — not,  however,  commencing  until  the  two  edges 
of  the  foetal  fissure  in  that  structure  and  in  the  choroid  have  become  united. 
Should  the  cleft  in  the  ciliary  body  remain  unclosed,  or  should  the  closure 
be  delayed,  then  either  no  iris  would  be  formed  in  the  position  of  the 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.       443 

cleft,  or  the  time  for  the  development  of  the  iris  in  that  position  would  be 
shortened,  and  it  would  not  attain  its  normal  length. 

To  this  explanation  it  may  be  objected  that  it  is  difficult  to  see  why  the 
defect  should  sometimes  be  situated  in  the  upper  part  of  the  iris,  the  foetal 
cleft  being  below.  Pfliiger  suggested  that  in  these  cases  some  rotation  of 
the  eye  occurred  during  foetal  life. 

Makrocki  supposes  the  choroidal  fissure  to  be  abnormally  placed. 

Neither  of  these  hypotheses  would  explain  the  existence  of  two  colobo- 
mata  in  one  eye,  or  of  a  horizontal  coloboma  of  the  iris  with  the  same  defect 
in  the  choroid  at  right  angles  to  it. 

It  has  been  suggested  that  irideremia  is  due  to  an  abnormal  adhesion  or 
late  separation  of  the  lens  and  cornea.  Should  this  adhesion  or  late  sepa- 
ration, instead  of  involving  in  the  cases  of  irideremia  the  whole  surface  of 
the  lens,  involve  only  a  portion  of  its  area,  then  the  iris  would  be  prevented 
from  developing  there,  but  would  be  formed  in  the  normal  way  in  the  rest 
of  its  circumference.  In  this  manner  a  coloboma  of  the  iris  might  occur 
in  any  position,  and  even  two  might  be  formed  in  the  same  eye. 

CHOROID   (CHORIOID). 

Congenital  abnormalities  of  the  choroid  comprise  developmental  defects 
in  the  region  of  the  choroidal  fissure,  and  macula, — i.e.,  colobomata,  defects 
of  pigmentation,  and  vascular  defects. 

COLOBOMA    OF   THE   CHOROID. 

This  defect  generally  consists  in  an  absence  of  the  choroid  along  the 
line  of  the  so-called  choroidal  fissure.  The  defective  area  is  usually  ovoid, 
with  its  long  diameter  parallel  with  the  antero-posterior  axis  of  the  globe ; 
posteriorly  it  may  extend  up  to,  or  even  beyond,  the  disk,  which  is  then  in- 
cluded in  the  area  ;  in  the  latter  case  there  is  a  coloboma  of  the  sheath  of 
the  nerve,  and  all  resemblance  to  the  normal  disk  surface  is  lost.  Clinically, 
the  anterior  extremity  of  the  defect  is  often  invisible ;  it  may  be  continuous 
with  a  coloboma  of  the  ciliary  body  and  iris. 

Ophthalmoscopically,  the  sclerotic,  which  is  left  exposed  in  the  region  of 
the  coloboma  through  the  absence  of  the  choroid,  is  of  a  pearly  white  color  ; 
here  and  there  a  little  pigment  and  a  few  ciliary  vessels,  which  are  occa- 
sionally crossed  by  a  retinal  vessel,  are  seen  on  its  surface.  The  floor  of 
the  coloboma  may  be  fairly  smooth,  or  in  parts  depressed,  with  the  bottom 
of  the  depression  several  millimetres  below  the  general  level  of  the  surface ; 
when  the  depressions  are  very  marked  they  may  form  cysts.  (See  Microph- 
thalmos.)  The  floor  may  also  be  raised  into  little  ridges,  whilst  occasionally 
it  is  subdivided  by  transverse  bands  of  normal  choroid.  The  margins  of 
the  area  are  sharply  defined  and  pigmented. 

The  retina  is  frequently  absent  from  the  region  of  the  coloboma  (Manz),1 

1  Graefe  and  Saemisch's  Handbook. 


444      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 

and  when  it  is  present  (Pause)1  the  hexagonal  epithelium  never  contains  any 
pigment  and  the  other  retinal  layers  may  not  be  completely  differentiated. 
The  occasional  presence  of  the  retina  accounts  for  there  not  being  in  every 
case  an  absolute  scotoma  corresponding  to  the  coloboma. 

Fuchs,  in  his  text-book,  states  that  it  is  hereditary  to  a  high  degree, 
and  attributes  the  defect  in  the  choroid  primarily  to  the  non-union  of  the 
edges  of  the  retinal  cleft,  which  does  not,  however,  explain  those  cases  in 
which  the  retina  exists  over  the  whole  area;  but  they  can  all  be  ex- 
plained if  it  is  considered  that  the  defect  is  due  to  an  abnormal  adhesion 
of  the  retina  to  the  mesoblast,  so  that  when  this  abnormal  adhesion  takes 
place  before  the  retinal  fissure  is  closed,  the  coloboma  is  devoid  of  a  cover- 
ing of  retina,  and  an  absolute  scotoma  exists,  whereas  when  it  occurs  after 
the  closure  of  the  fissure  the  retina  is  everywhere  present,  and  there  is  no 
scotoma. 

The  greater  frequency  of  the  defect  in  the  region  of  the  retinal  fissure  is 
thus  explained  ;  but  that  it  is  not  always  situated  in  this  region  is  shown  by 
the  occasional  occurrence  of  similar  defects  in  other  parts  of  the  fundus. 
Thus,  Frost2  records  a  case  of  coloboma  of  the  iris  and  choroid  on  the  tem- 


FIG.  16. 


Microscopical  appearances  of  the  posterior  half  of  a  microphthalmic  eye,  showing  a  break  in  the 
continuity  of  the  sclerotic  through  which  a  mass  of  nerve-tissue  is  protruding  (1) :  a  coloboma  of  the 
choroid  commencing  at  2;  an  adhesion  between  the  retina  and  the  uveal  pigment-layer  replaced  for 
a  short  distance  by  tissue  simulating  retina  (3).  (Trans.  Ophth.  Soc.,  vol.  xiii.) 

poral  side,  and  other  cases  are  described  in  which  congenital  defects  of  the 
choroid  have  been  met  with  in  various  positions  and  of  widely  different 
shapes  (Lang).3  An  isolated  patch  of  non-developed  choroid  may  occur 
in  the  same  eye  with  a  coloboma  of  the  iris,  choroid,  and  optic-nerve  sheath 
in  the  region  of  the  foetal  cleft  (AVood).4  The  explanation  of  all  the  con- 
genital defects  of  the  choroid,  wherever  situated,  is  the  same  :  an  adhesion 
forms  between  the  developing  retina  and  the  rnesoblast,  which  latter,  conse- 

1  Graefe's  Archiv,  1878. 

2  Trans.  Ophth.  Soc.,  vol.  xiii.  p.  144. 

3  Ibid.,  vol.  vi.  p.  439. 
*  Ibid.,  vol.  xii.  p.  173. 


FIG.  17. 


—•»>«». 


FIG.  18. 


Unusual  appearance  of  the  optic  disk,  due  to  the  abnormal  direction  of  the  head  of  the  optic  nerve. 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.       445 

quently,  fails  to  become  differentiated  into  the  choroid  and  sclerotic ;  where 
the  choroid  is  absent,  there  the  pigment  fails  to  be  formed  in  the  hexagonal 
epithelium. 

In  Fig.  16,  between  1  and  2,  the  retina,  the  pigment  epithelial  layer, 
the  choroid,  and  the  sclerotic  appear  normal,  except  that  the  choroid  is  not 
pigmented  and  contains  too  many  cells.  Between  2  and  4  the  choroid  ceases, 
and  the  epithelial  layer  is  unpigmented  and  is  adherent  to  the  sclerotic ; 
whilst  beyond  4  all  the  tissues  are  again  normal. 

COLOBOMA   OF   THE    MACULA. 

This  usually  consists  of  a  nearly  circular  defect  in  the  choroid,  which 
exposes  a  white  area  of  sclerotic,  upon  which  are  seen  a  few  scleral  vessels 
and  a  little  pigment.  (Fig.  17.)  The  choroid  is  deeply  pigmented  at  the 
margin,  where  it  forms  a  sharply  defined  line.  The  retina  passes  over  the 
defective  area,  as  may  occasionally  be  recognized  by  the  course  of  a  retinal 
vessel  and  by  the  absence  of  a  scotoma.  The  depth  of  the  floor  may  range 
between  one  and  several  millimetres. 

FUCHS'S   COLOBOMA. 

This  is  a  small  crescentic  defect  of  the  choroid  at  the  lower  border  of 
the  disk,  not  unlike  a  myopic  crescent,  except  in  its  position  ;  whilst  the  disk 
appears  as  if  it  were  twisted  around  its  antero-posterior  axis,  so  that  its  long 
diameter  is  horizontal  and  its  physiological  cup  is  directed  downward. 

The  retinal  vessels  usually  pass  over  it  in  an  uninterrupted  manner,  but 
occasionally  they  dip  down  into  a  depression  which  is  due  to  a  defect  in  the 
optic-nerve  sheath  or  the  sclerotic  at  the  same  point.  The  vision  of  these 
eyes  is  usually  below  normal. 

COLOBOMA   OF   THE   SHEATH   OF  THE   OPTIC   NERVE. 

This  may  be  partial  or  complete.  When  partial,  it  presents  the  appear- 
ance of  an  extremely  deep  cup  situated  at  the  inner  or  lower  part  of  the 
disk.  The  outline  of  the  disk  may  be  distinguished  in  part  of  or  in  its 
entire  circumference.  The  retinal  vessels,  on  reaching  the  margin  of  the 
colobomatous  area,  may  dip  down  and  at  once  be  lost  to  view,  or  they  may 
dip  down  and  reappear  again  as  they  mount  on  to  a  slightly  higher  portion 
of  the  disk.  In  other  cases  there  is  a  gradual  shelving  from  the  margin  to 
the  centre  of  the  coloboma,  and  the  vessels  do  not  abruptly  disappear. 

There  may  be  a  coexisting  coloboma  of  the  choroid  continuous  or  sepa- 
rated from  the  coloboma  of  the  nerve-sheath.  (Benson.) l 

Magnus 2  records  a  case  in  which  it  existed  with  microphthalmos  without 
coloboma  of  the  choroid.  In  a  case  of  Manz's 3  it  was  found  pathologically 

1  Dublin  Journal  Med.  Soc.,  1882. 

2  Klin.  Monatsb.  fiir  Augenheilk.,  1887. 

3  Knapp's  Archiv,  1892. 


446      CONGENITAL  MALFOEMATJONS  AND  ABNORMALITIES  OF  THE  EYE. 

that  the  optic  nerve-sheath  and  scleral  margin  bulged  backward  into  a  deep 
pocket,  over  which  extended  normal  retina.  A  condition  somewhat  similar 
is  seen  in  Fig.  16. 

The  defect  is  attributed  to  an  imperfect  closure  of  the  cleft  which  is 
originally  present  in  the  under  surface  of  the  optic  nerve. 

DEFECTS   IN   PIGMENTATION. 

The  pigment  of  the  choroid  and  retinal  epithelium  is  altogether  absent 
in  cases  of  complete  albinism,  and  the  choroidal  vessels  which  give  rise  to 
the  pink  eye  (see  Iris)  are  then  everywhere  visible  with  the  ophthalmoscope. 

An  angioma  of  the  choroid  associated  with  a  nsevus  of  the  lids  and  orbit 
has  been  described  in  pathological  specimens  by  Milles l  and  Lawford,2  but 
there  is  no  record  of  its  having  been  seen  with  the  ophthalmoscope. 

RETINA. 

Under  Congenital  Abnormalities  of  the  Retina  are  comprised  colobo- 
mata  and  cysts  associated  therewith  (page  421),  opaque  nerve-fibres,  certain 
small  bright  dots,  anomalies  of  pigmentation,  and  peculiarities  in  the  source 
and  arrangement  of  the  blood-vessels. 

OPAQUE    NERVE-FIBRES. 

The  appearance  presented  by  this  condition  is  that  of  a  very  bright  white 
area  which  is  usually  continuous  with  the  upper  or  the  lower  margin  of  the 
optic  disk,  from  whence  it  spreads  out  and  terminates  usually  in  a  feathery 
striated  border  ;  it  may  be  smaller  than  the  disk  or  many  times  larger,  when 
it  may  reach  beyond  the  macular  region,  or  it  may  completely  surround  the 
disk.  (Hartridge.) 3 

In  some  cases  a  patch  is  quite  separate  from  the  disk,  and  in  others 
the  nerve-fibres  show  the  characteristic  appearance  in  an  isolated  patch  in 
the  retina,  but  also  starting  from  the  disk.  The  retinal  vessels  coursing 
through  the  patch  partly  appear  on  its  surface  and  in  part  are  hidden  by 
the  opaque  fibres.  The  condition  is  due  to  the  retention  around  some  of 
the  axis-cylinders  of  the  medullary  sheaths,  which  normally  cease  at  the 
lamina  cribrosa. 

A  scotoma  in  the  field  of  vision  corresponds  to  the  opaque  area,  and  in 
extreme  cases  of  the  defect  the  eye  may  be  amblyopic. 

Marcus  Gunn  *  describes  a  change  in  the  retina  which  can  be  seen  only 
by  the  direct  method  and  a  low  illumination.  It  consists  of  a  number 
of  small  bright  "  crick  -dots,"  which  occur  near  the  disk  and  anterior  to 
the  retinal  vessels.  They  are  sometimes  hereditary,  and  may  affect  several 
members  of  a  family. 

1  Trans.  Ophth.  Soc.,  vol.  iv.  p.  168. 
3  Ibid.,  vol.  v.  p.  136. 
8  Ibid.,  vol.  v.  p.  177. 
*  Ophth.  Review,  1889. 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OP  THE  EYE.       447 


RETINAL   VESSELS. 

The  ordinary  vascular  supply  to  the  retina  by  means  of  the  arteria 
centralis  is  supplemented  in  seventeen  per  cent.  (Lang  and  Barrett)1  of 
cases  by  a  larger  or  smaller  branch  derived  from  the  posterior  ciliary 
arteries.  Nettleship2  first  called  attention  to  the  point,  and  Benson  has 
since  shown  what  a  benefit  the  possession  of  a  double  supply  confers 
when  the  central  artery  is  blocked  by  an  embolus.  The  cilio-retinal  branch 
may  take  the  place  of  one  of  the  temporal  divisions  of  the  arteria  cen- 
tralis, or  of  a  macular  branch.  It  is  characterized  by  the  fact  that  when 
it  reaches  the  disk-margin  it  immediately  bends  back  and  disappears, 
instead  of  proceeding  on  to  the  centre  of  the  disk.  It  is  usually  present 
in  one  eye  only. 

The  arrangement  of  the  blood-vessels  on  the  disk  is  very  variable. 
Occasionally  they  enter  the  nerve  at  the  margin  of  the  disk,  the  centre  being 
free  from  all  blood-vessels  and  very  pale  in  color.  (Lawford.)3  The  veins 
may  all  unite  into  one  branch,  which  forms  a  loop,  lying  partly  on  the  disk 
and  partly  on  the  retina,  before  disappearing  down  the  centre  of  the  nerve. 
(Lawford.)4  Or  a  branch  of  the  veins  may  form  a  loop  and  collect  other 
veins  (Werner),5  or  two  veins  may  be  joined  together  by  a  short  branch. 
(Frost.)6 

Just  before  reaching  the  disk,  vein  and  artery  may  be  coiled  round  each 
other  in  a  spiral  fashion.  Again,  the  artery  may  form  a  loop  which  passes 
forward  from  the  disk  into  the  vitreous  for  two  or  three  millimetres  and 
returns  again  to  the  disk  (Frost)  ;  or  a  vein  and  an  artery  may  communicate 
in  the  retina. 

Congenital  patches  of  pigment,  in  shape  and  arrangement  somewhat  like 
the  cells  of  growing  hyaline  cartilage,  and  occurring  in  a  sector-shaped  patch 
in  the  lower  half  of  the  retina,  are  pictured  and  described  by  Stephenson.7 
They  produce  no  symptoms,  and  are  discovered  only  on  a  systematic  exam- 
ination of  the  eye.  Stephenson  suggests  that  they  are  due  to  extension  of 
the  pigment  from  the  outer  into  the  inner  layers  of  the  retina.  Minute 
round  dots  of  pigment,  which  appear  to  be  the  size  of  a  pin's  point  by 
the  indirect  method  of  ophthalmoscopic  examination,  are  of  more  frequent 
occurrence.  For  Albinism,  see  page  435. 

Ketinitis  pigmentosa  and  glioma  of  the  retina,  which  occur  at  times  as 
congenital  defects,  do  not  differ  from  the  same  conditions  seen  at  a  later 
period. 

1  Royal  Lond.  Ophth.  Hosp.  Rep.,  vol.  xii.  p.  59. 

3  Ibid.,  vol.  viii. 

s  Trans.  Ophth.  Soc.,  vol.  xv. 

*  Ibid. 

5  Ibid.,  vol.  x. 

8  Ibid.,  vol.  xi. 

7  Ibid.,  vol.  xi.  p.  77. 


448      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 


OPTIC   NERVE. 

Apart  from  the  coloboma  of  its  sheath,  which  is  described  at  page 
445,  the  congenital  defects  of  the  optic  nerve  consist  of  alterations  in  the 
direction  of  the  nerve-head,  so  that  it  appears  to  be  reversed,  and  what  is 
ordinarily  the  outer  or  temporal  pale  part  of  the  disk  is  turned  towards  the 
nose  (see  Fig.  18),  of  pigment  particles  occurring  in  the  superficial  part  of 
the  disk,  and  of  absence  of  the  central  retinal  vessels,  of  which  a  case  has 
been  recorded  by  Berry. 

Congenital  atrophy  of  the  disk  does  not  ophthalmoscopically  differ  from 
post-natal  atrophy,  neither  does  the  congenital  glaucoma  cupping  differ 
from  that  condition  when  produced  after  birth.  (See  Buphthalmos.)  The 
appearances  produced  by  remains  of  the  hyaloid  artery  on  the  disk  are 
described  under  Abnormalities  of  the  Vitreous. 

CONGENITAL  ABNORMALITIES  OF  THE  LENS. 

The  lens  may  be  congenitally  absent  (aphakia).  It  may  be  altered  in 
size,  in  shape,  in  position,  and  in  transparency. 

APHAKIA. 

In  many  cases  in  which  the  lens  is  apparently  absent,  it  is  really  only 
displaced  out  of  sight.  In  fact,  it  is  very  doubtful  whether  a  seeing  eye 
could  be  produced  if  the  downgrowth  of  cuticular  epiblast  which  results  in 
the  formation  of  the  lens  failed  to  occur.  Cases  where  the  lens  appeared 
totally  absent  have  been  observed  in  microphthalmic  eyes  by  Seller,  von 
Ammon,  and  Hermann  Becker.1  In  the  case  reported  by  Becker  the  fol- 
lowing conditions  were  also  found  :  absence  of  pupil,  iris,  ciliary  body,  and 
anterior  chamber,  coloboma  of  retinal  pigment  epithelium  and  choroid,  and 
thinning  of  sclerotic. 

CONGENITAL   SMALLNESS  OF  THE  LENS. 

An  unusual  smallness  of  the  lens  is  detected  only  on  examination  of  the 
eye  with  the  ophthalmoscope  after  the  pupil  has  been  dilated  with  atropine. 
The  edge  of  the  lens  is  then  seen  as  a  dark  ring  standing  out  against  the 
red  reflex  of  the  fundus,  and  an  unusually  large  space  is  found  between  it 
and  the  margin  of  the  dilated  pupil.  The  anterior  chamber  in  such  cases 
is  usually  deeper  than  normal,  and  the  iris  is  tremulous. 

In  a  case  recorded  by  Hartridge 2  the  patient  was  highly  myopic. 

LENTICONUS. 

Lenticonus  is  the  term  applied  when  the  lens  presents  an  abnormal  cur- 
vature of  either  its  anterior  or  its  posterior  surface, — a  condition  of  things 

1  Archiv  fur  Ophth.,  Bd.  xxxiv.,  Abth.  3,  S.  103. 

2  Trans.  Ophth.  Soc.,  vol.  vi.  p.  489. 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.      449 

that  is  very  similar  to  keratoconus.  Conicity  of  the  surface  of  the  lens  is 
a  rare  form  of  abnormality ;  it  seems  to  be  more  frequently  met  with  in 
connection  with  its  posterior  than  with  its  anterior  surface,  for  there  are 
only  two  cases  recorded  in  which  the  latter  was  affected,  while  there  are 
twelve  of  the  former.  The  two  cases  of  anterior  lenticonus  are  described 
by  Webster  and  Placido ;  in  both  it  was  bilateral,  and  in  both  there  was 
some  doubt  as  to  whether  it  was  congenital  or  acquired.  It  is  a  condition 
which  can  be  easily  detected  by  focal  illumination. 

Posterior  lenticonus  may  be  diagnosed  by  seeing,  on  illumination  of  the 
fundus  with  the  ophthalmoscopic  mirror,  a  sharply  outlined  disk  in  the 
centre  of  the  illuminated  area,  having  the  appearance  of  an  oil-drop  in  the 
lens.  The  fundus  is  visible  through  the  central  disk,  but  it  is  found,  either 
by  estimation  with  the  direct  method  or  by  retinoscopy,  that  there  is  a 
difference  in  the  refraction  of  the  central  disk  and  of  the  sides  of  the  lens. 
The  condition  is  sometimes  associated  with  opacities  in  the  lens  at  the  pos- 
terior pole  or  elsewhere.  In  Meyer's  case  a  remnant  of  the  hyaloid  artery 
was  adherent  to  it ;  in  other  cases  the  lens  was  quite  clear.  These  latter 
cases  L.  Miiller l  believes  not  to  be  due  to  conicity  of  the  lens,  though  he 
asserts  that  the  former  are.  He  explains  the  appearances  seen  by  supposing 
that  there  is  some  undue  thickness  or  thinness  of  the  nucleus.  He  would 
prefer  to  describe  them  as  cases  of  lenses  with  a  double  focus. 

COLOBOMA   OF   THE   LENS. 

This  is  a  rare  form  of  malformation,  and  consists  of  a  defect  in  the 
margin  of  the  lens,  nearly  always  the  lower.  Two  exceptional  cases  are  on 
record  :  one  by  Schiess,2  where  it  was  situated  at  the  lower  and  outer  border, 
and  the  other  by  Lang,3  in  which  it  was  directly  outward.  (Fig.  13.)  The 
defect  consists  in  a  triangular  or  saddle-shaped  notch  extending  through  the 
whole  thickness  of  the  lens,  and  varying  in  amount  from  a  slight  indentation 
to  as  much  as  one-fourth  of  its  substance ;  or  in  the  lower  margin  of  the 
lens  presenting  instead  of  its  normal  curvature  a  straight  or  a  crenated  line. 

A  remarkable  case  has  been  described  by  Doyne,4  in  which  there  was  a 
coloboma  of  the  iris  and  choroid,  and  the  corresponding  margin  of  the  lens, 
instead  of  being  notched,  presented  a  projection. 

The  malformation  may  occur  in  one  eye  only  or  in  both,  and  is  fre- 
quently associated  with  other  abnormalities,  such  as  coloboma  of  the  iris, 
ciliary  body,  or  choroid.  A  defect  in  the  suspensory  ligament  correspond- 
ing to  the  notch  in  the  lens  is  occasionally  seen.  The  lenses  thus  affected 
may  also  be  congenitally  displaced,  smaller  than  normal,  or  jrartially  opaque. 

The  refraction  of  eyes  with  coloboma  lentis  is  usually  myopic.  In 
patients  who  have  had  the  defect  in  one  eye  only,  it  has  been  found  that  the 

1  Klinische  Monatsbl.  fur  Augenheilk.,  1894,  S.  178. 
*  Ibid.,  1871,  S.  99. 

3  Trans.  Ophth.  Soc.,  vol.  x.  p.  106. 

4  Ibid.,  vol.  xi.  p.  220. 
VOL.  I.— 29 


450      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OP  THE  EYE. 


defective  eye  is  myopic,  the  other  being  emmetropic.  Two  cases  have  been 
recorded  by  Bowman 1  and  Heyl 2  in  which  there  was  hypermetropia.  The 
abnormality  is  caused  by  some  defect  in  the  development  of  the  suspensory 
ligament.  As  has  been  already  mentioned,  this  is  developed  by  adhesions 
forming  between  the  sides  of  the  lens  and  the  ciliary  body  at  that  period  of 
foetal  life  when  they  lie  in  contact.  Should  some  of  these  adhesions  fail  to 
occur,  then,  as  the  eyeball  enlarged,  that  portion  of  the  capsule  to  which 
no  suspensory  ligament  was  attached  would  not  be  held  taut  and  made  to 
expand  like  the  remainder ;  consequently  there  would  be  a  depression  in 
the  lens  at  that  situation.  The  amount  and  shape  of  the  deficiency  would 
depend  on  the  extent  of  the  defect  in  the  suspensory  ligament.  The  most 
likely  cause  for  the  absence  of  adhesions  between  the  ciliary  body  and  the 
side  of  the  lens,  with  a  consequent  defect  in  the  suspensory  ligament,  would 
be  an  absence  of  the  ciliary  body ;  and,  as  we  have  already  said,  a  coloboma 
of  the  ciliary  body  is  frequently  found  associated  with  coloboma  of  the  lens. 


ECTOPIA   OF   THE   LENS. 


FIG.  19. 


Semi-diagrammatic  sketch  of  a  microphthalmic  eye. 
— /,  fibrous  tissue  in  centre  of  vitreous  holding  lens  back ; 
«,  suspensory  ligament  of  lens  stretched  and  attached  to 
elongated  ciliary  processes ;  r,  retina  much  folded ;  p, 
position  at  which  the  choroid  in  the  lower  part  of  the 
eye  commenced,  and  where  the  pigmented  epithelial 
layer  first  became  pigmented.  (Trans.  Ophth.  Soc.,  vol. 
xiii.) 

ward.     (Fig.  19.)    Displacement  backward 


Congenital  displacement 
of  the  lens  is  usually  bilateral, 
but  may  be  present  in  one  eye 
only.  It  is  not  a  malforma- 
tion which  is  commonly  met 
with :  out  of  fifty  thousand 
hospital  and  private  patients 
of  Knapp's,  only  3-^,  or  ten, 
of  these  cases  were  seen.  It  is 
sometimes  met  with  in  several 
members  of  one  family.  Mor- 
ton3 has  recorded  how,  pre- 
sumably, it  occurred  in  five 
successive  generations,  compris- 
ing seven  individuals.  The 
direction  in  which  the  lens  is 
displaced  varies ;  in  the  large 
majority  it  is  upward,  either 
directly  or  with  a  slight  incli- 
nation inward  or  outward.  It 
has  been  seen  displaced  hori- 
zontally, also  downward  and 
outward,  or  downward  and 
inward,  but  not  directly  down- 
occasionally  occurs  in  microph- 


1  Koyal  Lond.  Ophth.  Hosp.  Rep.,  vol.  v.  p.  12. 

2  Rep.  of  Fifth  Internat.  Ophth.  Cong.,  1876,  p.  16. 

3  Royal  Lond.  Ophth.  Hosp.  Rep.,  vol.  ix.  p.  435. 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.       451 

thalmic  eyes  :  this  is  due  to  some  defect  in  the  development  of  the  vitreous, 
and  will  be  further  dealt  with  in  connection  with  abnormalities  of  that 
structure. 

The  direction  in  which  the  lenses  are  displaced  is  usually  symmetrical  in 
the  two  eyes,  but  not  always  so  ;  for  example,  the  authors  have  recently  had 
under  their  observation  a  boy  in  whom  the  right  lens  was  displaced  directly 
upward  and  the  left  directly  outward.  Monocular  diplopia  is  not  a  common 
complication  :  Knapp  met  with  a  case  in  which  four  images  were  perceived 
simultaneously  with  the  two  eyes. 

On  examination  of  eyes  with  congenitally  displaced  lenses,  the  iris  will 
be  found  tremulous,  the  anterior  chamber  deep  and  often  of  uneven  depth 
in  different  parts,  it  being  deepest  in  the  part  from  which  the  lens  is  dis- 
placed. On  oblique  illumination  the  margin  of  the  lens  will  be  seen  as  a 
curved  line,  the  lens  itself  gray,  and  the  aphakic  part  at  its  margin  black. 
Sometimes  the  displacement  is  so  slight  that  the  margin  of  the  lens  cannot 
be  detected  until  the  pupil  has  been  dilated  with  atropine.  With  the 
ophthalmoscopic  mirror  the  curved  edge  of  the  lens  is  seen  as  a  dark  line ; 
it  is  not  always  quite  regular,  sometimes  having  slight  depressions  or  ele- 
vations on  it.  In  a  case  of  Marcus  Gunn's l  there  was  a  deep  notch  in  its 
margin,  constituting  a  coloboina. 

Displaced  lenses  are  often  smaller  than  normal,  and  altered  in  shape, 
being  rounder  than  they  should  be,  thus  resembling  the  festal  lens.  They 
are  usually  quite  clear,  but  may  have  opacities  in  them.  The  condition  of 
the  suspensory  ligament  between  the  margin  of  the  lens  and  the  ciliary 
body  from  which  it  is  displaced  varies ;  sometimes  it  is  absent  in  this  posi- 
tion ;  at  other  times  a  few  fibres  are  visible  here  and  there ;  in  other  cases, 
again,  except  for  the  stretching,  it  appears  normal.  When  the  suspensory 
ligament  is  absent  the  lens  is  mobile.  A  case  of  this  sort  is  recorded  by 
Sir  W.  Bowman,2  in  which,  apparently,  the  lens  at  times  swayed  forward, 
blocked  the  passage  of  fluids  through  the  pupil,  and  so  caused  increase  of 
tension.  The  most  common  malformations  of  the  eye  with  which  ectopia 
lentis  is  complicated  are  corectopia,  coloboma,  irideremia,  and  nystagmus. 

Congenital  displacement  of  the  lens,  like  coloboma  of  the  lens,  is  due  to 
some  defect  in  the  development  of  the  suspensory  ligament.  In  the  former 
the  defect  is  more  extensive  than  in  the  latter,  and  is  probably  occasioned 
by  a  failure  or  late  closure  of  the  ocular  cleft  in  the  ciliary  region,  so  that 
as  the  eye  expands  there  is  no  suspensory  ligament  to  hold  the  lens  down 
in  that  region,  and  it  consequently  becomes  drawn  in  the  opposite  direc- 
tion. The  most  common  position  in  which  lenses  are  displaced,  as  has 
been  already  mentioned,  is  upward,  either  directly  or  with  a  slight  in- 
clination inward  or  outward, — that  is,  opposite  to  that  in  which  the  foetal 
ocular  cleft  is  usually  met  with.  Displacements  might  also  be  occasioned 


1  Trans.  Ophth.  Soc.,  vol.  ix.  p.  166. 

*  Royal  Lond.  Ophth.  Hosp.  Rep.,  vol.  v.  p.  1. 


452      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 

by  the  adhesions  between  the  ciliary  body  and  the  margin  of  the  lens  being 
denser  on  one  side  and  less  elastic  than  normal,  so  that  they  expand  less 
readily  than  those  on  the  opposite  side ;  this  would  account  for  those  cases 
in  which  fibres  of  the  suspensory  ligament  can  be  seen  stretching  across  the 
aphakic  area. 

CONGENITAL   CATARACTS. 

Congenital  cataracts  may  be  divided  into  those  in  which  the  opacity 
extends  throughout  the  lens  and  those  in  which  only  a  portion  of  the 
lens  is  opaque.  The  partial  cataracts  may  be  further  subdivided  into 
(1)  anterior  polar  cataracts,  (2)  posterior  polar  cataracts,  (3)  lamellar  cata- 
racts, (4)  nuclear  cataracts,  (5)  dotted  cataracts. 

1.  Anterior  Polar  Cataracts. — The  term  anterior  polar  or  pyramidal 
cataract  should  be  confined  to  those  cases  in  which  an  opacity  is  situated  at 
the  anterior  pole  of  the  lens,  within  the  capsule, — that  is,  opacities  of  the 
lens  itself.     It  should  not  be  applied  to  those  in  which  the  opacity  is  ex- 
ternal to  the  capsule  and  due  to  a  persistence  of  a  portion  of  the  pupillary 
membrane.    Clinically,  it  is  often  very  difficult  to  localize  exactly  the  posi- 
tion of  the  opacity,  and  sometimes,  possibly,  the  two  forms  are  associated. 
That  a  true  anterior  polar  cataract  without  any  opacity  external  to  the 
capsule  may  be  congenital  is  proved  by  a  case  of  one  of  the  authors 
(E.  T.  C.),  in  which  the  eyeball  was  obtained  for  pathological  examination. 
In  it  the  lens-capsule  at  the  anterior  pole  was  raised  and  wrinkled,  it 
having  immediately  beneath  it  a  mass  of  laminated  hyaline  substance  and 
scattered  epithelial  cells.      This  mass  presented  microscopical  appearances 
precisely  similar  to  those  seen  in  pyramidal  cataracts  which  occur  after 
ulceration  of  the  cornea. 

2.  Posterior  Polar  Cataracts. — This  form  of  cataract  is  sometimes  said 
to  be  congenital.     It  is  possible,  however,  that  the  cases  so  recorded  have 
been  ones  in  which  portions  of  the  posterior  fibro- vascular  sheath  of  the 
lens  have  remained  persistent  and  attached  to  the  external  surfaces  of  the 
capsule ;  therefore  not  really  opacities  of  the  lens  itself. 

3  and  4.  Zonular  and  Nuclear  Cataracts. — Zonular  or  lamellar  cataract 
is  the  name  given  to  that  form  of  opacity  of  the  lens  in  which  there  is  a 
layer  of  opaque  substance  situated  between  a  clear  nucleus  and  a  clear 
cortex.  It  is  distinguished  from  nuclear  cataract  by  illumination  of  the 
eye  with  the  ophthalmoscopic  mirror,  when  the  margin  of  the  disk  is  seen 
to  be  darker  than  the  centre  ;  whereas  when  the  nucleus  is  opaque  the  centre 
of  the  disk  is  as  dark  as  or  darker  than  the  edge.  In  some  rare  cases  a 
second  complete  or  partial  layer  of  opacity  is  present,  the  two  being  sepa- 
rated by  clear  substance.  Frequently  on  the  surface  of  the  disk  of  opacity 
streaks  or  dots  of  denser  opacity  are  seen,  which  may  project  beyond  it 
into  the  clear  cortex,  when  they  produce  much  the  appearance  of  the 
handles  of  the  steering-wheel  of  a  ship.  There  has  been  much  discussion 
as  to  whether  lamellar  cataract  is  congenital,  or  whether  it  occurs  during 
the  first  few  years  of  infancy.  Measurements  of  the  zone  of  opacity  and 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OP  THE  EYE.       453 

measurements  of  the  foetal  lens  show  that  the  former  is  never  larger  than 
the  lens  at  birth,  that  it  may  be  about  the  size  of  it  at  that  time,  and  that 
it  is  usually  smaller,  sometimes  very  much  smaller.  From  this  it  must  be 
concluded  that  lamellar  cataract  is  produced  before  birth,  or  that  the  part 
affected  at  the  time  the  change  occurs  is  not  the  most  peripheral,  as  has 
generally  been  supposed.  Certainly  lamellar  cataract  occasionally  comes  on 
after  birth,  for  Graefe  has  recorded  a  case  in  which  the  opacity  was  caused 
by  iritis  and  synechia,  Beselin  and  Schirmer  recorded  cases  following  ulcer- 
atiou  of  the  cornea,  and  one  of  the  writers  of  this  article  has  seen  a  case  in 
which  the  opacity  was  brought  about  in  the  same  way. 

Von  Graefe l  and  Ja'ger z  were  the  first  to  bring  forward  anatomical 
proof  of  the  lamellar  character  of  this  form  of  opacity ;  they  found,  on 
section  of  lenses  presenting  the  appearances  described,  which  had  been  ex- 
tracted, that  there  was  an  opaque  whitish  line  separating  a  clear  nucleus 
from  a  clear  cortex.  During  the  last  few  years  much  has  been  written  on 
the  microscopical  appearances  of  lamellar  cataract  by  Deutschmann,3  Bese- 
lin,4 Schirmer,5  Lawford,6  Hess,  and  Treacher  Collins.7  The  outcome  of 
these  observations  seems  to  be  that  there  are  three  sorts  of  changes  met  with. 
First,  fissures  between  the  lens-fibres  which  may  or  may  not  contain  a  gran- 
ular substance,  and  which  run  concentric  to  the  nucleus,  separating  it  from 
the  cortex.  Second,  small  vacuoles,  the  average  size  of  which  is  .005  mil- 
limetre across ;  they  are  mostly  round  or  oval,  but  in  places  where  they 
seem  to  have  run  into  one  another  they  are  elongated  and  beaded.  Some 
of  them  contain  a  hyaline  substance,  which  after  prolonged  immersion  in 
logwood  stains  deeper  than  the  surrounding  lens-fibres.  Third,  spaces  larger 
than  the  so-called  vacuoles,  measuring  on  an  average  .02  millimetre  across, 
mostly  circular,  with  very  irregular  margins,  and  containing  a  granular 
substance  which  stains  deeply  with  logwood.  Apparently  some  degenera- 
tion of  lens-substance  has  occurred  in  their  formation.  These  three  changes 
correspond  to  the  appearances  seen  clinically  in  these  cataracts, — viz.,  radi- 
ating spokes,  the  uniform  ha/e,  and  dots. 

5.  Dotted  Cataracts. — Numerous  small,  scattered  congenital  opacities 
are  sometimes  seen  in  the  lens,  in  the  form  either  of  circular  patches  or  of 
little  streaks.  They  are  most  frequently  situated  in  the  peripheral  parts, 
and  do  not  give  rise  to  any  defect  in  the  acuity  of  vision.  The  condition 
is  generally  hereditary. 

Complete  Congenital  Cataracts. — Complete  congenital  cataracts  may  be 
divided  into  three  classes,  according  to  their  consistency  : 

1  Archiv  fur  Ophth.,  Bd.  i.  S.  236. 

2  Staar  und  Staar-Operationen,  1854,  S.  17. 

3  Archiv  fur  Ophth.,  Bd.  xxxii.,  Abth.  2,  S.  295. 

4  Archiv  fur  Augenheilk.,  Bd.  xviii.  S.  71. 

5  Archiv  fur  Ophth.,  Bd.  xxxv.,  Abth.  1,  S.  147. 

8  Royal  Lond.  Ophth.  Hosp.  Rep.,  vol.  xii.  p.  184. 
7  Lancet,  December,  1894. 


454      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 

1.  Those  in  which  it  is  quite  fluid,  the  capsule  being  simply  a  bag 
containing  the  opaque,  liquid,  degenerate  lens-substance,  which?  when  the 
former  is  punctured,  runs  out  and  can  be  at  once  evacuated. 

2.  Cases  in  which  the  lens  presents  a  milky-white,  uniform  opacity,  but 
in  which  it  retains  much  of  the  normal  gelatinous  consistency  of  a  juvenile 
lens. 

3.  Cases  in  which  the  opacity  is  densely  white,  and  where  the  lens  is 
apparently  shrunken  and  flattened ;  in  these  the  pupil  will  often  not  re- 
spond well  to  a  mydriatic.     A  very  probable  explanation  of  such  cases  is 
that  the  opacity  and  failure  in  development  of  the  lens  are  secondary  to  a 
persistence  and  thickening  of  the  posterior  fibro-vascular  sheath,  together 
with  persistence  of  the  central  artery  of  the  vitreous.     The  opaque  mem- 
brane remaining  after  discission  is  the  thickened  fibro-vascular  sheath  or 
abnormally  developed  vitreous,  which,  being  composed  of  fibrous  tissue, 
would  not  be  acted  upon  by  the  aqueous  humor.     (See  Abnormalities  of 
the  Vitreous.) 

CONGENITAL  ABNORMALITIES  OF  THE  VITREOUS. 

The  congenital  abnormalities  of  the  vitreous  humor  which  are  met 
with  proceed  from  the  persistence  of  some  portion  of  the  vascular  system 
which  exists  in  it  during  foetal  life,  or  from  what  has  been  aptly  termed 
by  Hess  atypical  embryonic  development  of  the  mesoblastic  tissue  from 
which  it  is  derived.  The  blood-vessel  which  in  foetal  life  courses  through 
the  vitreous  is  continuous,  as  has  been  already  stated,  with  the  central 
artery  of  the  optic  nerve.  It  lies  in  a  canal  bounded  by  a  hyaline  mem- 
brane, and  for  a  portion  of  its  extent  is  surrounded  by  a  cellular  sheath  ; 
it  is  generally  continued  as  a  single  vessel  to  the  posterior  surface  of  the 
lens,  where  it  breaks  up  into  branches  which  supply  the  posterior  vascular 
sheath  of  that  structure.  In  some  eyes,  however,  it  branches  dichotomously 
several  times  in  its  course  through  the  vitreous. 

It  is  important  to  bear  these  several  points  in  mind,  because  so  many 
different  anomalies  are  produced  by  persistence  of  one  or  more  portions  of 
this  system.  These  various  anomalies  may  be  classed  as  follows  : 

1.  That  in  which  the  whole  artery,  with  the  cellular  sheath  around  the 
posterior  part  of  it,  remains  as  in  the  foetal  eye,  and  continues  to  carry 
blood.  (Figs.  20  and  21.)  A  few  such  cases  have  been  seen  with  the 
ophthalmoscope,  the  presence  of  blood  in  the  artery  having  been  diag- 
nosed by  the  red  color  which  it  presented.  In  some  of  these  certainly, 
and  probably  in  all,  a  portion  of  the  posterior  fibro-vascular  sheath 
of  the  lens  also  existed,  the  vessels  of  which,  by  retaining  their  con- 
nections with  those  of  the  ciliary  body,  allowed  of  the  exit  of  the  blood 
traversing  the  abnormal  artery.  In  several  eyes  in  which  a  persistent 
and  patent  hyaloid  artery  was  found  on  dissection,  it  ended  in  a  mass  of 
fibrous'  tissue  at  the  back  of  the  lens.  (Vassaux,  Haab,  Hess,  Treacher 
Collins.) 


CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.      455 

These  cases  will  be  referred  to  again  in  considering  atypical  develop- 
ment of  the  Vitreous. 

2.  That  in  which  the  artery  persists  as  a  band  through  its  whole  length, 
but  does  iiot  carry  blood.  In  such  cases  the  band  can  be  seen  with  the 
ophthalmoscope  stretching  from  the  optic  disk  to  the  posterior  pole  of 
the  lens,  to  both  of  which  it  is  adherent.  It  is  frequently  noticed  to 
oscillate  on  movement  of  the  globe,  and  is  generally  larger  at  its  two 
extremities  than  in  the  centre.  In  a  few  cases  the  band  coming  from  the 


FIG.  20. 


FIG.  21. 


Diagrammatic  representation  of  the  section 
of  an  eye  with  a  persistent  and  patent  hyaloid 
artery  which  terminated  in  a  thick  fibrous  mem- 
brane at  the  back  of  the  lens.  (Royal  Lond. 
Ophth.  Hosp.  Rep.,  vol.  xiii.) 


Diagrammatic   representation   of   a   case  very 
similar  to  that  depicted  in  Fig.  20. 


optic  disk  has  been  observed  to  divide  several  times  in  the  vitreous  before 
reaching  the  back  of  the  lens.  When  an  undivided  band  is  present,  its 
point  of  attachment  to  the  lens  is  not  always  central,  but  is  frequently  a  little 
to  one  side  or  the  other ;  sometimes  there  is  a  stellate  opacity  of  greater  or 
less  density  in  which  it  terminates. 

3.  That  in  which  there  is  a  remnant  of  the  artery  attached  to  the  optic 
disk  which  ends  in  a  free  extremity  in  the  vitreous,  and  in  which  there  is 
also  an  opacity  at  the  back  of  the  lens.     The  remnant  attached  to  the  disk 
is  often  a  long,  thin  cord  which  is  seen  by  the  ophthalmoscope  to  lash  about 
in  an  undulating  way  on  movement  of  the  eye. 

An  interesting  case  of  the  second  class  is  recorded  by  Unterhamscheit l 
in  a  myopic  lad  aged  fourteen,  which  three  years  later  was  found  to  have 
become  converted  into  one  of  the  third  class,  the  band  having  given  way 
on  account  of  the  elongation  of  the  eye  from  increase  of  the  myopia. 

4.  That  in  which  the  lenticular  end  of  the  artery  has  alone  remained,  its 
hindermost  end  being  free  and  mobile  in  the  vitreous.     It  is  a  variety 
which  is  rarely  met  with.     De  Beck 2  could  find  only  eight  recorded  cases 
of  it. 


1  Klinische  Monatsb.  fur  Augenheilk.,  Bd.  xx.  S.  240. 

2  Persistent  Kemains  of  the  Foetal  Hyaloid  Artery,  Cincinnati,  1890. 


456      CONGENITAL  MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE. 


FIG.  22. 


Shrunken  globe  in  which  a  tag  of  a 
persisteut  hyaloid  artery  was  found  ad- 
herent to  the  optic  nerve  on  pathological 
examination. 


5.  That  in  which  the. neural  end  of  the  artery  remains,  the  lenticular  ex- 
tremity having  completely  disappeared.     (Fig.  22.)     This  is  the  commonest 

variety  of  any.  A  thin,  narrow  cord  is 
seen  with  the  ophthalmoscope  attached  by 
one  end  to  the  optic  disk,  the  other  ending 
free  in  the  vitreous.  Its  length  varies 
considerably  in  different  cases.  It  may 
be  of  a  dark  color  and  quite  easily  seen, 
or  of  a  light  gray  and  semi-transparent 
tint,  so  that  it  is  readily  overlooked.  The 
free  extremity  of  the  cord  is  sometimes 
rounded  and  knob-like,  at  other  times 
fine  and  tapering;  occasionally  the  band 
may  bifurcate  once  or  oftener.  It  gener- 
ally oscillates  with  an  undulating  motion 
on  movement  of  the  eye,  but  occasionally 
is  fixed  and  immobile. 

6.  As  stated  above,  the  posterior  part 
of  the  central  hyaloid  artery  is  surrounded 
by  a  cellular  sheath.  The  artery  may 

become  obliterated,  but  portions  of  this  sheath  may  remain  as  fibrous 
membranes  or  shreds  of  tissue  attached  to  the  optic  disk,  filling  up  the 
depression  normally  left  in  the  nerve-head  by  the  divergence  of  the  nerve- 
fibres.  Such  gray  membranes  or  tags  of  tissue  are  quite  commonly  seen 
ophthalmoscopically  in  otherwise  perfectly  normal  eyes,  and  one  of  the 
authors  has  examined  two  of  them  microscopically,  and  found  them  com- 
posed of  cells  and  fibres  similar  to  those  forming  the  sheath  of  the  artery 
in  the  fetal  state. 

7.  Little  rounded  bodies  of  a  steel-gray  color,  which  appear  to  be  fluid- 
containing  cysts  attached  to  the  optic  disk,  are  occasionally  met  with  as  a 
congenital  abnormality,  and  must  be  in  some  way  connected  with  the  fetal 
vascular  apparatus  for  the  vitreous.     It  is  not  evident  whether  they  are 
cystic  distentions  of  the  persistent  sheath  of  the  artery  or  cystic  distentions 
of  the  artery  itself. 

8.  Some  observers  have  seen  appearances  in  the  vitreous  which  they 
have  thought  pointed  to  persistence  of  the  hyaline  canal  in  which   the 
artery  lies,  it  and  its  cellular  sheath  having  disappeared.     It  is  usually 
believed  that  part  of  this  canal  exists  in  the  normal  eye  (the  canal  of  Stil- 
ling), but  that  it  is  not  visible  with  the  ophthalmoscope.    Alteration,  either 
in  its  contents  or  in  its  extent,  may  possibly  in  some  cases  render  it  so. 

9.  Congenital  opacities  at  the  posterior  pole  of  the  lens  are  probably 
many  of  them  due  to  persistence  of  portions  of  the  fibre-vascular  sheath 
of  the  lens,  the  vessels  of  which  are  derived  in  the  fatal  state  from  the 
division  of  the  central  hyaloid  artery. 

Several  cases  have  now  been  recorded  in  which  the  vitreous  humor  has 


CONGENITAL  .MALFORMATIONS  AND  ABNORMALITIES  OF  THE  EYE.       457 

been  found  in  a  part  of  its  extent  to  have  been  replaced  by  a  mass  of  fibres 
and  cells,  and  this  in  eyes  in  which  the  other  structures  showed  not  the  least 
sign  of  active  or  past  inflammation.  This  fibrous  and  cellular  tissue  has 
been  found  located  in  different  positions,  sometimes  forming  a  thick  mem- 
brane behind  the  lens  (Figs.  20  and  21),  in  other  eyes  a  mass  in  the  region 
of  the  ocular  cleft  or  a  thick  band  passing  through  the  centre  of  the  globe 
from  before  backward.  (Fig.  19.)  The  eyes  in  which  it  was  found  were 
many  of  them  microphthalmic,  and  the  central  artery  of  the  vitreous  was 
also  usually  persistent  and  patent. 

When  the  abnormal  tissue  is  situated  as  a  membrane  behind  the  lens, 
if  that  structure  has  remained  clear,  a  yellowish-gray  reflex  is  obtained 
from  immediately  posterior  to  it,  and  the  appearances  of  glioma  of  the 
retina  are  closely  simulated,  which  has  several  times  led  to  a  mistaken 
diagnosis  and  excision  of  the  globe.  When  it  has  formed  a  band  passing 
through  the  vitreous,  it  has  sometimes  held  the  lens  back  while  the  globe 
has  increased  in  size,  and  so  brought  about  a  congenital  displacement  of  the 
lens.  Hess  explains  the  presence  of  this  fibrous  and  cellular  tissue  by 
supposing  that  the  mesoblastic  tissue  which  grows  inward  to  form  the 
vitreous,  instead  of  expanding  into  that  structure  in  the  normal  way,  has 
undergone  what  he  terms  an  atypical  form  of  development. 


THE  DIOPTRICS  OF  THE  EYE. 

BY  EDWARD  JACKSON,  A.M.,  M.D., 

Professor  of  Diseases  of  the  Eye  in  the  Philadelphia  Polyclinic ;   Special  Lecturer  on 

Physiological  Optics  in  the  University  of  Colorado,  Denver, 

Colorado,  U.S.A. 


LIGHT,  according  to  the  undulatory  theory,  .which  best  correlates  its 
phenomena,  consists  of  series  of  vibrations  of  the  extremely  tenuous  and 
elastic  ether  which  pervades  space,  interpenetrating  all  substances,  and  being 
present  in  a  so-called  vacuum.  These  vibrations  travel  with  extreme 
rapidity  (186,380  miles  per  second  in  vacuum),  and  are  very  minute.  The 
following  table  gives  the  number  of  complete  vibrations  or  wave-spaces 
for  light  of  different  colors  contained  in  a  single  millimetre  of  space.  The 
letters  indicate  the  Fraunhofer  lines,  giving  the  exact  position  of  the  light 
in  the  solar  spectrum. 

Color  of  Light.  Fraunhofer  Line.  Waves  per  Millimetre. 

Red B  1305 

Orange C  1524 

Yellow D  1697 

Green E  1898 

Blue .   .   .  F  2058 

Indigo G  2331 

Violet H  2540 

Substances  which  permit  the  passage  through  them  of  the  luminous 
vibrations  of  ether  are  said  to  be  transparent.  Those  that  do  not  permit 
the  passage  of  such  vibrations  are  opaque.  Any  transparent  substance  may 
be  a  dioptric  medium.  When  the  surface  of  such  a  medium  is  smooth,  light 
can  pass  freely  through  it,  and  it  constitutes  a  dioptric  surface.  When  the 
surface  of  a  dioptric  medium  is  such  that  it  turns  the  vibrations  that  were 
about  to  pass  from  it  back  into  the  medium,  it  is  a  reflecting  surface. 

Waves  of  light,  like  all  other  waves,  move  with  always  the  same  velocity 
in  the  same  medium,  and  move  in  a  direction  perpendicular  to  the  wave- 
crest  or  wave-front.  The  line  along  which  are  moving  the  corresponding 
points  of  the  successive  waves  is  called  a  ray  of  light.  A  number  of  adjoin- 
ing rays  constitute  a  pencil  of  rays,  the  path  of  an  appreciable  portion  of 
successive  waves.  Rays  falling  on  a  dioptric  or  reflecting  surface  are  called 
incident  rays. 

From  each  luminous  point  light  passes  off  equally  in  all  directions, 

459 


460 


THE    DIOPTRICS   OF   THE   EYE. 


unless  interrupted  by  some  opaque  substance ;  and,  each  part  of  it  travel- 
ling with  equal  rapidity  from  the  point  of  origin,  a  wave-front  constitutes 
the  surface  of  an  enlarging  sphere,  of  which  the  rays  of  light  are  radii. 
Taking  a  limited  pencil  of  rays,  a  limited  part  of  the  wave-fronts,  the 
farther  we  go  from  the  point  of  origin  the  flatter  the  wave-fronts  become ; 
and  the  less  divergent  the  more  nearly  parallel  are  the  included  rays.  This 
is  illustrated  by  comparing  the  wave-fronts  of  light,  and  the  rays  repre- 
sented as  passing  the  equal  openings  at  A,  B,  (?,  and  Dy  in  Fig.  1 .  In  ophthal- 

FIG.  1. 


mology  we  have  to  consider  the  pencil  of  rays  entering  the  pupil.  When 
this  comes  from  a  distance  of  twenty  feet  or  more,  it  is  customary,  although 
not  strictly  accurate,  to  speak  of  the  rays  as  parallel. 

CATOPTRICS,   OR  THE   REFLECTION   OF   LIGHT. 

All  dioptric  surfaces  are  also  reflecting  surfaces,  the  proportion  of  light 
transmitted  through  them  and  the  proportion  reflected  from  them  varying 
with  the  substances  which  they  separate  and  the  angle  at  which  the  vibra- 
tions strike.  Other  than  dioptric  surfaces  are  also  reflecting  surfaces,  as 
the  polished  surfaces  of  opaque  bodies  and  all  irregular  surfaces.  From  a 
polished  surface  light  is  reflected  in  a  definite  direction,  the  vibrations  pre- 
serving their  original  characters :  this  is  called  regular  reflection.  This  kind 
of  reflection  enables  one  to  see  the  object  from  which  the  light  came  to  the 
surface.  But  from  irregular  surfaces  (and  from  polished  surfaces  in  so  far 
as  their  polish  is  imperfect)  light  is  reflected  in  all  directions,  often  with 
considerable  alteration  of  the  vibrations.  This  is  irregular  reflection.  It 
enables  us  to  see  the  reflecting  surface.  All  bodies  that  do  not  of  them- 
selves emit  light  are  thus  rendered  visible.  For  instance,  of  the  light 
thrown  into  the  eye  with  the  ophthalmoscope,  the  bulk  is  irregularly  re- 
flected from  the  retina  and  choroid,  enabling  the  surgeon  to  see  these  mem- 
branes ;  but  a  part  is  regularly  reflected  from  the  smooth  surface  of  the 
cornea  and  a  part  from  the  smooth  surface  of  the  retina,  giving  the  annoy- 
ing corneal  reflex  and  the  instructive  retinal  reflexes,  both  of  them  more  or 
less  imperfect  images  of  the  source  of  light.  We  are  here  chiefly  concerned 
with  regular  reflection. 

In  regular  reflection  the  wave-fronts  have,  after  reflection,  an  inclination 
to  the  reflecting  surface  opposite  in  direction  but  equal  in  amount  to  their 


THE  DIOPTRICS  OF  THE  EYE. 


461 


inclination  to  that  surface  before  striking  it.     This  is  illustrated  in  Fig.  2,  in 
which  A  A',  BB',  CO',  DD',  EE',  FF',  and  GG'  represent  successive  wave- 
fronts,  or  successive  posi- 
tions of  the  same  wave-  ^10.  2- 
front,  coming  from  the 
direction   of  /,  and   re- 
flected by  the  surface  S 
in  the  direction  R.     IS 
represents     an    incident 
ray,  and  SR  a  reflected 

ray.     PS  is  the  perpen-  ^  £  J. 

dicular  to  the  reflecting 

surface  at  the  point  S.  IS  and  SR  necessarily  lie  in  the  same  plane  perpen- 
dicular to  the  reflecting  surface.  The  angle  which  the  incident  wave-fronts 
make  with  the  reflecting  surface,  or  its  equal,  the  angle  ISP,  which  the 
incident  ray  makes  with  the  perpendicular  to  that  surface,  is  called  the  angle 
of  incidence.  The  angle  which  the  reflected  wave-front  makes  with  the 
reflecting  surface,  or  the  angle  PSR  which  the  reflected  ray  makes  with  the 
perpendicular,  is  called  the  angle  of  reflection.  TJie  angle  of  incidence  always 
equals  the  angle  of  reflection. 

Reflection  by  a  Plane  Mirror. — The  perpendiculars  or  normals  to  a  plane 
surface,  PS,  P'S',  and  P"S",  Figs.  3  and  4,  are  all  parallel.  Hence  when 
parallel  rays  fall  on  a  plane  reflecting  surface,  each  forms  with  the  normal 
at  its  point  of  incidence  the  same  angle  of  incidence,  and,  consequently,  the 
same  angle  of  reflection  ;  and  they  all  pass  off,  after  reflection,  still  parallel. 
Thus,  in  Fig.  3,  the  rays  coming  from  /and  reflected  from  Sf  pass  off  towards 


FIG.  3. 


Fio.  4. 


R,  still  parallel.  Again,  if  the  incident  rays  be  divergent,  they  will  continue 
after  reflection  equally  divergent.  Thus,  in  Fig.  4,  rays  divergent  from  I 
are  reflected  to  R,  as  though  they  were  diverging  from  /'  situated  in 
the  same  perpendicular  to  the  mirror  as  /,  and  equally  distant  from  it  on 
the  opposite  side.  If  the  incident  rays  are  convergent,  they  continue  so 
after  reflection.  Thus,  if  in  Fig.  4  the  incident  rays  are  supposed  to  come 
from  R,  converging  towards  /',  they  will,  after  reflection,  continue  to 


462 


TITE   DIOPTRICS   OF   THE   EYE. 


Fio.  5. 


converge  to  /.     Reflection  by  a  plane  mirror  does  not  alter  the  parallelism, 
divergence,  or  convergence  of  rays. 

Reflection  by  a  Concave  Spherical  Mirror. — When  the  reflecting  surface 
is  spherical  and  concave,  like  the  ordinary  ophthalmoscopic  mirror,  the 
normals  to  it  are  radii  of  curvature,  which  converge  and  meet  in  front  of  it 
at  the  centre  of  curvature.  The  rays  reflected  from  such  a  surface,  under 
the  law  of  equal  angles  of  incidence  and  reflection,  are  rendered  relatively 
convergent.  Let  us  consider  first  what  occurs  when  the  incident  rays  are 
parallel.  This  is  represented  in  Fig.  5,  in  which  AM  represents  the  section 

of  a  concave  spherical  mirror  and  C  its  centre  of 
curvature.  Of  a  pencil  of  parallel  rays  falling 
on  the  mirror,  the  one  passing  through  C  and 
incident  at  A  is  perpendicular  to  the  reflecting 
—  surface  and  is  reflected  back  upon  itself.  Another 
ray,  OM,  incident  at  M,  makes  with  the  normal 
— .  (the  radius)  CM  an  angle  of  incidence  OMC,  and 
is  reflected  towards  F,  making  an  angle  of  reflec- 
tion CMF  equal  to  OM C.  In  the  triangle  CMF  the  angle  FCM  is  also 
equal  to  OMC,  because  OM  is  parallel  to  CA  and  CM  common  to  the  two 
angles.  Hence  FCM  and  FMC  are  equal,  and  therefore  FM  equals  FC. 
When  AM  is  a  comparatively  small  arc,  AF  is  very  nearly  equal  to  FM  or 
FC,  and  F  may  be  regarded  as  midway  between  A  and  C.  All  rays  parallel 
to  A  C  and  sufficiently  near  the  central  ray  will  thus  be  reflected  to  F,  which 
is  called  the  principal  focus  of  the  mirror.  Its  distance  AF  from  the  mirror 
is  the  principal  focal  distance,  and  is  half  the  radius  of  curvature.  For 
rays  farther  removed  from  the  central  ray,  and  therefore  striking  the  re- 
flecting surface  more  obliquely,  AF  becomes  decidedly  shorter  than  FC,  so 
that  rays  are  perfectly  focussed  by  a  concave  mirror  only  when  they  fall 
nearly  perpendicular  to  its  surface. 

If  the  incident  rays,  as  in  Fig.  6,  instead  of  being  parallel,  are  divergent 


PIG.  6. 


FIG.  7. 


from  any  point  beyond  C,  as  A,  the  angles  of  incidence  and  reflection  are 
smaller,  and  the  focus  falls  farther  from  the  mirror  and  nearer  the  centre  of 
curvature  at  A'.  If  the  rays  diverge  from  the  centre  of  curvature  C,  they 
fall  perpendicularly  upon  the  mirror  and  are  reflected  back  to  that  point. 


THE   DIOPTRICS   OF  THE   EYE. 


463 


From  points  between  C  and  F  they  will  be  reflected  to  points  beyond  <?,  as 
from  A'  to  A.  From  F  the  mirror  renders  them  parallel ;  and  from  points 
between  F  and  the  mirror,  as  B,  they  remain  divergent  after  reflection,  as 
though  from  a  point  B'  back  of  the  mirror,  called  a  virtual  focus.  When 
rays  from  one  point  are  reflected  to  a  focus  at  a  second  point,  rays  from  the 
second  are  reflected  to  a  focus  at  the  first.  Such  points  are,  therefore,  said 
to  be  conjugate  foci.  The  point  within  the  focal  distance  from  which  rays 
diverge,  and  the  point  back  of  the  mirror  from  which,  after  reflection,  they 
appear  to  diverge,  as  B  and  B',  have  the  same  relation. 

Reflection  by  a  Convex  Spherical  Mirror. — This,  as  it  occurs  from  the 
surface  of  the  cornea,  is  the  subject  of  study  by  the  ophthalmometer.  It  is 
illustrated  in  Fig.  7,  in  which  the  lettering  corresponds  to  that  of  Fig.  5. 
The  ray  perpendicular  to  the  surface  is  reflected  upon  itself,  and  the  ray 
OM  is  caused  to  diverge,  as  from  F,  the  principal  (virtual)  focus  of  the 
convex  mirror.  Rays  divergent  when  they  strike  such  a  mirror  are  ren- 
dered more  divergent  by  it.  Only  rays  more  convergent  before  reflection 
than  the  normals  to  the  points  on  which  they  fell  remain  after  reflection 
convergent  to  an  actual  focus  in  front  of  the  mirror. 

Formation  of  Images  by  Mirrors. — For  the  plane  mirror  this  is  illus- 
trated in  Fig.  8.  In  general,  corresponding  points  of  an  object  and  its 


FIG.  8. 


Fio.  9. 


image  are  symmetrical  as  regards  the  reflecting  surface,  and  equally  distant 
from  that  surface.  This  causes  a  sort  of  lateral  reversal  of.  the  image  as 
compared  with  the  object,  but  not  any  reversal  in  the  direction  parallel  to 
the  mirror.  A  familiar  example  of  this  is  the  "  reversal"  of  print  or 
manuscript  seen  in  a  mirror.  The  symmetry  of  corresponding  points  also 
causes  the  image  exactly  to  equal  the  object  in  size. 

With  the  concave  mirror  the  image  of  each  point  of  an  object  is  formed 
on  the  ray  that  passes  from  that  point  through  the  centre  of  curvature  of 
the  mirror.  This  leads  to  a  complete  reversal  of  the  image  as  compared 
with  the  object  when  these  are  on  opposite  sides  of  this  centre  of  curvature, 
but  only  a  partial  or  lateral  reversal  like  that  of  the  plane  mirror  when  both 
are  on  the  same  side  of  this  centre  of  curvature. 

In  Fig.  9  the  object  AB  is  situated  beyond  the  centre  of  curvature. 


464 


THE   DIOPTRICS   OF   THE    EYE. 


Hence  the  image  ab  is  situated  between  the  centre  of  curvature  and  the 
principal  focus.  When  the  object  is  situated  between  the  centre  of  curva- 
ture and  the  principal  focus,  as  at  ab,  the  image  is  in  front  of  the  mirror 
and  beyond  the  oentre  of  curvature,  as  at  AB.  When  the  object  is  situ- 
ated between  the  principal  focus  and  the  mirror,  its  image  is  virtual  and 
situated  behind  the  mirror,  as  shown  in  Fig.  10,  and  the  farther  the  object 
from  the  mirror  (the  less  divergent  the  rays)  the  farther  will  the  image  be 
from  it. 

With  a  convex  spherical  mirror,  the  object  being  always  in  front  of  the 
mirror  and  the  rays  divergent,  the  image  will  always  be  back  of  the  mirror 
(see  Fig.  1 1) ;  and  since  the  nearer  the  object  to  the  mirror  the  more  divergent 
the  incident  rays,  the  more  divergent  will  the  reflected  rays  be  also,  and  the 
closer  the  image  to  the  mirror.  In  general,  the  image  is  closer  to  the  mirror 


FIG.  11. 


FIG.  10. 


than  the  principal  focus  of  the  mirror ;  so  that,  object  and  image  being  both 
on  the  same  side  of  the  centre  of  curvature,  the  image  is  erect. 

The  relative  size  of  the  image  formed  by  a  spherical  mirror  is  propor- 
tioned to  its  relative  distance  from  the  centre  of  curvature  of  the  spherical 
surface.  Thus,  in  Figs.  9,  10,  and  11  (the  lettering  being  the  same  in  all) 
we  have  ABC&ud  abC  similar  triangles,  in  which 

ab:AB  :  :  aC :  AC. 


DIOPTRICS,   OR  THE  REFRACTION   OF   LIGHT. 

Light-waves  move  at  different  rates  in  different  dioptric  media.  We 
have  already  seen  that  the  rate  of  movement  in  vacuum  is  186,380  miles 
per  second.  Taking  as  unit  the  length  of  time  it  requires  to  go  a  certain 
distance  in  vacuum,  the  following  table  gives  the  length  of  time  it  would 
require  to  go  the  same  distance  in  the  substances  named.  This  number 
indicating  the  relative  length  of  time  it  takes  light  to  travel  a  certain  distance 
in  a  given  substance  is  called  the  index  of  refraction  of  the  substance.  It 
is  customary  and  convenient  in  ophthalmology  to  take  the  index  of  refrac- 
tion of  air  as  unit ;  but,  since  this  differs  from  that  of  vacuum  only  in 
the  fourth  decimal  place,  the  difference  may  for  all  practical  purposes  be 
disregarded. 


THE   DIOPTRICS   OF   THE   EYE. 


465 


Substance.  todex  of  Refraction. 

Vacuum 1_ 

Air 1.000294 

Water  at  0°  Cent 1.3330 

Water  at  40°  Cent 1.8297 

Alcohol    .    .  1.372 

Canada  balsam 1.532 

Aqueous  humor 1.33C5 

Vitreous  humor 1.3365 

Cornea     1.3365 

Crystalline  lens      1.39  to  1.43 

Crown  glass 1.51  to  1.54 

Flint  glass 1.55  to  1.72 

Rock  crystal 1.65  to  1.57 

Diamond 2.47  to  2.75 

It  is  common  to  speak  of  differences  of  index  of  refraction  as  differences 
of  "  density."  But  index  of  refraction  must  not  be  confused  with  specific 
gravity,  which  is  usually  meant  when  the  word  "  density"  is  used,  and  with 
which  it  has  no  direct  or  constant  relation.  For  instance,  water,  with  a 
specific  gravity  of  1,  has  an  index  of  refraction  of  1.333,  while  alcohol, 
with  a  specific  gravity  of  only  0.728,  has  an  index  of  refraction  of  1.372. 

If  we  suppose  a  succession  of  light-waves  passing  from  one  medium 
having  a  lower  to  another  having  a  higher  index  of  refraction,  as  from  air 
into  glass,  the  distances  travelled  by  each  wave- front  in  a  unit  of  time  will 
be  the  reciprocals  of  the  indexes  of  refraction  ;  that  is,  while  a  wave-front 
is  passing  a  distance  j-  in  the  air,  it  passes  only  T^  (or  a  little  less  than 
two-thirds  the  distance)  in  the  glass.  If  all  parts  of  each  wave-front  enter 
the  glass  at  the  same  time, — that  is,  if  the  wave-fronts  are  parallel  (the  rays 
perpendicular)  to  the  dioptric  surface, — their  direction  is  unaltered  by  the 
passage.  When,  however,  the  wave-fronts  strike  the  dioptric  surface  ob- 
liquely, one  portion  still  moving  in  the  first  medium,  while  another  part  of 
the  same  wave-front  has  passed  into  the  second  medium,  the  slower  move- 
ment in  the  latter  causes  a  change  of  direction  in  the  wave-fronts  and  a 
change  in  the  direction  of  the  movement 
of  the  light,  since  this  is  always  perpen- 
dicular to  the  wave-fronts.  Such  change 
of  direction  is  called  the  refraction  of 
light  The  general  nature  and  extent  of 
such  a  change  of  direction  are  indicated 
in  Fig.  12.  AB  represents  a  dioptric 
surface  separating  air  and  glass;  IB,  a 
wave-front  falling  on  the  glass  from  the 
air;  AR,  a  wave-front  passing  in  the 
glass  ;  I  A  and  I'B,  incident  rays,  and 
BR  and  AR',  refracted  rays ;  and  PP',  a  perpendicular  to  the  dioptric 

surface  at  A.     Since  the  light  travels  in  the  glass  only  j-^  as  far  as  in  the 
VOL.  I.— 30 


FIQ.  12. 


466 


THE   DIOPTRICS   OF   THE    EYE. 


-— 
1 


.±SL, 

1.53 


But  since  the  rays  are  perpendicular  to  the  wave-fronts. 


ABI  and  ABR  are  right-angled  triangles  in  which  IB  A  is  the  angle  of 
incidence  and  BAR  the  angle  of  refraction.  Taking  the  common  side  AB 
as  radius,  we  have  IA  =  sine  ABI  and  BR  =  sine  BAR.  Hence 

sine  of  the  angle  of  incidence  _  sine  of  the  angle  of  refraction 
~ 


or 


sine  of  the  angle  of  incidence  :  sine  of  the  angle  of  refraction  : :  1  53 : 1. 


In  general,  this  relation  is  thus  stated  :  The  sine  of  the  angle  of  incidence 
is  to  the  sine  of  the  angle  of  refraction  as  the  index  of  refraction  of  the  medium 
into  which  the  light  passes  is  to  the  index  of  refraction  of  the  medium  from 
which  it  passes. 

If  we  consider  the  angle  of  incidence  between  the  ray  IA  and  the  per- 
pendicular PPr,  and  the  angle  of  refraction  between  the  ray  AR'  and  the 
same  perpendicular,  it  appears  that  the  ray  is  bent  towards  the  perpendicular 
on  passing  from  a  less  refracting  to  a  more  refracting  medium.  If,  on  the 
other  hand,  the  light  is  supposed  to  pass  from  the  glass  into  the  air,  it  is 
evident  that  the  ray  is  equally  bent  from  the  perpendicular  in  passing  from  a 
more  refracting  to  a  less  refracting  medium. 

Refraction  by  a  plate  of  glass  with  parallel  surfaces,  when  held  in  the  air, 
is  equal  at  both  surfaces,  because,  the  angle  of  incidence  at  the  second  surface 
being  the  same  as  the  angle  of  refraction  at  the  first  surface,  the  angle  of 
refraction  at  the  second  surface  must  be  equal  to  the  angle  of  incidence  at 
the  first  surface,  and  the  direction  of  any  ray  after  leaving  the  second  surface 
is  parallel  to  the  direction  of  the  same  ray  before  reaching  the  first  surface. 
Refraction  by  a  Prism. — When  a  portion  of  a  refracting  medium  is 
bounded  by  two  plane  surfaces  inclined  towards  one  another,  it  is  called  a 
prism,  and  by  passing  through  it  the  directions  of  the  wave-fronts  and  rays 
are  permanently  altered.  This  is  illustrated  in  Fig.  13,  in  which  AB  and 

AC  represent  the  bounding  surfaces 
of  a  prism  of  glass,  or  other  medium, 
more  refractive  than  the  air  by  which 
it  is  surrounded.  The  edge  in  which 
these  surfaces  intersect  at  A  is  the 
apex  or  edge  of  the  prism,  and  the 
angle  at  which  they  meet  is  the  re- 
fracting angle.  The  thickest  part  of 
the  prism  BC  is  the  base.  II'  shows 
the  direction  of  the  wave-front  before 
entering  the  prism,  I"P  its  direction 

while  in  the  prism,  and  RR"  its  direction  after  leaving  the  prism.  II' f 
shows  the  direction  of  a  ray  before  entering  the  prism,  I"R'  its  direction 
in  the  prism,  and  R'R  its  direction  after  leaving  the  prism.  Obviously, 


THE   DIOPTRICS   OF  THE    EYE.  467 

whatever  the  original  direction  of  the  wave-front,  the  part  of  it  that  goes 
through  the  thicker  part  of  the  prism  will,  on  the  whole,  be  more  retarded 
than  the  part  that  goes  through  the  thinner  part  of  the  prism.  It  will  be 
turned  towards  the  base  of  the  prism,  with  a  corresponding  change  in  the 
direction  of  the  ray.  The  amount  of  this  change  of  direction  is  the  angle 
of  deviation.  The  amount  of  deviation  produced  by  a  prism  varies  with 
the  angle  at  which  the  incident  ray  strikes  it.  When  the  angle  of  the  inci- 
dent ray  with  the  first  surface  is  just  equal  to  the  angle  of  the  refracted  ray 
with  the  second  surface,  and  the  ray  within  the  prism  is  perpendicular  to 
the  plane  bisecting  the  refracting  angle,  this  deviation  is  the  least,  and  is 
called  the  minimum  deviation.  This  position  of  the  prism  is  called  the 
position  of  minimum  deviation.  When  the  strength  of  a  prism  is  spoken 
of,  it  is  generally  understood,  unless  otherwise  indicated,  to  mean  its  power 
of  minimum  deviation.  The  effect  of  rotating  a  prism  from  its  position  of 
minimum  deviation  can  be  readily  observed  by  looking  through  it  at  a  line 
parallel  to  its  edge,  and  noting  the  change  in  the  apparent  position  of  the 
line  produced  by  rotation.  This  change  of  strength  by  rotation  is  due  to 
the  fact  that  the  nearer  an  angle  approaches  to  90°  the  more  is  it  increased 
for  a  given  increase  of  its  sine,  and  rotation  of  the  prism  from  the  position 
of  minimum  deviation,  while  it  may  make  the  angle  of  incidence  less  atone 
surface,  always  makes  it  greater  at  the  other,  and  the  gain  to  the  larger  angle 
always  causes  a  greater  increase  of  deviation  than  the  equal  diminution 
occasioned  by  the  reduction  of  the  smaller  angle.  For  the  same  reason  a 
strong  prism  has  more  deviating  power  in  proportion  to  its  refracting  angle 
than  a  weak  prism.  Thus,  a  prism  having  a  refracting  angle  of  8°  will 
cause  a  minimum  deviation  of  4°  16',  while  one  having  a  refracting  angle 
of  80°  causes  a  deviation  of  79°  4'.  The  latter  prism,  with  ten  times  the 
refracting  angle  of  the  former,  has  over  eighteen  times  its  refracting  power. 
To  ascertain  the  effect  of  a  prism  upon  a  ray  of  light  passing  through 
it,  we  must  for  most  positions  of  the  prism  calculate  the  deviation  produced 
both  on  entering  and  on  leaving  the  prism,  and  get  the  algebraic  sum  of  the 
two.  When,  however,  the  ray  passes  within 
the  prism  perpendicular  to  one  of  its  surfaces, 
it  is  refracted  only  at  the  other  surface,  and  it 
is  only  necessary  to  determine  its  deviation 
there.  Thus,  in  Fig.  14,  suppose  the  incident 
ray  perpendicular  to  the  surface  through  which 
it  enters.  It  will  be  refracted  only  at  the  sur- 
face of  exit,  where  the  angle  of  incidence  i  will 
(because  the  sides  of  the  two  angles  are  mutually 

perpendicular)  be  equal  to  the  refracting  angle  of  the  prism.  If  the  prism 
be  of  ordinary  optical  glass  with  an  index  of  refraction  of  1.53,  we  have, 
letting  r  represent  the  angle  of  refraction, 

sin  i  X  1.58  =  sin  r. 


468  THE   DIOPTRICS   OF   THE   EYE. 

Suppose  the  refracting  angle  of  the  prism  to  be  10°.     The  sine  of  10°  = 
0.1736.     Hence 

sin  r  =  0. 1736  X  1.53=0.2656, 

which  is  the  sine  of  15.4°,  the  angle  of  refraction.     The  angle  of  deviation, 
d  =  r  —  i, 

15.4  —  10  =  5.4°, 

the  angle  of  deviation  for  a  prism  having  a  refracting  angle  of  10°. 

When  it  is  desired  to  ascertain  its  minimum  deviation,  a  prism  may  be 
regarded  as  made  up  of  two  equal  parts,  in  each  of  which  the  refraction  is 
all  at  one  surface,  as  in  the  preceding  example.  Calculating  the  deviation 
for  one  of  these  halves,  and  doubling  it,  gives  the  deviation  of  the  whole 
prism. 

If,  in  the  above  example,  with  the  refraction  all  at  one  surface,  we  had 
taken,  instead  of  10°,  a  refracting  angle  of  40°  49',  the  angle  of  refraction 
would  have  been  90°  ;  that  is,  the  ray  after  refraction  would  have  remained 
in  the  plane  of  the  surface  and  would  not  have  escaped  from  the  prism. 
This  is  called  the  limiting  angle.  Light  striking  the  surface  with  a  greater 
angle  of  incidence  than  this  does  not  pass  out,  but  is  totally  reflected  within 
the  prism.  If  we  took  a  prism  of  double  this  refracting  angle,  or  81°  38', 
no  light  could  pass  through  it. 

When  rays  fall  upon  a  prism  parallel,  having  all  the  same  angle  of  in- 
cidence, they  continue  parallel  after  refraction.  When,  however,  they  fall 
upon  the  prism  divergent  or  convergent,  having  different  angles  of  inci- 
dence, they  are  not  equally  refracted,  but  their  divergence  or  convergence  is 
slightly  altered.  This  alteration  is,  however,  for  ordinary  ophthalmological 
prisms  so  slight  when  the  rays  fall  at  a  small  angle  of  incidence  that  it 
may  for  practical  purposes  be  disregarded.  For  a  more  complete  account 
of  prisms,  the  reader  is  referred  to  the  article  "  Prisms  and  Prismometry." 
Only  such  matters  have  been  discussed  here  as  are  related  to  refraction  by 
lenses. 

Dispersion  of  Light. — We  have  thus  far  spoken  of  the  index  of  refrac- 
tion as  though  each  dioptric  medium  had  but  one  fixed  index.  But  not 
only  does  this  index  vary  with  changes  of  density  that  accompany  changes 
of  temperature,  but  it  is  different  for  light  of  different  wave-lengths.  In 
vacuum,  all  kinds  of  light  appear  to  travel  at  the  same  rate ;  but  in  most 
dioptric  media  they  are  unequally  retarded,  and  there  seems  to  be  no  con- 
stant ratio  between  the  indexes  of  refraction  for  light  of  different  colors  in 
different  substances.  Commonly  the  light  with  short  wave-lengths  (near 
the  violet  end  of  the  spectrum)  is  most  retarded  and  most  refracted ;  but 
by  some  substances,  as  the  vapor  of  iodine,  red  light  is  more  retarded 
and  refracted  than  blue. 

The  following  table  gives  the  indexes  of  refraction  of  crown  and  flint 
glass  for  light  of  different  colors,  the  letters  giving  Fraunhofer  lines  that 
exactly  locate  the  light  in  the  spectrum  : 


THE    DIOPTRICS    OF   THE    EYE. 


469 


Color  of  Light. 

Red B 

Yellow D 

Green E 

Blue F 

Violet                                              .  H 


Fraunhofer  Line.    Crown  Glass.  Flint  Glass. 

1.5136  1.6167 

1.5171  1.6224 

1.6203  1.6289 

1.6231  1.6347 

1.5328  1.6662 


These  differences  of  refractive  index  cause  the  notable  separation  or  disper- 
sion of  light  of  different  colors  when  strongly  refracted  by  prisms  or  lenses, 
giving  rise  to  chromatic  aberration,  which  requires  very  careful  correction 
in  such  optical  instruments  as  the  telescope  and  the  microscope,  and  which 
interferes  somewhat  with  the  use  of  the  strongest  ophthalmic  prisms  and 
lenses. 

Refraction  by  Convex  Spherical  Lenses. — When  light  passes  from  one 
dioptric  medium  to  another  through  a  spherical  surface,  in  such  a  way  that 
a  part  of  each  wave-front  is  in  one  medium  while  a  part  is  in  the  other 
medium,  the  shape  of  the  wave-fronts  and  the  direction  of  the  rays  along 
which  they  move  are  changed.  Thus,  in  Fig.  15  let  S  represent  a  spherical 
surface  separating  two  media  and  convex  towards  the  less  refractive  medium 
from  which  comes  the  light,  the  wave-fronts  being  represented  by  the  parallel 

FIG.  15. 


lines.  The  centre  of  each  wave-front  first  enters  the  more  refractive  medium 
and  is  retarded,  while  the  parts  on  either  side  passing  on  at  the  more  rapid 
rate  get  ahead  of  it,  so  that  when  the  whole  wave-front  has  entered  the 
second  medium  it  is  concave  forward,  and  its  motion  perpendicular  to  the 
direction  of  the  wave-front  causes  it  to  concentrate  to  a  single  point  F,  its 
centre  of  curvature,  after  which  it  again  begins  to  spread  out. 

In  Fig.  15  the  representation  has  been  as  though  the  wave-front,  after 
passing  the  surface,  were  perfectly  spherical  in  form,  concentrating  accurately 
to  a  single  point.  But  the  passage  through  a  spherical  surface  does  not 
effect  this.  Only  the  part  of  the  wave-front  near  the  centre  of  the  lens— the 
part  wiiere  the  angle  of  incidence  is  very  small— sufficiently  approximates 
the  spherical  form  to  be  so  regarded.  The  more  peripheral  portions  of  the 
wave-front  are  really  bent  round  too  much,  so  that  their  centres  of  cur- 
vature lie,  and  tend  to  concentrate,  in  front  of  the  centre  for  the  middle 
portion.  What  really  happens  is  shown  in  Fig.  16,  where,  for  simplicity, 
only  the  rays  are  represented.  They  are  supposed  to  pass  parallel  in  the 
more  refractive  medium. 


470 


THE    DIOPTRICS    OF    THE    EYE. 


Fro.  16. 


The  rays  passing  through  the  central  part  of  the  surface  are  concentrated 
to  practically  a  single  point  F,  but  others  passing  through  the  periphery 

are  concentrated  at  various 
points  nearer  the  surface. 
This  failure  of  a  spherical 
surface  accurately  to  concen- 
trate the  rays  that  fall  upon 
it  obliquely  is  called  spheri- 
cal aberration,  or  monochro- 
matic aberration,  in  con- 
tradistinction to  chromatic 
aberration  due  to  the  dis- 
persion of  light.  Since  the  whole  value  of  the  lens  in  physiological  optics 
is  to  cause  the  rays  of  a  pencil  all  to  converge  towards  or  diverge  from  a 
certain  point,  the  whole  optical  theory  of  lenses  has  to  be  based  on  the 
assumptions  that  the  lens  surface  includes  only  a  very  small  portion  of  the 
sphere  of  which  it  is  a  part,  and  that  the  rays  fall  upon  it  nearly  perpen- 
dicular to  its  surface.  Working,  then,  under  the  above  assumptions,  the 
point  at  which  the  pencil  of  light  is  concentrated  is  called  a  focus.  The 
focus  for  a  pencil  of  parallel  rays  (flat  wave-fronts)  is  called  the  principal 
focus  of  the  lens,  and  its  distance  from  the  lens  its  principal  focal  distance, 
commonly  spoken  of  as  the  focal  distance. 

To  ascertain  the  principal  focal  distance  of  a  lens,  we  may  consider  what 
happens  to  a  single  refracted  ray,  shown  in  Fig.  17,  which  represents  a  por- 
tion of  a  spherical  surface  through  which  pass  two  parallel  rays  AF,  going 
through  the  centre  O  of  the  surface,  and  therefore  unrefracted,  and  BIF 
refracted  at  L  For  the  latter  the  angle  of  incidence  is  i,  the  angle  of 
refraction  r,  and  the  angle  of  deviation  d.  The  radius  of  curvature  of  the 
surface  is  R,  and  the  focal  distance  SF  is  /.  Let  the  index  of  refraction 
for  the  first  medium  be  n,  and  for  the  second  medium  n'.  The  accurate 


focussing  of  light  by  a  spherical  surface  is  possible  only  when  the  angles 
i,  r,  and  d  are  so  small  that  we  may  consider  them  as  identical  with  their 
sines,  and  DS  is  so  short  that  it  may  be  disregarded,  leaving  FD  or  IF 
equal  to  FS.  We  have,  then,  in  the  right-angled  triangles  IDC  and  IDF, 
ID=  CI  X  sin  i  =  R  X  sin  i,  and  ID  =  IFX  sin  d=fX  sin  d.  Hence 


THE    DIOPTRICS   OF   THE   EYE.  471 

R  X  sin  i  =/  X  sin  d. 


8\nd 

But  d  =  r  —  i,  or  sin  d  =  sin  r  —  sin  i,  and,  by  the  general  law  of  refraction, 

n  X  sin  i  =  n'x  sin  r. 

Hence 


f=R 


sin  r  —  sin  i 
sin  i  r>     1 


^/Sini-8ini          *_1  (A) 

n  n' 


Now,  if  we  have  the  first  medium  glass  and  the  second  air,  we  have  n  =  1.53 
and  n'  =  1  ;  and  the  focal  distance  of  a  glass  lens  in  air,  where  the  refraction 
was  all  at  one  surface  (a  plano-convex  lens,  with  rays  perpendicular  to  the 
plane  surface),  would  be 


f=_R_        =  _R_ 
J     1.53  .53' 


For  a  biconvex  lens,  by  which  the  light  is  refracted  equally  on  entering  and 
on  leaving  the  lens,  the  formula  would  be 

/=     *     =JL  (B) 

.53  X  2        1.06  V    ' 

The  focal  distance  of  a  lens  is  directly  proportional  to  the  radius  of  curva- 
ture, and  inversely  proportional  to  the  difference  between  the  refractive  indexes 
of  the  two  media,  or  its  refractive  power. 

If  we  designate  the  refractive  power  or  "  strength"  of  a  lens  by  S,  and 
its  focal  distance  by  F,  we  have 

•-£-'•4  (C) 

When  lenses  are  combined  so  as  to  add  their  strengths,  as  S  -\-  S'  +  S",  etc., 
we  can  express  the  same  thing  thus  : 

I  +  JL  +  J_,etc. 
F      F'      F" 

Or  we  may  conceive  a  lens  strength  S  as  made  up  of  S'  +  S",  in  which  case 


Conjugate  Foci.  —  Formula  (D)  indicates  the  relations  between  the  differ- 
ent focal  distances  of  a  lens.  Let  Fig.  18  represent  a  lens  L,  whose  principal 
focal  distance  is  /,  and  which,  on  receiving  rays  from  Fr,  focusses  them  at 


472 


THE    DIOPTRICS    OF   THE    EYE. 


Fio.  18. 


F".  The  lens  L  may  be  regarded  as  made  up  of  two  parts,  L'  having  a  prin- 
cipal focal  distance/',  and  L"  having  a  principal  focal  distance/".  Rays 

diverging  from  F'  are 
rendered  parallel  by  L', 
and  the  parallel  rays  are 
converged  to  F"  by  L". 
Or,  conversely,  rays 
from  F"  are  rendered 
parallel  by  L"  and 
focussed  at  F'  by  L'. 
F'  and  F"  are  called 
conjugate  foci,  and  their  relation  to  the  principal  focus  of  the  lens  is  expressed 
by  Formula  (D).  It  is  evident  that  for  each  value  of/'  there  is  a  correspond- 
ing value  for  /".  As  /'  diminishes  /"  increases,  until  when  /'  equals  /  /" 
equals  infinity.  The  focus  conjugate  to  the  principal  focus  is  at  an  infinite 
distance.  When  /'  is  twice  /,  /"  is  also  twice /;  that  is,  when  the  conjugate 
foci  are  the  same  distance  from  the  lens,  this  distance  is  double  the  principal 
focal  distance  of  the  lens.  When  /'  becomes  less  than  /  /"  becomes  negative 
and  diminishes  with  /';  the  conjugate  foci  are  both  on  the  same  side  of  the 
lens,  and  one  of  them  is  a  virtual  focus.  Given  the  principal  focal  distance 
and  one  conjugate  focus,  we  can  by  the  formula  find  the  other  conjugate ;  or 
given  both  conjugate  focal  distances,  we  can  find  the  principal  focal  distance. 
Refraction  by  Concave  Lenses. — If  light  passes  from  one  dioptric  medium 
to  another  through  a  spherical  surface  concave  towards  the  less  refractive 
medium,  the  effect  upon  it  is  that  represented  by  Fig.  19.  The  central  por- 
tion of  each  wave  remains  in  the  less 
refractive  medium,  moving  at  its  previous 
rate  after  the  peripheral  portions  have 
entered  the  more  refractive  medium  and 
are  being  retarded  by  it.  This  allows 
the  central  portion  of  each  wave  to  get 
ahead  of  the  peripheral  portions,  so  that 
when  the  whole  wave-front  has  entered 
the  second  medium  it  is  convex  forward, 
as  though  the  light  had  emanated  from 
a  point  F,  the  centre  of  curvature  of  the 
successive  waves  after  refraction.  All 

rays,  as  those  shown  in  the  figure,  after  refraction  diverge  from  the  same 
point.  This  point  from  which  the  light  after  refraction  appears  to  emanate 
is  a  focus  of  the  concave  lens.  Since  the  light  does  not  really  pass  through 
it,  it  is  called  a  virtual  focus,  in  contrast  with  a  real  focus,  like  F  in  Fig. 
15,  through  which  the  light  will  actually  pass.  As  for  convex  lenses,  the 
distance  of  the  focus  from  the  lens  is  the  focal  distance,  the  focus  for  rays 
previously  parallel  is  the  principal  focus,  and  its  distance  from  the  lens  is 
the  principal  focal  distance.  The  formulas  given  for  convex  lenses,  with 


FIG  19. 


F 


THE   DIOPTRICS   OF   THE   EYE. 


473 


the  needful  changes  of  signs,  the  existence  of  spherical  aberration,  and  the 
relations  of  conjugate  foci,  all  hold  for  concave  lenses. 

Different  Forms  of  Lenses.— Six  general  classes  of  lenses  differing  in 
their  essential  forms  are  illustrated  in  Fig.  20.  The  representation  is  of 
lenses  of  about  the  same  strength. 

FIG.  20. 


A.  Plano-convex :  One  side  convex,  the  other  plane. 

B.  Double  convex :  Both  sides  convex.     When  both  sides  are  equally 
convex,  as  represented  in  the  figure,  the  lens  is  called  biconvex. 

C.  Concavo-convex :  One  side  concave,  the  other  more  convex. 

D.  Plano-concave :  One  side  concave,  the  other  plane. 

E.  Double-concave :  Both  sides  concave.     When  they  are  equally  con- 
cave, as  in  the  figure,  the  lens  is  called  biconcave. 

F.  Convexo-concave :  One  side  convex,  the  other  more  concave. 
Either  C  or  F  is  called  a  meniscus,  or  periscopic  lens. 

In  each  lens  there  is  a  certain  point  where  its  bounding  surfaces  are 
parallel.  This  is  called  the  optical  centre  of  the  lens.  Lenses  thickest  at 
the  optical  centre  act  as  convex  lenses,  and  those  thinnest  at  the  optical  centre 
as  concave  lenses.  Any  lens  may  be  regarded  as  a  series  of  prisms  growing 
progressively  stronger  from  the  optical  centre  to  the  edge  of  the  lens.  For 
a  convex  lens  the  prisms  have  their  bases  all  turned  towards  the  centre  of  the 
lens ;  for  a  concave  lens  their  bases  are  turned  from  the  centre.  Hence 
the  former  are  converging,  the  latter  diverging  lenses. 

To  ascertain  the  total  effect  of  any  lens,  it  is  needful  to  consider  the 
refraction  at  both  surfaces.  Thus,  in  the  plano-convex  lens  we  can  consider 
the  refraction  of  the  plane  surface  as  zero,  and  the  total  converging  power 
of  the  lens  to  be  that  of  its  convex  surface.  A  double  convex  lens  is  to  be 
regarded  as  a  combination  of  two  plano-convex  lenses  of  the  appropriate 
curvatures,  the  sum  of  whose  strengths  gives  the  strength  of  the  whole  lens. 
A  concavo-convex  lens,  however,  is  equal  to  a  combination  of  a  plano-con- 
cave lens  with  a  plano-convex  lens,  and  the  total  effect  is  the  excess  of  the 
converging  over  the  diverging  action.  Concave  lenses  of  various  forms  are 
to  be  regarded  in  the  same  way. 

The  Numbering  of  Lenses. — At  first,  lenses  were  numbered  according  to 
the  radius  of  curvature  in  inches  of  the  lens  surface.  But  lenses  having 
different  forms  would  by  this  plan  be  known  by  different  numbers,  although 


474  THE   DIOPTRICS   OF   THE   EYE. 

they  might  have  the  same  optical  effect.  On  this  account  convex  lenses  of 
other  forms  came  to  be  known  by  the  number  of  the  equivalent  biconvex 
lens,  and  other  concave  lenses  by  the  number  of  the  equivalent  biconcave 
lens,  this  number  being  the  number  of  inches  in  the  radius  of  curvature  of 
the  equally  curved  surfaces. 

Jt  will  be  noticed  that  for  a  biconvex  lens  Formula  (B)  gives  the  focal 
distance  as  the  radius  of  curvature  divided  by  1.06.  Hence  the  numbers 
of  lenses  on  the  old  plan  closely  approximated  the  focal  distances  of  the 
lenses  in  inches.  For  people  using  the  English  inch  as  a  unit  of  distance 
this  approximation  was  still  closer.  The  first  sets  of  trial-lenses  were  chiefly 
made  in  Paris,  and  numbered  by  the  radius  of  curvature  in  Paris  inches, 
and  the  Paris  inch  is  1.066  times  longer  than  the  English  inch.  Hence  the 
number  of  Paris  inches  in  the  radius  of  curvature  almost  exactly  coincided 
with  the  number  of  slightly  shorter  English  inches  in  the  slightly  shorter 
focal  distance. 

To  the  user  of  lenses  the  radius  of  curvature  of  its  surfaces  is  not  a 
matter  of  direct  importance.  The  things  which  are  of  direct  and  principal 
importance  are  the  strength  and  focal  distance  of  the  lens ;  hence  the 
numbers  of  lenses  came  to  be  regarded  as  indicating  their  respective  focal 
distances.  We  have  seen — Formula  (C) — that  the  strength  of  a  lens  is 
the  reciprocal  of  its  focal  distance.  If  the  focal  distance  be  expressed  in  a 
whole  number,  whether  of  inches  or  centimetres,  the  strength  is  expressed 
by  a  fraction, — one  divided  by  that  number.  In  working  with  lenses,  it 
is  very  frequently  necessary  to  add  and  subtract  their  strengths  or  refrac- 
tive powers,  and  any  system  of  numbering  by  focal  distances  compels  the 
addition  and  subtraction  of  vulgar  fractions,  which  have  to  be  reduced  to 
a  common  denominator. 

To  avoid  the  difficulty  of  so  working  with  vulgar  fractions,  it  was  neces- 
sary to  number  lenses  according  to  their  strengths,  using  only  whole  numbers 
or  fractions  having  a  common  denominator. 

For  this  purpose  the  refracting  power  of  a  metre-lens — a  lens  having 
a  focal  distance  of  one  metre — was  chosen  as  the  unit,  and  was  called  a 
dioptre  ;  and  the  system  of  numbering  based  on  this  is  called  the  metric  or 
dioptric  system.  The  interval  of  a  whole  dioptre  being  too  great  for  a 
complete  series  of  ophthalmic  lenses,  intermediate  lenses  were  introduced 
at  regular  intervals  of  one- quarter  dioptre  and  numbered  in  decimals,  so 
that  their  addition  or  subtraction  is  as  simple  as  that  of  whole  numbers. 
Later,  when  additional  lenses  were  added  at  irregular  intervals,  the  writer 
urged  that,  in  "  adding  to  the  metric  series,  the  simplicity  of  the  system 
should  be  carefully  preserved  by  making  the  interval  one-eighth  dioptre, — 
an  exact  division  of  the  interval  previously  adopted."  *  This  plan  has  since 
been  generally  followed. 

While  in  practical  work  it  is  commonly  the  strength  of  lenses  which  we 

1  Transactions  of  the  Amer.  Ophthalmol.  Soc.,  1887,  vol.  iv.  p.  596. 


THE   DIOPTRICS   OF  THE   EYE. 


475 


wish  to  add  to  or  subtract  from,  it  is  sometimes  necessary  to  add  to  or  sub- 
tract from  the  focal  distance.  This  it  is  easier  to  do  for  lenses  numbered 
by  their  focal  distance,  since  by  the  metric  system  the  focal  distance  is 
always  a  fraction,  one  divided  by  the  strength.  It  is  therefore  worth  while 
to  have  both  sets  of  numbers  in  mind,  and  to  be  able  to  convert  the  one 
into  the  other.  Having  the  dioptric  number,  by  dividing  it  into  40  or  39 
(there  are  39.37  inches  in  a  metre)  we  obtain  the  number  according  to 
inches  of  focal  distance;  or  having  the  number  of  inches  focal  distance  of  a 
lens  and  dividing  it  into  39  or  40,  we  get  the  number  of  the  lens  in  the 
dioptric  system.  The  following  table  gives  the  numbers  for  a  fairly  com- 
plete series  of  trial-lenses  with  their  focal  distances  and  the  nearest  equiva- 
lent lens  of  the  older  series.  It  will  be  noted  that  the  stronger  lenses  are 
separated  by  longer  intervals  because  they  are  less  frequently  required,  and 
intermediate  strengths  can  be  gained  by  combinations,  or  the  effect  se- 
cured by  varying  the  distance  of  the  lens  from  the  eye  (see  Correction  of 
Hyperopia). 


Number  by  Dioptres. 

Focal  Distance  In 
Millimetres. 

Focal  Distance  in 
English  Inches. 

Number  in  Old 
Series. 

0.12  (0.125) 

8000 

315. 

0.25 

4000 

157.5 

144 

0.37 

2667 

105. 

100 

0.50 

2000 

78.7 

72 

0.62 

1600 

63. 

60 

0.75 

1333 

52.5 

48 

0.87 

1143 

45. 

1. 

1000 

39.37079 

40 

1.12 

889 

35. 

36 

1.25 

800 

31.5 

80 

1.37 

727 

28.6 

1.50 

667 

26.2 

24 

1.75 

571 

22.5 

2. 

500 

197 

20 

2.25 

444 

17.5 

18 

2.50 

400 

15.7 

16 

2.75 

364 

14.3 

14 

3. 

333 

13.1 

3.25 

308 

12.1 

12 

3.50 

286 

11.2 

11 

4. 

250 

9.8 

10 

4.50 

222 

8.7 

9 

5. 

200 

7.9 

8 

5.50 

182 

7.2 

7 

6. 

167 

6.5 

6 

7. 

143 

5.6 

5J 

8. 

125 

4.9 

6 

9. 

111 

4.4 

4J 

10. 

100 

3.9 

4 

11. 

91 

3.6 

8* 

12. 

83 

3.3 

.  . 

13. 

77 

3. 

3 

14. 

71 

2.8 

2J 

15. 

67 

2.6 

2} 

16. 

62  • 

2.4 

17. 

59 

2.3 

2* 

18. 

55 

2.2 

19. 

52 

2.1 

20. 

50 

2. 

2 

476 


THE   DIOPTRICS   OP   THE   EYE. 


Axes,  Nodal  Points,  and  Images. — A  straight  line  passing  through  the 
centres  of  curvature  of  both  surfaces  of  a  lens,  and  therefore  perpendicular 
to  those  surfaces,  is  called  the  principal  axis  of  the  lens.  A  ray  of  light 
passing  in  the  direction  of  the  principal  axis  is  not  refracted  at  either  sur- 
face. All  other  rays  are  refracted ;  but  from  any  point  outside  of  the  prin- 
cipal axis  one  ray  may  fall  on  the  lens  in  such  a  way  that,  although  it  strikes 
the  surfaces  obliquely,  the  two  surfaces  are  parallel  at  the  points  at  which  it 
enters  and  leaves  the  lens,  so  that  the  lens  has  upon  it  the  effect  of  a  plate 
of  glass  with  parallel  sides.  The  direction  of  the  ray  after  leaving  the  lens 
is  parallel  to  its  direction  before  entering  the  lens.  Such  a  ray  is  called  a 
secondary  axis.  A  point  upon  the  primary  axis  towards  which  such  a  ray  is 
directed  before  entering  the  lens  is  called  the  first  nodal  point.  The  point 
upon  the  primary  axis  from  which  it  passes  after  leaving  the  lens  is  the  second 
nodal  point.  All  the  secondary  axes  of  a  lens  pass  towards  and  from  the  same 
nodal  points.  Their  location  is  shown  for  the  different  forms  of  convex  lenses 
in  Fig.  21,  and  for  the  different  forms  of  concave  lenses  in  Fig.  22.  In  each 

FIG.  21 . 


FIG.  22. 


figure  AA  is  the  primary  axis  and  BB  a  secondary  axis  of  the  lens,  and 
N1  the  first  and  .ZV2  the  second  nodal  point  for  rays  passing  from  left  to 
right.  Where  the  medium  through  which  the  light  passes  before  entering 
and  after  leaving  the  lens  is  the  same,  the  nodal  points  are  also  the  principal 
points  of  the  lens.  If  through  each  of  the  principal  points  a  plane  be 
passed  perpendicular  to  the  primary  axis,  these  planes  will  be  the  principal 
planes.  Any  ray  passing  before  entering  the  lens  towards  a  certain  point 
in  the  first  principal  plane  will,  after  leaving  the  lens,  pass  through  or  from 
a  point  in  the  second  principal  plane,  similarly  situated  with  reference  to  the 
primary  axis. 


THE    DIOPTRICS    OF   THE    EYE. 


477 


The  focus  for  a  pencil  of  rays  coming  from  any  point  on  the  primary 
axis  will  be  another  point  on  the  primary  axis.  The  focus  for  rays  from  a 
point  on  any  secondary  axis  will  be  another  point  on  that  same  secondary 
axis.  From  each  luminous  point  of  an  object  passes  a  secondary  axis,  on 
which,  at  a  conjugate  focus,  are  focussed  the  rays  emanating  from  it  that 
pass  through  the  lens.  Each  point  of  an  object  thus  has  its  image,  and  the 
assemblage  of  these  images  forms  the  image  of  the  object.  Fig.  23  illus- 

FIQ.  23. 


trates  this  for  a  convex  lens,  the  object  AB  being  situated  farther  from  the 
lens  than  its  principal  focus.  In  this  case  the  object  and  the  image  are  on 
opposite  sides  of  the  nodal  points,  and  therefore  the  image  is  inverted  u-ith 
reference  to  the  object.  The  image  is  a  real  image.  Fig.  24  represents  the 
formation  of  an  image  by  a  convex  lens  when  the  object  is  closer  to  the  lens 
than  its  principal  focus,  and  the  image  and  the  object,  therefore,  are  on  the 
same  side  of  the  nodal  points.  In  this  case  the  image  is  virtual  and  erect. 


FIG.  24. 


FIG.  25. 


In  Fig.  25  is  represented  the  formation  of  an  image  by  a  concave  lens. 
Here  also  the  object  and  the  image  are  on  the  same  side  of  the  nodal  points, 
and  the  image  is  erect  and  virtual. 

The  relative  sizes  of  the  object  and  the  image  are  proportioned  to  their 
distances  from  the  nodal  points :  thus,  in  Figs.  23,  24,  and  25  we  have  the 
similar  triangles  ASN1  and  abN*  (their  sides  being  mutually  parallel),  in 
which 

AN1 :  N*a  ::  AB:  ab. 

It  may  be  noted  that  in  Fig.  24  the  image  ab  is  farther  from  the  nodal 
point  than  the  object  AB  ;  therefore  the  image  is  larger  than  the  object. 


478 


THE    DIOPTRICS    OF   THE    EYE. 


But  in  Fig.  25  ab  is  nearer  the  lens  than  AB :  the  image  is  smaller  than 
the  object.  Therefore  Fig.  24  represents  a  magnifying  and  Fig.  25  a 
minifying  lens. 

Refraction  by  a  Cylindrical  Lens. — A  lens  may  be  bounded  by  curved 
surfaces  other  than  spherical.  Of  especial  interest  in  ophthalmology  is  a 
lens  one  or  both  surfaces  of  which  are  segments  of  a  cylinder  or  cylinders. 
Such  a  lens  is  called  a  cylindrical  lens.  Cylindrical  lenses  may  have  any  of 
the  general  forms  enumerated  for  spherical  lenses,  but  are  commonly  made 
plano-convex  and  plano-concave.  These  forms  and  the  way  in  which  they 
refract  light  are  represented  in  Figs.  26  and  27. 


FIG.  26. 


FIG.  27. 


£A* 


The  line  A  A'  passing  in  the  centre  of  the  cylindrical  surface,  parallel  to 
the  direction  of  the  axis  of  the  cylinder  of  which  the  surface  is  a  part,  is 
called  the  axis  of  the  cylindrical  lens.  The  lens  in  any  plane  perpendicular 
to  the  axis  has  a  curvature  and  a  refractive  power  similar  to  those  of  the 
spherical  lens  in  all  directions,  but  in  any  plane  parallel  to  the  axis  it  has 
no  refractive  power. 

Thus,  in  Fig.  26,  suppose  parallel  rays  to  fall  on  the  lens.  Of  those 
lying  in  a  plane  passed  through  A  perpendicular  to  the  axis,  the  ray 
passing  through  A  is  not  refracted,  and  the  other  rays  are  bent  towards  it 
so  that  they  meet  in  F.  So,  too,  in  the  plane  A'F'  the  rays  are  brought  to 
a  focus  at  F' ;  but  the  rays  in  the  one  plane  are  not  bent  towards  or  from 
the  rays  in  the  other  plane.  The  same  is  true  of  all  intermediate  planes 
perpendicular  to  the  axis,  the  rays  in  each  being  brought  to  a  focus  in  a 
corresponding  point  of  FF';  so  that  the  line  FF'  is  the  focus  of  the  cylin- 
drical lens.  In  the  same  way  the  concave  cylindrical  lens  represented  in 
Fig.  27  refracts  the  rays  that  pass  through  it,  except  that,  being  a  concave 
lens,  it  causes  the  rays  to  diverge  as  though  they  had  come  from  the  line 
FF',  its  virtual  focus. 

As  to  refraction  by  a  cylindrical  lens  in  the  plane  perpendicular  to  its 
axis, — the  plane  in  which  it  has  a  lens  action, — what  has  been  said  in  con- 
nection with  spherical  lenses  will  apply.  Its  power  of  accurately  focussing 
rays  is  similarly  confined  to  those  which  fall  nearly  perpendicular  to  its 
surface  ;  its  principal  focus  and  focal  distance  have  the  same  relation  to  it 
and  to  its  strength.  The  relation  of  its  conjugate  foci  to  the  principal  focus 
is  the  same,  and  the  system  of  numbering  is  the  same.  Its  primary  and 
secondary  axes  and  nodal  points  have  the  same  relations ;  but  there  are  a 


THE   DIOPTRICS   OF   THE   EYE. 


479 


primary  axis,  two  nodal  points,  and  a  full  system  of  secondary  axes  in  each 
plane  perpendicular  to  the  cylinder  axis,  those  of  one  plane  being  similar  to 
all  the  others.  The  formation  of  images,  the  determination  of  their  size 
and  their  reversal  are  the  same  as  for  spherical  lenses,  except  that  they  are 
confined  to  the  one  plane. 

The  fact  that  the  cylindrical  lens  acts  only  in  one  plane  makes  it  neces- 
sary always  to  recognize  in  connection  with  it  the  direction  in  which  it  is 
turned.  This  is  done  in  ophthalmic  work  by  describing  an  imaginary  circle 
in  the  plane  of  the  face  around  each  pupil  as  a  centre,  and  laying  this  off 
in  degrees  from  0  to  180,  the  two  halves  of  the  circle  being  similarly  num- 
bered. Trial-frames,  test-cards,  etc.,  are  then  graduated  to  conform  to  this 
circle. 

Different  systems  of  graduation  have  been  proposed.  That  in  most 
general  use  in  America,  when  viewed  from  in  front,  as  by  the  surgeon 
facing  the  patient,  conforms  to  the  graduation  of  the  circle  followed  in 
other  departments  of  mathematics.  It  is  shown  hi  Fig.  28. 


FIG.  28. 


120 


90 


.60 


90 


18° 


R.  E. 


180 


L.  E. 


30 


30 


90 
Right  eye. 


90 
Left  eye. 


Starting  with  0  at  the  right  of  the  horizontal  diameter,  it  goes  upward, 
90°  being  vertical,  and  over  to  180°  at  the  left  of  the  horizontal  diameter. 
It  is  the  same  for  both  eyes.  Other  plans  of  graduation  have  been  pro- 
posed to  make  the  numbering  symmetrical  with  reference  to  the  vertical 
meridian  of  the  eye  or  the  median  plane  of  the  body.  Harlan  would 
reverse  the  direction  of  graduation  for  the  left  eye,  making  it  start  from  the 
nasal  end  of  the  horizontal  meridian  for  both  eyes  and  go  upward.1  Knapp 
suggests  counting  from  the  upper  end  of  the  vertical  meridian  down  the 
nose  to  its  lower  end,  from  0°  to  180°  for  each  eye.2  Snelleu,  followed  by 
Laudolt  and  many  other  European  surgeons,  starts  from  the  upper  extremity 
of  the  vertical  meridian,  and  numbers  from  0°  to  90°  both  to  the  nasal  and 
to  the  temporal  side. 

To  ascertain  the  Strength  of  a  Lem.  —  For  a  convex  lens  the  strength 
may  be  ascertained  by  noting  at  what  distance  it  focusses  parallel  rays,  as 


1  Archives  of  Ophthalmology,  vol.  xxii.  p.  250. 
*  Loc.  cit. 


480  THE    DIOPTRICS    OF   THE    EYE. 

the  rays  of  light  from  the  sun.  Or  this  may  be  calculated  from  the  position 
of  conjugate  foci,  Formula  (D),  as  a  point  of  light  on  one  side  of  the  lens  and 
its  image  on  the  other  side, 'the  distances  of  which  can  readily  be  measured. 
The  strength  of  a  concave  lens  may  be  ascertained  by  combining  it  with  a 
stronger  convex  lens  of  known  strength  and  ascertaining  how  much  that 
strength  is  diminished  thereby.  When  the  index  of  refraction  of  the  glass 
is  known,  the  strength  of  the  lens  can  be  learned  by  measuring  the  curvature 
of  its  surfaces. 

The  common  practical  method,  however,  is  by  finding  what  lens  of  the 
opposite  kind  is  required  to  neutralize  its  optical  effect.  When  a  convex 
lens  is  held  before  the  eye,  but  not  beyond  its  focal  distance,  and  moved 
from  side  to  side,  objects  seen  through  it  appear  to  move  in  an  opposite 
direction.  When  the  same  is  done  with  a  concave  lens,  objects  appear 
through  it  to  move  in  the  same  direction  as  the  lens.  When  a  convex  lens 
is  closely  applied  to  a  concave  lens  of  equal  strength,  no  such  apparent 
movement  occurs.  To  ascertain,  then,  the  strength  of  a  convex  lens,  it  is 
only  needful  to  ascertain  by  trial  the  number  of  the  concave  lens  from  the 
trial-case  which  neutralizes  it.  In  the  same  way  the  strength  of  a  concave 
lens  is  known  by  that  of  the  convex  lens  which  just  neutralizes  it.  Spherical 
lenses  are  to  be  neutralized  by  spherical  lenses.  Cylindrical  lenses  may  be 
neutralized  by  cylindrical,  care  being  taken  to  have  their  axes  turned  the 
same  way ;  or  they  and  combinations  of  spherical  and  cylindrical  lenses 
may  be  neutralized  by  one  spherical  in  the  direction  of  the  axis  of  the 
cylinder,  and  then  by  a  different  spherical  at  right  angles  to  that  axis  :  the 
difference  between  these  gives  the  strength  of  the  cylindrical  lens.  For 
neutralization,  plano-convex  and  plano-concave  lenses  are  better  than  the 
biconvex  and  biconcave  lenses  often  found  in  trial-cases.  The  superiority 
of  the  former  is  very  marked  for  strong  lenses,  because  with  them  the 
neutralizing  surfaces  may  always  be  directly  applied  to  one  another. 

To  determine  the  Axis  of  a  Cylindrical  Lens. — On  account  of  its  in- 
fluence on  the  nodal  points  of  the  eye  (see  Astigmatism,  latter  part  of 
this  article),  a  cylindrical  lens  held  before  the  eye  changes  the  apparent 
direction  of  all  lines  seen  through  it  that  are  not  parallel  to  its  cylinder  axis 
or  perpendicular  to  the  same.  To  determine  the  axis,  hold  a  cylindrical 
lens  so  as  to  see  through  it  a  part  of  some  straight  line,  as  a  window-sash, 
the  continuation  of  which  can  be  seen  above  and  below  the  lens.  Then 
"turn  the  lens  in  its  own  plane  until  the  part  of  the  line  seen  through  it 
appears  continuous  with  the  parts  above  and  below.  The  axis  of  the  lens 
is  then  parallel  to  this  line  or  perpendicular  thereto. 

To  find  the  Optical  Centre  of  a  Lens. — Hold  it  as  when  fixing  the  direc- 
tion of  a  cylinder  axis,  and  when  the  line  seen  through  it  appears  continuous 
with  the  parts  above  and  below  it,  draw  on  the  lens  a  line  where  the  line 
looked  at  appears  to  cross  it.  Turn  the  lens  so  that  the  line  drawn  on  it 
will  be  perpendicular  to  the  line  looked  at.  Their  apparent  intersection  is 
the  optical  centre  of  the  lens. 


THE    DIOPTRICS   OF   THE    EYE.  481 

Many  practical  points  with  reference  to  ophthalmic  lenses  and  their 
mounting  cannot  be  discussed  here.  The  reader  is  referred  to  the  little 
work,  "  Spectacles  and  Eye-Glasses :  their  Forms,  Mounting,  and  Adjust- 
ment," by  R.  J.  Phillips,  M.D.,  for  a  fuller  account  of  the  subject. 

THE    REFRACTION   OF   THE    EYE. 

The  difference  between  bare  light-perception  and  the  full  normal  vision 
of  man  consists  in  ability  to  receive  distinct  visual  impressions  from  differ- 
ent objects,  and  rests  on  the  power  of  the  eye  to  assort  the  light  entering  it. 
The  eyeball  may  be  defined  as  an  apparatus  to  assort  light  and  sustain  the 
part  of  the  nervous  system  specialized  for  the  reception  of  luminous  impres- 
sions in  a  favorable  position  to  receive  those  of  assorted  light.  Under  the 
conditions  of  perfect  vision,  light  entering  the  pupil  is  so  assorted  that  all 
coming  from  a  single  luminous  point  of  the  object  looked  at  is  concen- 
trated to  make  its  impression  on  a  single  point  of  the  retina.  This  assort- 
ing of  the  light  is  effected  by  the  action  of  the  dioptric  surfaces  and  media 
of  the  eye,  resembling  that  of  a  convex  lens. 

Light  entering  the  eye  passes  from  air  having  an  index  of  refraction  of 
1,  to  the  corneal  tissue  having  an  index  of  refraction  of  1.3365,  through  a 
surface  convex  towards  the  air.  The  rays  of  an  incident  pencil  are  by  this 
passage  rendered  relatively  convergent.  Continuing  their  course,  they  pass 
through  the  aqueous  humor,  which  has,  however,  the  same  index  of  refrac- 
tion as  the  cornea,  and  is  therefore,  as  regards  its  dioptric  qualities,  a  con- 
tinuation of  that  medium.  Reaching  the  anterior  surface  of  the  crystalline 
lens,  the  light  again  passes  from  a  less  refractive  to  a  more  refractive  medium, 
through  a  surface  convex  towards  the  former,  and  thus  its  rays  are  rendered 
still  more  convergent.  Of  the  index  of  refraction  of  the  lens  and  its  effect 
on  the  refraction  of  the  eye  more  will  be  said  presently.  At  the  posterior 
surface  of  the  lens  the  light  passes  from  the  more  refractive  lens-substance 
into  the  vitreous  having  the  same  refractive  index  as  the  cornea  and  aqueous, 
but  again  through  a  surface  convex  towards  the  less  refractive  medium,  so 
that  the  rays  are  still  farther  converged.  They  then  pass  on  directly  through 
the  vitreous  and  superficial  layers  of  the  retina  to  focus  (in  the  properly 
proportioned  or  emmetropic  eye)  in  the  percipient  layer,  the  slight  variation 
of  refractive  index  in  the  retina  being  practically  nullified  by  the  comparative 
flatness,  and  nearness  to  the  focus,  of  its  surface. 

Returning  to  the  refraction  of  light  by  the  crystalline  lens,  we  find  that 
it  is  not  a  simple  matter  of  refraction  at  two  surfaces.  The  index  of 
refraction  varies  from  layer  to  layer,  increasing  from  its  surfaces  to  its 
centre ;  and  as  light  passes  from  one  layer  of  the  lens  to  another  it  is  re- 
fracted in  correspondence  to  the  slight  change  of  refractive  index.  The 
surfaces  separating  the  layers  being,  like  the  surfaces  of  the  lens,  always 
convex  towards  the  less  refracting  medium,  the  change  is  in  each  instance 
towards  greater  convergence.  To  determine  by  exact  calculation  these 
minute  changes  of  direction  that  occur  to  the  rays  during  their  whole 

VOL.  I.— 31 


482  THE    DIOPTRICS   OF   THE   EYE. 

passage  through  the  lens  is  manifestly  impossible.     We  have  not  methods 

for  accurately  determining  the  index  of  refraction  of  any  single  layer  or 
of  accurately  measuring  its  thickness  during  life.  The  best  we 
can  do  is  to  determine  what  refraction  will,  on  the  whole,  most 
nearly  represent  the  effect  of  the  average  crystalline  lens. 

For  this  purpose  physiologists  have  agreed  to  consider  the 
lens  as  though  it  were  composed  of  three  layers  having  each  its 
own  index  of  refraction,  different  from  those  of  the  other  layers, 
but  uniform  through  its  whole  extent.  These  three  parts  are 
called  nuclear,  intermediate,  and  external.  Their  relations  are 
shown  in  Fig.  29. 
The  mean  index  of  refraction  for  each  of  these  different  layers  varies 

considerably,  as  determined  in  different  eyes  by  different  observers,  but 

approximates  for  the 

External  layer 1.40 

Intermediate  layer 1.42 

Nucleus 1.43 

This  division  of  the  lens  into  three  layers  has  been  used  chiefly  as  a 
basis  from  which  to  calculate  the  index  of  refraction  and  curvatures  of 
surface  which  would  give  a  refractive  effect  equal  to  that  of  the  crystalline 
lens  as  it  is,  if  that  lens  possessed  the  same  index  of  refraction  throughout. 
From  his  later  calculations,  Helmholtz  reached  the  conclusion  that  the 
equivalent  of  the  crystalline  lens  having  a  curvature  on  its  anterior  surface 
of  ten  millimetres'  radius  and  on  its  posterior  surface  a  curvature  of  six 
millimetres'  radius  should  have  an  index  of  refraction  of  1.4371.  (He  had 
earlier  adopted  an  index  of  1.4545.) 

It  will  be  noted  that  this  index  for  the  whole  lens  is  not  a  mean  or 
average  of  the  indexes  of  the  different  portions  of  the  lens,  but  is  higher 
even  than  the  observed  index  of  the  nucleus.  The  reason  for  this  will 
appear  from  a  careful  examination  of  Fig.  29.  We  have  in  the  crystalline, 
first,  a  double  convex  lens,  the  nuclear  portion,  having  surfaces  that  are 
more  convex  than  the  surfaces  of  the  whole  lens ;  second,  in  front  and 
behind  this  are  a  series  of  menisci  which  are  thinnest  at  the  centre,  and 
would,  therefore,  if  acting  alone,  tend  to  diverge  the  rays  passing  through 
them.  These  menisci  partially  neutralize  the  effect  of  the  double  convex 
nuclear  portion.  If  these  menisci  had  the  same  index  of  refraction  as  the 
nuclear  portion,  they  would  neutralize  its  effect  to  a  certain  extent,  and 
would  leave  the  total  effect  of  the  lens  that  of  a  double  convex  leus  having 
the  curvatures  of  the  crystalline  with  the  index  of  refraction  of  the  nucleus. 
These  menisci,  however,  have  a  lower  index  of  refraction,  and  hence  have 
less  power  to  neutralize  the  converging  effect  of  the  nuclear  part,  so  that  the 
total  converging  effect  of  the  lens  is  left  greater  than  it  would  be  were  the 
index  of  refraction  uniformly  that  of  the  nucleus.  Now,  as  we  adopt  the 
curvatures  of  the  actual  crystalline  for  the  curvatures  of  the  equivalent  lens 


THE   DIOPTRICS  OF   THE   EYE.  483 

possessing  this  stronger  converging  power,  the  only  thing  to  be  done  to 
make  it  properly  equivalent  is  to  assign  it  a  higher  index  of  refraction. 
Briefly,  the  convergence  of  rays  accomplished  by  the  crystalline  is  not  done 
by  a  lens  having  its  exterior  curves,  but  by  the  more  convex  nuclear  por- 
tion ;  and  as  the  outer  layers  are  (on  account  of  lower  refractive  index)  not 
able  wholly  to  neutralize  the  excess  of  this  action,  we  can  only  ascribe  the 
effect  of  the  more  convex  nucleus  to  the  less  convex  lens  total  by  assuming 
the  higher  index  of  refraction.  This  substitution  of  a  conventional  convex 
lens  with  two  known  surfaces  and  a  uniform  index  of  refraction  for  the 
complex  dioptric  apparatus  of  the  actual  crystalline  lens  is  one  step  in  the 
process  of  working  out  a  "  schematic  eye"  which  is  the  average  dioptric 
equivalent  of  the  normal  emmetropic  eye,  and  through  which  we  are  able 
to  solve  various  problems  in  practical  physiological  optics.  With  the  sub- 
stitution of  the  equivalent  or  reduced  crystalline  lens  we  are  able  to  consider 
the  refraction  of  the  eye  as  occurring  at  three  surfaces, — the  anterior  surface 
of  the  cornea,  the  anterior  surface  of  the  lens,  and  the  posterior  surface  of 
the  lens.  After  the  removal  of  the  crystalline  lens,  as  for  cataract,  there 
remains  but  a  single  refracting  surface,  the  anterior  surface  of  the  cornea. 
This  is  also  the  condition  of  the  so-called  reduced  eye,  to  be  described  later. 

In  such  an  aphakic  eye,  assuming  the  curvature  of  the  cornea  to  be 
spherical,  we  have  the  problems  of  ocular  refraction  reduced  to  their  simplest 
terms. 

The  Aphakic  Bye. — The  straight  line  F1F2  passing  through  the 
centre  of  curvature  N  of  the  corneal  surface  and  through  the  centre  of  the 
surface  C  in  Fig.  30  is  the  primary  or  principal  axis  of  the  dioptric  system, 

or  the  optic  axis.      The  point  C 

v,  .         .       .  .,          f  FIG.  30. 

where  this  axis  pierces  the  refract- 
ing surface  is  called  the  principal 
point,  and  a  plane  HH  passed 
through  the  principal  point  per- 
pendicular to  the  axis  is  the  prin- 
cipal plane. 

In  any  consideration  of  the  re- 
fraction of  such  an  eye  it  is  assumed 

that  the  bending  of  rays  all  takes  place  at  this  principal  plane.  A  glance 
at  the  figure  shows  that  this  assumption  is  not  strictly  accurate,  for  the  plane 
manifestly  does  not  coincide  throughout  with  the  anterior  surface  of  the 
cornea,  where  the  bending  really  occurs.  But  for  a  small  space  at  the  centre 
of  the  cornea  the  principal  plane  and  the  corneal  surface  so  nearly  coincide 
that  the  assumption  is  accurate  enough  for  all  practical  purposes,  and  our 
whole  optical  theory  of  lenses  is  based  on  such  assumptions.  To  abandon 
such  assumptions  and  attempt  to  work  out  a  formula  that  would  apply  with 
perfect  accuracy  would  be  to  make  the  subject  infinitely  more  complex ; 
and  even  then  we  should  still  be  working  with  an  assumed  eye  that  would 
in  some  respects  differ  from  every  actual  eye  in  existence. 


484  THE   DIOPTRICS   OF  THE   EYE. 

Rays  passing  in  the  air  parallel  to  the  axis  F1F2  and  entering  the  eye 
are  refracted  (at  the  principal  plane  HH,  it  is  assumed)  towards  a  certain 
point  F2  on  the  axis.  This  point  is  called  the  posterior  or  second  principal 
focus.  The  anterior  or  first  principal  focus  is  the  point  Fl  towards  which 
would  be  refracted  rays  passing  in  the  vitreous  parallel  to  the  principal  axis 
(see  broken  lines  on  Fig.  30)  and  emerging  from  the  eye.  Conversely,  rays 
coming  from  the  second  principal  focus  and  passing  into  the  air  are  rendered 
parallel  to  the  axis,  and  rays  coming  from  the  first  principal  focus  and 
entering  the  eye  pass  parallel  in  the  vitreous.  The  distance  Fl  C  from  the 
first  principal  focus  to  the  principal  plane  is  the  first  or  anterior  focal  distance. 
The  distance  CF2  from  the  principal  plane  to  the  second  principal  focus  is 
the  second  or  posterior  focal  distance.  These  focal  distances  are  directly 
proportional  to  the  indexes  of  refraction  of  the  media  in  which  they  are 
measured.  With  the  index  of  air  1,  and  that  of  the  cornea,  aqueous,  and 
vitreous  1.337, 

F1C:  CF*:  :  1  :  1.337. 

The  centre  of  curvature  N  of  such  an  eye  is  also  the  nodal  point  of  the 
eye.  All  rays  coming  towards  it,  or  from  it,  pass  through  the  surface  of  the 
cornea  unrefracted  because  they  are  perpendicular  to  that  surface.  Each  of 
these  rays  is  the  axial  ray  of  a  pencil  coming  to  the  eye  from  the  point  from 
which  it  emanates.  Fig.  31  represents  the  eye  with  its  principal  plane,  at 

which  the  refraction  is 

IG'  assumed  to  occur,  receiv- 

ing rays  from  one  point 
A  on  the  primary  axis 
which  are  converged  to 
a,  and  rays  from  another 
point  B  outside  of  the 
primary  axis,  the  ray 
BN  passing  through  the 

nodal  point  and  being  met  by  the  other  rays  from  B  at  b.  BN  holds  the 
same  relation  to  the  rays  emanating  from  B  as  the  primary  axis  holds  to 
rays  from  A,  and  is  called  a  secondary  axis. 

If  we  have  the  line  AB  and  draw  a  line  joining  a  and  6,  from  each 
point  of  AB  rays  will  enter  the  eye,  to  be  converged  to  a  corresponding 
point  on  ah  ;  and  one  of  these  rays  passing  through  the  nodal  point  N,  and 
therefore  unrefracted,  constitutes  for  each  point  of  AB  a  secondary  axis, 
and  determines  the  direction  in  which  each  pencil  of  rays  will  converge. 
Each  point  of  ab  is  then  the  image  of  a  corresponding  point  of  AB,  and 
the  whole  line  ab  is  the  image  of  the  whole  line  A  B. 

The  relative  size  of  an  image  depends  on  its  relative  distance  from  the 
nodal  point ;  for  in  the  triangles  ABN  and  abN,  the  sides  being  parallel, 
the  triangles  are  similar  and  the  sides  proportional.  Thus, 

AN  :  AB  :  :  aN  :  db. 


THE   DIOPTRICS   OF   THE   EYE. 


485 


Having  any  three  terms  of  this  proportion,  the  fourth  is  readily  found.  For 
instance,  having  the  size  of  an  object  and  its  distance  from  the  nodal  point 
of  the  eye,  and  knowing  the  distance  of  the  nodal  point  N  from  the  retina, 
we  have : 

Size  of  object  X  distance  of  retina  from  N 

distance  of  object  from  N~  f  ima£e'  (E) 

The  relations  of  the  nodal  point  to  the  position  and  size  of  images  give  it 
practical  importance. 

The  Schematic  Eye. — For  the  aphakic  eye  we  could  assume  without 
important  error  that  the  refraction  all  took  place  in  one  principal  plane. 
For  the  schematic  eye,  with  its  three  refractive  surfaces  situated  some 
distance  apart,  that  assumption  must  be  abandoned.  We  may,  however,  for 
such  an  eye,  or  for  any  number  of  spherical  surfaces  having  the  same  prin- 
cipal axis, — having  their  centres  of  curvature  in  the  same  straight  line, — 
assume  with  practical  accuracy  that  the  refraction  of  the  lens  system  occurs 
in  two  planes,  the  position  of  which  will  depend  on  the  relative  position 
and  lens-action  of  these  different  surfaces.  These  principal  planes  are 
separated  by  a  certain  interval,  and  there  go  with  them  two  nodal  points 
separated  by  the  same  interval. 
The  relation  of  these  planes  and 
points  in  the  schematic  eye  is 
shown  in  Fig.  32. 

F1F2  is  the  primary  or  prin- 
cipal or  optic  axis, — the  straight 
line  in  which  are  located  the 
centres  of  curvatures  of  the 
three  surfaces.  HlHl,  perpen- 
dicular to  the  optic  axis,  is  the 
first  or  anterior  principal  plane,  its  intersection  with  the  optic  axis  being  the 
first  principal  point.  H2H2,  also  perpendicular  to  the  optic  axis,  is  the 
second  or  posterior  principal  plane,  and  its  intersection  with  that  axis  is  the 
second  principal  point.  Fl  is  the  first  or  anterior  principal  focus, — the 
focus  for  rays  parallel  in  the  vitreous, — and  is  situated  the  anterior  focal 
distance  in  front  of  the  first  principal  plane.  F2  is  the  second  or  pos- 
terior principal  focus,  situated  the  posterior  focal  distance  behind  the  second 
principal  plane.  The  relation  of  these  focal  distances  is  in  general  the  same 
as  the  relation  of  the  index  of  refraction  of  the  first  medium  (in  this  case 
air)  to  the  index  of  refraction  of  the  last  medium  (in  this  case  vitreous 
humor). 

The  first  or  anterior  nodal  point  is  Nl,  and  the  second  or  posterior  nodal 
point  is  N2.  A  secondary  axis  Bb  does  not,  for  such  a  system,  constitute 
a  single  straight  line,  but  consists  of  two  parts, — one,  EN1,  a  straight  line 
extending  from  the  source  of  the  light  towards  the  first  nodal  point  Nl, 
until  it  reaches  the  first  principal  plane  at  A1,  and  the  other,  h2b,  extending 
from  h2  in  the  second  principal  plane  through  the  second  nodal  point,  N*. 


486  THE    DIOPTRICS    OF   THE    EYE. 

The  points  h}  and  A2  are  equally  distant  from  the  optic  axis  and  in  the  same 
direction  (that  is,  hlh?  is  parallel  to  the  optic  axis),  and  h  *B  is  parallel  to 
J5A1.  In  general,  all  secondary  axes  pass  before  refraction  towards  the  first 
nodal  point,  and  after  refraction  through  and  from  the  second  nodal  poinf; 
in  a  direction  parallel  to  their  first  direction.  Or,  conversely,  all  rays 
passing  towards  the  first  nodal  point  or  from  the  second  nodal  point  con- 
tinue parallel  to  their  original  direction,  and  are  secondary  axes.  Between 
the  principal  planes  the  course  of  all  rays  is  assumed  parallel  to  the  prin- 
cipal axis,  whatever  their  direction  in  front  of  the  first  principal  plane  or 
beyond  the  second  principal  plane. 

The  following  are  means  of  the  careful  ophthalmometric  measurements 
of  many  eyes : 

Millimetres. 

Kadius  of  curvature  of  cornea 7.829 

Badius  of  curvature  of  anterior  surface  of  lens,  the  ciliary  muscle 

being  relaxed 10. 

Radius  of  curvature  of  posterior  surface  of  lens 6. 

Distance  from  summit  of  cornea  to  anterior  pole  of  lens 3.6 

Thickness  of  crystalline  lens 3.6 

The  indexes  of  refraction  as  determined  for  the  other  media,  and  the 
equivalent  calculated  for  the  crystalline  lens,  are : 

Millimetres. 

For  cornea,  aqueous  humor,  and  vitreous  humor 1.3365 

Equivalent  crystalline  lens  (Helmholtz's  latest) 1.4371 

Equivalent  crystalline  lens  (Listing,  adopted  by  Bonders  and  for- 
merly by  Helmholtz) 1.4545 

Equivalent  crystalline  lens  (Aubert  and  Matthiessen) 1.4480 

Taking  the  above  dimensions,  with  the  later  index  of  Helmholtz  for 
the  equivalent  crystalline  lens,  calculation  gives  for  the  schematic  eye  the 
following  distances : 

Millimetres. 

Summit  of  cornea  to  first  principal  point 1.7532 

Summit  of  cornea  to  second  principal  point 2.1101 

Summit  of  cornea  to  first  nodal  point      6.9685 

Summit  of  cornea  to  second  nodal  point 7.3254 

Distance  between  the  principal  planes  equal  the  distance  between 

the  nodal  points    ....        0.3569 

First  nodal  point  in  front  of  posterior  pole  of  lens 0.2315 

Second  nodal  point  behind  posterior  pole  of  lens 0.1254 

Anterior  focal  distance  (measured  from  first  principal  plane)     .    .    .  15.4983 

Posterior  focal  distance  (measured  from  second  principal  plane)   .    .  20.7136 

Anterior  focus  in  front  of  summit  of  cornea 13.7451 

Posterior  focus  behind  summit  of  cornea 22  8237 

Posterior  nodal  point  to  posterior  focus 154983 

If  the  eye  be  emmetropic,  the  retina  is  situated  at  the  posterior  focus, 
its  distance  from  the  anterior  surface  of  the  cornea  is  the  antero-posterior 
axis  of  the  eyeball,  and  the  distance  from  the  posterior  nodal  point  to  that 


THE   DIOPTRICS   OF  THE   EYE.  487 

focus  is  the  distance  of  the  retina,  where  images  are  formed,  from  the  nodal 
point, — the  distance  which  determines  the  size  of  the  retinal  image. 

The  Reduced  Eye. — It  will  be  observed  that,  among  the  values  given 
for  the  schematic  eye,  the  distance  between  the  principal  planes  or  the  distance 
between  the  nodal  points  is  a  little  over  one-third  of  a  millimetre,  and  also 
that  this  space  is  in  various  ways  a  space  where  nothing  happens.  It  does 
not  enter  into  the  anterior  or  posterior  focal  distances  or  into  conjugate  focal 
distances.  Since  in  it  rays  remain  parallel  to  the  axial  ray,  it  does  not  affect 
the  size  of  circles  of  diffusion.  Neither  does  it  affect  the  relative  sizes  of 
images,  for  these  depend  on  the  distance  of  the  object  from  the  anterior 
nodal  point  and  the  distance  of  the  image  or  the  retina  from  the  posterior 
nodal  point. 

For  most  purposes  of  practical  dioptrics  we  may  therefore  bring  the  two 
principal  planes  together,  and  the  two  nodal  points  likewise,  by  making  the 
further  assumption  that  the  refraction  of  the  standard  schematic  eye  is  repre- 
sented by  that  of  a  reduced  eye  with  but  a  single  refractive  surface,  that  of 
the  cornea.  The  most  important  requirements  regarding  such  an  eye  are 
that  it  shall  closely  correspond  to  the  schematic  eye  in  its  focal  distances  and 
in  the  distance  of  its  nodal  point  from  the  retina.  To  secure  these  it  is  neces- 
sary to  assume  for  the  cornea  of  the  reduced  eye  either  a  greater  curvature  or 
a  higher  index  of  refraction  than  the  natural  cornea  possesses. 

Four  such  reduced  eyes,  differing  in  some  respects  from  one  another, 
have  been  proposed  by  Listing,  Bonders,  Stammeshaus,  and  von  Hasner,  and 
are  found  referred  to  in  the  literature  of  the  subject.  Their  respective  dimen- 
sions, cardinal  points,  and  focal  distances  are  given  below.  It  will  be  borne 
in  mind  that  in  such  an  eye  the  nodal  point  is  at  the  centre  of  curvature, 
and  its  distance  back  of  the  cornea  equals  the  radius  of  curvature ;  also  that 
in  such  an  eye,  supposed  to  be  emmetropic,  the  antero-posterior  axis  equals 
the  posterior  focal  distance.  The  figures  indicate  distances  in  millimetres. 

Listing.  Donders.  Stammeshaus.  Von  Hasner. 

Radius  of  curvature  of  cornea  .   .    .    6.077            6.  5.2152  7.6 

Index  of  refraction 1.337            1.33$  1.3365  1.5 

Anterior  focal  distance 16.036  15.  15.4983  15. 

Posterior  focal  distance 20.113  20.  20.7135  22.5 

Distance  from  nodal  point  to  retina  .  15.036  15.  15.4983  15. 

On  comparing  these  assumed  optical  equivalents  of  the  human  eye,  it 
will  be  noted  that  the  first  three  have  adopted  approximately  the  index  of 
refraction  of  the  cornea,  with  a  much  shortened  radius  of  curvature.  Von 
Hasner,  however,  has  adopted  a  curvature  approximating  that  of  the  average 
cornea,  and  a  much  higher  index  of  refraction,  one  approaching  that  of 
ordinary  optical  glass.  This  gives  a  length  of  eyeball  approximating  the 
usual  length  of  the  eye,  but  an  excessive  posterior  focal  distance.  It  will 
be  seen  that  the  values  decided  on  by  Listing  and  Stammeshaus  approximate 
very  closely  those  of  the  schematic  eye,  while  those  of  Donders  and  von 
Hasner  are  rougher  approximations,  which  are  very  much  easier  to  remem- 


488  THE   DIOPTRICS   OF   THE   EYE. 

ber  and  work  with.  As  a  chief  value  of  the  reduced  eye  is  to  enable  one 
to  solve  quickly  by  mental  calculation  some  of  the  problems  that  arise  in 
practical  work,  the  extreme  simplicity  of  the  latter  eyes  gives  them  the 
greater  practical  value. 

Suppose,  for  instance,  it  is  desired  to  find  the  height  of  the  image  formed 
on  the  retina  by  a  man  1.8  metres  high,  6  metres  from  the  eye.  By  the 
formula  (E)  we  have,  reducing  all  to  millimetres, 

1800X15_=15 
6000 

Or  suppose  a  certain  scotoma  causes  blindness  over  a  space  30  millimetres 
in  diameter  on  a  surface  (the  arc  of  a  perimeter)  300  millimetres  away, 

30  x  15  =  15  millimetres 
300 

the  actual  diameter  of  the  scotoma. 

Circumstances  influencing  if  e,  Positions  of  the  Cardinal  Points. — As  has 
been  indicated,  the  dimensions  given  are  those  for  an  assumed  standard  or 
average  eye,  from  which  any  given  eye  will  vary ;  and  just  what  the  varia- 
tions are  in  a  particular  case  can  be  ascertained  only  by  careful  opthalmo- 
metrical  measurements  of  both  the  cornea  and  the  crystalline  lens,  such  as 
are  practicable  only  for  a  comparatively  few  eyes  in  the  well-equipped 
laboratory.  In  general,  increased  convexity  of  the  surfaces  will  cause 
shortening  of  the  focal  distances  ;  their  comparative  flattening  will  increase 
those  distances.  Absolute  increase  in  the  curvature  of  the  cornea  alone,  or 
relative  increase  in  its  curvature  as  compared  with  the  surfaces  of  the  lens, 
or  a  more  anterior  position  of  the  lens,  will  bring  the  principal  and  nodal 
points  closer  to  the  cornea.  A  greater  depth  of  the  lens,  or  its  relatively 
greater  curvature  as  compared  with  the  cornea,  will  cause  the  principal  and 
nodal  points  to  fall  deeper  within  the  eye.  The  position  of  these  points  is 
also  influenced  by  ametropia  and  the  lenses  used  to  correct  it,  as  will  be 
indicated  in  the  account  of  these. 

Emmetropia. — The  discussion  of  the  refraction  of  light  within  the 
eye  is  thus  far  complete,  without  taking  any  account  of  the  position  of  the 
retina.  Although  the  position  of  the  retina  with  reference  to  the  cardinal 
points  of  the  eye,  especially  the  posterior  principal  focus  and  the  nodal 
point,  is  of  the  highest  practical  importance,  the  bending  and  course  of  the 
rays  are  in  no  way  dependent  on  it. 

The  position  of  the  retina  with  reference  to  the  focus  of  the  dioptric 
media  is  important,  because  the  whole  focussing  apparatus  exists  to  furnish 
the  retina  with  assorted  light,  and  the  assorting  of  the  light  is  complete 
only  at  the  focus  of  the  dioptric  system.  In  front  of  this  the  rays  of  the 
same  pencil  still  occupy  a  certain  area  and  are  intermingled  with  rays  from 
other  pencils,  and  beyond  the  focus  they  again  spread  out  and  intermingle. 

When  the  refracting  surfaces  of  the  eye  are  such  that  without  effort  of 


THE    DIOPTRICS   OF   THE    EYE. 


IX!) 


FIG.  33. 


the  ciliary  muscle  they  can  bring  rays  of  light  parallel  in  the  air  to  a  perfect 
focus,  and  the  retina  is  situated  at  that  focus,  the  eye  .is  said  to  be  emme- 
tropic. When  the  dioptric  surfaces  do  not  perfectly  focus  the  light  passing 
through  them,  or  when  they  do  so  focus  it  but  the  retina  is  situated  else- 
where, the  eye  is  said  to  be  ametropic.  Emmetropia  gives  the  eye  certain 
advantages  which  will  appear  by  contrast  with  the  disadvantages  of  the 
various  forms  of  ametropia.  The  various  departures  of  the  eye  from 
emmetropia  are  called  errors  or  anomalies  of  refraction. 

It  must  be  borne  in  mind,  however,  that  in  no  eye  is  the  whole  retina  so 
situated  as  to  receive  perfectly  focussed  rays  that  have  been  parallel  in  the 
air.  We  may  more  accurately  define  emmetropia  to  be  the  state  of  ocular 
refraction  in  which  the  part  of  the  retina  pierced  by  the  optic  axis,  the  region 
of  the  macula,  receives  perfectly  focussed  the  rays  which  were  parallel  in 
the  air. 

That  only  a  limited  part  of  the  retina  can  be  so  situated  is  illustrated 
by  Fig.  33,  which  represents  the  reduced  eye,  to  which  is  added  an  arc  (the 
broken  line)  described  with  a  radius  NF* 
about  the  nodal  point  as  a  centre.  Evi- 
dently it  is  the  different  points  of  this  arc, 
and  not  of  the  retina,  that  are  distant  the 
posterior  focal  distance  behind  the  cornea. 
Hence  rays  parallel  to  any  secondary  axis 
Bb  will  be  focussed  on  this  arc  and  behind 
the  portion  of  the  retina  on  which  they  fall. 

Thus,  in  the  "  emmetropic  eye"  only  the 

central  part  of  the  retina  is  emmetropic,  other  parts  being  hyperopic.  By 
a  similar  construction  one  may  demonstrate  that  in  hyperopic  eyes  the 
eccentric  portions  of  the  retina  are  more  hyperopic  than  the  central,  and 
that  in  myopic  eyes,  aside  from  unequal  bulging  of  the  coats,  the  eccentric 
parts  of  the  retina  are  less  myopic,  emmetropic,  or  even  hyperopic.  The 
refraction  of  an  eye  as  it  is  commonly  spoken  of  refers  only  to  the  position 
of  the  central  part  of  the  retina  relative  to  the  posterior  principal  focus. 

Circles  of  Diffusion. — The  pencil  of  rays  entering  the  eye  from  any 
luminous  point  is  limited  by  the  outline  of  the  pupil,  approximately  circu- 
lar. If  we  suppose  this  circle  to  be  situated 
in  the  second  principal  plane,  the  rays  after 
leaving  this  plane  occupy  a  cone  with  its  base 
the  pupil  and  its  apex  the  focus  towards 
which  they  tend.  If  at  any  point  in  front 
of  or  behind  this  focus  the  rays  be  inter- 
cepted by  a  plane  parallel  to  the  base,  as  by 
the  retina,  they  will  be  found  to  occupy  a 
circle  the  diameter  of  which  is  directly  proportional  to  the  diameter  of  the 
pupil  and  to  the  distance  of  the  intercepting  plane  from  the  focus,  and 
inversely  proportional  to  the  distance  of  the  second  principal  plane  from 


FIG.  34. 


490  THE   DIOPTRICS   OF   THE   EYE. 

the  focus.     Thus,  in  Fig.  34  we  have  the  similar  triangles  .AP^Fand  apF, 
in  which 

AF:  aF::  AP:  ap, 
or 

APX  aF 
ap==.       AF 

In  calculating  the  size  of  diffusion  circles  formed  on  the  retina,  situated 
a  known  distance  from  the  focus,  one  may,  for  all  practical  purposes,  regard 
the  plane  of  the  pupil  as  coincident  with  the  principal  plane  of  Donders's 
reduced  eye. 

AMETROPIA. 

Hyperopia. — Hyperopia  is  the  term  suggested  by  Helmholtz,  and 
hypermetropia  the  one  subsequently  adopted  by  Donders,  to  designate  that 
condition  of  the  refraction  of  the  eye  in  which,  the  accommodation  being 
at  rest,  the  retina  intercepts  the  optic  axis  in  front  of  the  principal  focus. 
The  condition  may  be  due  to  unusual  flatness  of  the  dioptric  surfaces  of 
the  eye,  in  which  case  it  is  called  hyperopia  of  curvature  ;  or  to  the  absence 
of  the  crystalline  lens,  aphakic  hyperopia ;  but  is  most  commonly  due  to 
undue  shortness  of  the  antero-posterior  axis  of  the  eyeball,  axial  hyperopia. 

Whatever  its  cause,  its  optical  and  its  clinical  results  are  the  same. 
Pencils  of  rays  parallel  before  entering  the  eye  are  not  focussed  on  the 
retina,  but  form  upon  it  circles  of  diffusion  ;  rays  divergent  before  entering 
the  eye  form  on  the  retina  still  larger  circles  of  diffusion  ;  and  only  rays  con- 
vergent when  they  strike  the  surface  of  the  cornea  can  be  accurately  focussed 
on  the  retina.  These  different  conditions  are  illustrated  in  Fig.  35. 

FIG.  35. 


Only  those  rays  having  a  certain  degree  of  convergence,  those  con- 
verging towards  a  certain  point  back  of  the  eye,  will  be  focussed  on  the 
retina.  Rays  more  convergent  will  be  focussed  in  front  of  the  retina,  and 
those  less  convergent  back  of  the  retina.  This  point  towards  which  the 
rays  must  be  converging  in  order  that  the  dioptric  system  of  the  eye  can 
focus  them  upon  the  retina  is  a  focus  conjugate  to  the  position  of  the  retina. 
It  is  called  the  far  point  of  the  hyperopic  eye.  Rays  coming  from  a  point 
on  the  retina  will,  after  emerging  from  the  eye,  diverge  as  though  they  had 
started  from  this  focus.  Thus,  in  Fig.  35,  if  rays  converging  towards  R 
are  focussed  at  r,  rays  from  r  will  pass  into  the  air  divergent  as  if  from  R. 

To  secure  the  focussing  of  parallel  rays  upon  the  retina  they  must  be 


THE   DIOPTRICS   OF   THE    EYE.  491 

rendered  sufficiently  convergent  before  entering  the  cornea,  as  by  passing 
through  a  convex  lens.  The  placing  before  the  eye  of  a  lens  that  will  give 
parallel  rays  the  proper  convergence,  and  thus  secure  their  focussing  on 
the  retina,  is  called  correcting  the  hyperopia,  and  the  leus  which  does  it  the 
correcting  lens.  It  is  convenient  in  practical  work  always  to  think  of  hyper- 
opia as  a  deficiency  of  refracting  power  in  the  eye  ;  and  the  degree  or  amount 
of  hyperopia  is  the  refracting  power  of  the  infinitely  thin  convex  lens  which 
when  placed  at  the  surface  of  the  cornea  enables  the  eye  to  focus  parallel 
rays  on  its  retina.  It  is  evident  that  a  convex  lens  that  will  give  parallel 
rays  the  proper  convergence — turn  them  towards  the  focus  conjugate  to  the 
retina — must  be  a  lens  with  its  principal  focus  at  that  point,  or  that  any 
convex  lens  so  placed  before  the  eye  that  its  principal  focus  falls  at  the 
conjugate  to  the  retina  will  correct  the  hyperopia.  Thus,  in  Fig.  36,  if  we 

FIG.  36. 


suppose  the  focus  R  to  be  three  inches  from  the  lens  L  placed  before  the 
eye,  this  lens  would  need  to  have  a  focal  distance  of  three  inches  (or  13  D. 
of  refracting  power)  to  correct  the  hyperopia.  But  if  another  lens  L'  were 
used  in  a  position  one  inch  farther  from  the  eye,  it  would  need  to  have  a 
focal  distance  of  four  inches  (a  refracting  power  of  only  10  D.)  to  correct 
the  hyperopia.  If  the  convex  correcting  lens  is  to  be  worn  a  certain  dis- 
tance in  front  of  the  eye,  it  must  have  a  certain  strength ;  nearer  to  the  eye 
it  needs  to  be  stronger,  farther  from  the  eye  it  must  be  weaker. 

In  young  persons  hyperopia  may  be  corrected  by  increased  curvature  of 
the  crystalline  lens,  having  the  effect  of  a  supplementary  convex  lens  within 
the  eye,  as  well  as  by  the  convex  lens  placed  in  front  of  the  eye,  and  is  com- 
monly so  corrected. 

The  position  of  the  second  nodal  point  is  affected  by  hyperopia  and  its 
correction  in  a  way  that  is  of  some  practical  importance.  Hyperopia  of 
curvature  causes  but  little  change  in  it,  and  may  be  disregarded.  In  axial 
hyperopia,  the  position  of  the  nodal  point  remaining  the  same  as  in  emme- 
tropia  with  regard  to  the  corneal  surface,  the  shortening  of  the  antero- 
posterior  axis  of  the  eyeball  brings  the  retina  by  so  much  nearer  to  it, 
and  proportionately  decreases  the  size  of  the  retinal  images.  In  aphakia 
the  nodal  point  comes  to  be  the  centre  of  curvature  of  the  cornea,  usually 
slightly  farther  back  than  the  second  nodal  point  of  the  reduced  eye ;  but 
the  effect  of  this,  taken  by  itself,  is  trifling. 

The  correction  of  the  hyperopia  by  a  convex  lens  placed  before  the  eye 
produces  an  important  change  in  the  position  of  the  nodal  point,  carrying 


492 


THE   DIOPTRICS   OF   THE   EYE. 


it  forward  to  an  extent  directly  proportioned  to  the  amount  of  the  hyper- 
opia  corrected  and  the  distance  of  the  correcting  lens  before  the  eye. 
Of  two  lenses  correcting  a  given  amount  of  hyperopia,  the  weaker  lens 
placed  farther  from  the  eye  produces  the  greater  effect  on  the  position  of  the 
nodal  point.  In  axial  hyperopia  the  placing  of  a  correcting  lens  at  the 
anterior  focus  (about  the  distance  in  front  of  the  eye  that  it  is  usually 
placed)  will  cause  the  nodal  point  to  fall  just  the  same  distance  in  front  of 
the  retina  that  it  does  in  the  emmetropic  eye,  and  will  give  retinal  images 
of  corresponding  size.  In  hyperopia  of  curvature  or  aphakia  the  correct- 
ing lens  brings  the  nodal  point  farther  from  the  retina  and  gives  larger 
retinal  images.  On  account  of  the  effect  on  the  nodal  point  of  the  correct- 
ing lens  placed  at  the  anterior  focus,  it  has  been  proposed  to  consider  the 
strength  of  this  lens  the  measure  of  the  hyperopia ;  and  in  practical  work 
and  the  reporting  of  cases  the  degree  spoken  of  corresponds  to  this  position 
of  the  lens  rather  than  to  one  at  the  corneal  surface.  The  correction  of 
hyperopia  by  accommodation,  and  its  effect  on  the  region  of  accommoda- 
tion, will  be  discussed  under  accommodation. 

The  change  in  the  length  of  axis  required  to  produce  a  certain  degree  of 
axial  hyperopia  is  shown  in  the  following  table,  and  also,  for  purposes  of 
comparison,  the  axial  changes  that  cause  similar  degrees  of  myopia.  The 
first  column  gives  the  degree  of  ametropia,  the  second  the  length  of  the  axis 
in  hyperopia,  the  third  the  length  of  the  axis  in  myopia,  the  fourth  the 
diminution  of  the  axis  in  hyperopia,  and  the  fifth  the  increase  in  the  axis 
in  myopia.  Hyperopia  of  over  10  or  12  D.  is  rarely  seen,  aside  from 
aphakia ;  while  myopia  upward  of  20  D.,  almost  always  axial,  is  quite 
frequently  encountered.  A  glance  at  the  table  indicates  the  enormously 
greater  distortion  of  the  eyeball  in  the  latter  condition.  The  figures  of  the 
first  column  indicate  dioptres,  those  of  the  other  columns  millimetres. 


Dioptres. 

Hyperopia.    Axis. 

Myopia.    Axis. 

Hyperopia. 
Diminution. 

Myopia.    Increase. 

0 

22.824 

22.824 

1 

22.51 

23.14 

.31 

.32 

2 

22.20 

23.48 

.62 

.66 

3 

21.90 

23.83 

•   .92 

1.01 

4 

21.61 

24.19 

1.21 

1.37 

5 

21.32 

24.56 

1.50 

1.74 

6 

21.06 

24.95 

1.76 

2.13 

7 

20.80 

25.34 

2.03 

2.52 

8 

20.64 

25.75 

2.28 

2.93 

9 

20.29 

26.17 

2.53 

3.35 

10 

20.04 

26.02 

2.78 

3.80 

11 

19.80 

27.08 

3.02 

4.26 

12 

19.57 

27.55 

3.25 

4.73 

13 

19.36 

28.05 

3.47 

5.23 

14 

19.13 

28.56 

3.69 

5.74 

15 

18.91 

29.10 

3.91 

6.28 

16 

18.71 

29.65 

4.11 

6.83 

17 

18.50 

30.23 

4.32 

741 

18 

18.30 

30.85 

4.52 

8.03 

19 

18.11 

31.47 

4.71 

8.65 

20 

17.92 

32.13 

4.90 

9.31 

THE   DIOPTRICS   OF   THE    EYE. 


493 


Myopia  is  that  condition  of  the  refraction  of  the  eye  in  which  the 
principal  focus  falls  in  front  of  the  retina.  It  may  be  due  to  excessive 
curvature  of  one  or  more  of  the  dioptric  surfaces,  myopia  of  curvature ;  or 
to  increase  in  the  index  of  refraction  of  the  nucleus  of  the  lens  index 
myopia;  but  commonly  it  is  caused  by  excessive  length  of  the  antero- 
posterior  axis  of  the  eyeball,  axial  myopia.  In  any  case  a  pencil  of  rays 
parallel  in  the  air,  on  entering  the  eye  is  so  refracted  as  to  focus  in  the 
vitreous,  and,  diverging  again  from  its  focus,  forms  on  the  retina  a  circle  of 
diffusion.  Rays  convergent  before  entering  the  eye  are  focussed  still  farther 
in  front  of  the  retina,  and  form  upon  it  a  correspondingly  larger  circle  of 
diffusion.  Only  rays  reaching  the  cornea  with  a  certain  degree  of  diver- 
gence can  be  accurately  focussed  upon  the  retina,  rays  more  divergent  or 
less  divergent  being  focussed  behind  or  in  front  of  the  retina.  The  rays 
coming  from  R  in  Fig.  37  have  the  proper  degree  of  divergence  to  be 

FIG.  37. 


focussed  on  the  retina  at  r.  R  and  r  have  the  relation  of  conjugate  foci. 
Rays  emanating  from  r,  on  passing  out  of  the  eye  will  be  rendered  so  con- 
vergent as  to  be  focussed  at  R,  the  focus  conjugate  to  the  position  of  the 
retina.  R  is  the  far  point  of  the  myopic  eye. 

To  secure  the  focussing  of  parallel  rays  upon  the  retina  of  the  myopic 
eye,  they  must  be  rendered  divergent,  as  though  from  the  far  point  R. 
This  is  accomplished  by  a  concave  lens  which  has  its  principal  focus  at  the 
far  point,  and  which  is  called  a  correcting  lens.  Evidently  lenses  of  differ- 
ent strengths  are  equally  capable  of  correcting  a  given  amount  of  myopia 
if  placed  at  their  focal  distances  from  R.  Thus,  in  Fig.  38,  if  a  lens  L 

FIG.  38. 


placed  at  the  anterior  surface  of  the  cornea  is  four  inches  from  R,  it  must 
have  a  focal  distance  of  four  inches  (refracting  power  of  10  D.)  to  correct 
the  myopia;  and  another  lens  L'  placed  one  inch  in  front  of  the  cornea 
would  be  only  three  inches  from  R,  and  would  require  a  focal  distance  of 


494  THE   DIOPTRICS   OF   THE   EYE. 

three  inches  (refracting  power  of  13  D.)  to  correct  the  same  myopia.  The 
nearer  a  concave  lens  is  to  the  cornea  the  more  myopia  does  it  correct,  or 
the  weaker  it  can  be  to  correct  a  given  amount  of  myopia.  Strictly  speak- 
ing, the  infinitely  thin  lens  which  corrects  the  myopia  when  placed  at  the 
surface  of  the  cornea  is  the  lens  whose  strength  measures  the  amount  of 
myopia.  But  in  practice  the  lens  which  corrects  it  when  placed  in  front  of 
the  eye  in  the  ordinary  position  of  the  correcting  lens  is  regarded  as  the 
measure  of  the  amount.  For  the  higher  degrees  of  myopia  it  should  always 
be  indicated  which  is  meant,  for  the  difference  between  the  two  is  important. 
Thus,  in  the  case  supposed  for  Fig.  38,  strictly  speaking,  the  amount  is 
10  D. ;  yet  in  practice  it  would  be  corrected  by  an  11  D.  or  11.50  D.  lens, 
and  would  be  so  recorded  in  the  case-book  or  in  reporting  the  case. 

The  position  of  the  second  nodal  point  with,  reference  to  the  retina  is  thus 
affected  by  myopia  and  concave  correcting  lenses.  The  elongation  of  the 
antero-posterior  axis  of  the  eyeball  in  axial  myopia  carries  the  retina  farther 
from  the  second  nodal  point  than  is  its  position  in  the  emmetropic  eye, 
causing  a  corresponding  enlargement  of  retinal  images.  The  increase  of 
curvature  in  the  dioptric  surfaces  which  causes  myopia  of  curvature  brings 
the  nodal  point  somewhat  closer  to  the  cornea  and,  therefore,  farther  from 
the  retina,  also  enlarging  the  retinal  images.  Hence  the  myopic  eye,  for 
such  distances  as  it  can  see  clearly  without  a  concave  lens,  has  larger  retinal 
images  than  the  emmetropic  eye. 

A  concave  lens  added  in  front  of  the  dioptric  system  of  the  eye  causes 
the  second  nodal  point  to  be  displaced  farther  from  the  cornea  and  nearer 
to  the  retina.  The  amount  of  such  displacement  increases  directly  with  the 
strength  of  the  concave  lens  and  its  distance  in  front  of  the  cornea.  If 
the  correcting  lens  be  placed  at  the  anterior  focus,  its  effect  will  be  such  that 
in  axial  myopia  the  nodal  point  will  fall  the  same  distance  from  the  retina 
as  in  the  emmetropic  eye.  In  curvature  myopia  the  effect  of  a  correcting 
lens  in  front  of  the  cornea  is  always  to  give  a  nodal  point  closer  to  the  retina 
and  a  smaller  retinal  image  than  in  emmetropia. 

Astigmatism. — We  have  thus  far  assumed  that  the  dioptric  surfaces 
of  the  eye  were  small  portions  of  spherical  surfaces, — that  they  curved 
equally  in  all  parts  and  in  all  directions.  In  reality,  while  they  approach 
the  spherical  in  form,  they  never  exactly  attain  it  throughout  their  ex- 
tent. They  more  nearly  approach  in  form  an  ellipsoid  of  two  axes,  and 
still  more  nearly  an  ellipsoid  developed  on  three  axes,  because  the  tri-axial 
ellipsoid  permits  of  those  differences  of  curvature  in  different  directions 
and  in  different  portions  of  the  surface  which  are  to  be  observed  in  the 
dioptric  surfaces  of  the  eye.  These  variations  of  form  give  rise  to  varie- 
ties of  curvature  ametropia,  some  of  which  are  of  the  highest  practical 
importance. 

When  the  curvature  of  one  or  more  of  the  dioptric  surfaces  of  the  eye 
is  the  same  in  different  parts  of  the  surface,  but  is  different  in  different 
directions,  in  such  a  way  that  the  direction  in  which  it  is  most  convex  is 


THE   DIOPTRICS   OF   THE   EYE. 


495 


perpendicular  to  the  direction  in  which  it  is  least  convex,  it  gives  rise  to  the 
form  of  ametropia  known  as  regular  astigmatism.  The  writer  is  accustomed 
to  illustrate  such  a  curvature  by  that  of  the  edge  of  a  watch,  the  curve  of 
least  convexity  being  in  the  plane  parallel  to  the  dial  of  the  watch  and  the 
curve  of  greatest  convexity  in  a  plane  perpendicular  to  the  dial.  Another 
common  illustration  is  the  convex  surface  of  the  bowl  of  a  spoon.  The 
dioptric  surface  which  most  constantly  and  markedly  presents  this  anomaly 
of  curvature  is  that  of  the  cornea.  It  may,  however,  reside  in  either  sur- 
face of  the  lens,  or  the  same  effect  may  be  produced  by  obliquity  of  one 
or  more  of  these  surfaces  to  the  entering  pencil  of  rays. 

When  a  pencil  of  rays  enters  the  eye  through  a  surface  of  the  kind 
described,  since  the  refraction  of  the  rays  is  dependent  on  the  curve  of  the 
surface,  it  will  be  greater  in  the  direction  of  the  curve  with  the  shorter  radius 
than  in  the  direction  of  the  curve  with  the  longer  radius,  and  the  refraction 
at  any  particular  point  of  the  cornea  may  be  regarded  as  made  up  of  these 
two  unequal  factors.  The  changes  occurring  in  a  pencil  of  rays  by  reason 
of  such  refraction  may  be  understood  by  supposing  a  case  of  astigmatism  in 
which,  according  to  the  rule,  the  curve  with  the  shorter  radius  is  vertical 
and  the  curve  with  the  longer  radius  is  horizontal.  In  Fig.  39,  let  ABAB 

FIG.  39. 


represent  this  cornea  receiving  a  pencil  of  parallel  rays,  A  A  being  a  vertical 
and  BB  a  horizontal  section  of  it.  On  account  of  the  greater  curve  in  that 
direction,  rays  will  be  turned  up  and  down  more  than  they  will  be  turned 
in  from  the  sides  :  they  will  converge  faster  vertically  than  horizontally. 
Consequently,  when  at  F2,  the  posterior  principal  focus  for  the  vertical  cur- 
vature, the  rays  entering  the  lower  half  of  the  cornea  have  come  up  to  the 
level  of  the  central  ray,  and  the  rays  entering  the  upper  half  of  the  cornea 
have  come  down  to  that  level,  they  will  still  remain  spread  out  the  length 
of  F2  horizontally.  Passing  F2,  they  begin  to  diverge  again  vertically,  the 
rays  that  entered  the  lower  half  of  the  cornea  spreading  out  above  the  cen- 
tral ray,  and  those  that  entered  the  upper  half  spreading  out  below  the  cen- 


496 


THE   DIOPTRICS   OF   THE   EYE. 


tral  ray  ;  but  from  side  to  side  they  still  converge  towards  the  middle,  until 
those  entering  the  right  half  of  the  cornea  have  come  over  to  the  central  ray, 
and  those  entering  the  left  half  have  come  over  to  the  central  ray  at  F*,  the 
posterior  principal  focus  for  the  horizontal  curvature.  By  this  time  the  rays 
have  diverged  considerably  up  and  down,  and  after  passing  F*  they  diverge 
laterally  also.  The  form  of  the  pencil  of  rays  at  different  points — that  is, 
the  form  of  the  diffusion  areas  to  which  it  gives  rise  if  intercepted — is  shown 
in  Fig.  40,  in  which  sections  of  the  pencil  taken  at  c,  d,  F2,  I,  m,  n,  F*, 

FIG.  40. 


and  o  are  represented.  The  lettering  in  the  two  figures  corresponds 
throughout.  In  all  of  these  areas  the  position  of  the  rays  that  entered 
through  the  different  quadrants  is  indicated  by  the  same  numbering,  which 
serves  to  indicate  the  course  of  the  rays  as  they  pass  backward  :  thus  the  rays 
indicated  by  1  start  in  the  lower  left  quadrant,  continuing  there  to  F2,  when 
they  pass  into  the  upper  left  quadrant,  and  at  F*  into  the  upper  right  quad- 
rant. On  comparing  these  different  diffusion  areas  with  Fig.  39,  it  will  be 
noted  that  at  c,  while  the  pencil  has  grown  smaller  in  both  directions,  the  ver- 
tical diameter  cc  is  shorter  than  the  horizontal  diameter  c  V,  making  the  diffu- 
sion area  an  ellipse  with  the  long  axis  horizontal ;  at  d  this  difference  is  still 
more  pronounced ;  at  F2  the  vertical  diameter  is  zero,  and  the  rays  are  all 
collected,  not  into  a  point  as  by  a  spherical  lens,  but  into  a  line  called  the 
first  or  anterior  focal  line.  Back  of  F2  the  horizontal  diameter  continues 
to  shorten  with  the  continued  convergence  of  the  rays  from  side  to  side,  but 
the  vertical  diameter  begins  to  lengthen,  and  the  rays  that  were  in  the  lower 
quadrants  are  now  in  the  upper.  At  a  certain  point  h  (nearer  F2  than  F4, 
because  the  rays  intersect  at  a  greater  angle  vertically  than  horizontally)  the 
vertical  and  horizontal  diameters  become  equal,  and  the  diffusion  area  takes 
the  form  of  a  circle.  Beyond  this,  the  vertical  diameter  increasing  while 
the  horizontal  still  diminishes,  the  area  becomes  an  ellipse  with  its  long  axis 
vertical,  and  it  continues  to  become  longer  and  narrower  until  the  horizontal 
axis  becomes  zero,  and  we  have  at  F4  a  vertical  line,  the  second  or  posterior 
focal  line.  Here  the  rays  that  were  in  the  right  quadrants  go  over  to  the 
left,  and  those  that  were  in  the  left  quadrants  cross  to  the  right.  From  F* 
the  rays  diverge  laterally  as  well  as  vertically,  and  at  any  point  o  give  the 
•diffusion  area  of  an  ellipse  with  its  long  axis  vertical. 

It  is  seen,  then,  that  a  dioptric  system  the  seat  of  astigmatism  cannot 
bring  the  rays  from  a  single  point  together  again  at  another  point,  but  can 


THE   DIOPTRICS   OF  THE   EYE.  497 

only  collect  them  into  focal  lines.  These  focal  lines  are  perpendicular  to 
one  another,  and  are  situated  at  different  distances  from  the  cornea,  the 
distance  of  the  horizontal  line  being  determined  by  the  vertical  curvature 
and  the  position  of  the  vertical  line  by  the  horizontal  curvature.  The 
interval  F2  F*  between  these  focal  lines  is  called  the  focal  intei-val  of  Sturm. 
The  greater  this  interval  the  longer  the  focal  lines,  the  shorter  the  interval 
the  shorter  the  lines  ;  and  when  (the  curvatures  becoming  equal  in  the  two 
directions)  the  focal  lines  come  together,  they  merge  in  the  single  point  to 
which  the  non-astigmatic  dioptric  system  can  bring  rays  that  come  from 
a  single  point.  The  direction  of  greatest  curvature  and  the  direction  of 
least  curvature  are  called  the  principal  meridians  of  the  astigmatic  cornea 
or  astigmatic  eye,  or  the  meridians  of  astigmatism. 

The  appearance  of  lines  to  the  astigmatic  eye  depends  on  their  direction 
relative  to  the  meridians  of  astigmatism.  A  line  consists  of  a  succession  of 
points,  each  of  which  makes  its  own  impression  on  the  retina,  and  the  series 
of  these  impressions  constitutes  the  impression  of  the  line.  In  the  non- 
astigmatic  eye  the  light  from  one  point  of  the  line  is  focussed  to  one  point 
within  the  eye,  and  if  the  retina  be  properly  situated,  each  point  of  the  line 
makes  its  distinct  impression  on  the  retina  without  overlapping  neighboring 
points.  In  the  astigmatic  eye,  however,  the  best  that  can  be  done  with  the 
rays  from  a  single  point  is  to  bring  them  together  into  a  focal  line,  which 
must  overlap  the  impressions  of  points  adjoining  it  in  the  direction  of  the 
line.  This  is  illustrated  in  Fig.  41,  in  which  A  represents  the  impression 


FIQ.  41. 


§       I 


made  by  a  point  on  the  non-astigmatic  eye,  the  points  above  and  below  it 
not  confused  with  it ;  while  B,  the  impression  made  on  the  retina  of  the 
astigmatic  eye,  so  situated  as  to  receive  the  vertical  focal  lines,  overlaps  and 
confuses  the  impressions  of  the  points  above  and  below  it.  The  impression 
of  a  line  in  the  non-astigmatic  eye  is  represented  at  C  as  that  of  a  succession 
of  independent  points.  When  in  the  astigmatic  eye  the  line  happens  to  run 
in  the  same  direction  as  the  focal  line  falling  on  the  retina,  the  impression 
made  by  any  one  point  of  the  line  overlaps  the  impressions  of  adjoining 
points  of  the  line,  and  not  the  space  on  either  side  of  the  line,  so  that  the 
impression  is,  like  D,  that  of  a  distinct  line  with  shaded  ends.  When,  how- 
ever, the  line  looked  at  does  not  run  in  the  direction  of  the  focal  line  on  the 
retina,  this  focal  line  overlaps  not  the  line  itself,  but  the  space  above  and 
below  it,  giving  the  impression  of  a  broad,  indistinct  band  E.  By  the 
VOL.  I.— 32 


498  THE   DIOPTRICS   OF   THE   EYE. 

astigmatic  eye  lines  are  seen  distinctly  only  when  they  run  in  the  direction 
of  the  focal  line  falling  on  the  retina. 

Correction  of  Astigmatism.  Cylindrical  Lenses. — The  correction  of 
astigmatism  consists  in  compensating  the  difference  between  the  curvatures 
of  the  two  principal  meridians  by  an  appropriate  cylindrical  lens,  which 
will,  if  convex,  add  its  effect  to  that  of  the  less  convex  meridian  and  thus 
equal  the  refraction  of  the  more  convex  meridian,  or,  if  concave,  will,  by 
partly  neutralizing  the  effect  of  the  more  convex,  leave  a  remainder  just 
equal  to  the  effect  of  the  less  convex  meridian.  For  the  correction  of  a 
given  case  of  astigmatism  a  cylindrical  lens  must  be  selected  the  refractive 
power  of  which  equals  the  difference  between  the  refractive  powers  of  the 
principal  meridians  of  the  eye.  It  may  be  either  convex  or  concave.  If 
convex,  its  curve  must  be  placed  parallel  to  the  less  convex  meridian  of  the 
eye,  its  axis  parallel  to  the  focal  line  the  position  of  which  the  curve  of 
that  meridian  determines.  If  concave,  its  curve  must  be  in  the  direction 
of  the  more  convex  meridian  of  the  cornea,  its  axis  parallel  to  the  focal 
line  whose  position  is  determined  by  that  curve. 

Combinations  of  Cylindrical  Lenses. — It  should  be  borne  in  mind  that 
any  case  of  regular  astigmatism  can  be  corrected  by  a  single  cylindrical 
lens  (either  convex  or  concave)  of  the  proper  strength  and  properly  placed. 
If  two  cylindrical  lenses  are  employed  before  the  same  eye,  with  their  axes 
perpendicular  the  one  to  the  other,  they  act,  when  of  the  same  kind  (both 
convex  or  both  concave),  like  a  cylindrical  lens  with  a  refractive  power  equal 
to  the  difference  between  the  two,  combined  with  a  spherical  lens  with  a 
refractive  power  equal  to  that  of  the  weaker.  When  of  opposite  kinds 
(one  convex,  the  other  concave),  they  act  like  a  cylindrical  lens  equal  in 
refractive  power  to  the  sum  of  their  refracting  powers,  combined  with  a 
spherical  lens  of  the  opposite  kind,  equal  to  one  of  them  in  refractive 
power.  When  two  cylinders  are  combined  with  their  axes  oblique  one  to 
the  other,  they  produce  an  optical  effect  exactly  equivalent  to  that  of  a 
certain  sphero-cylindrical  lens.  This  has  been  mathematically  demon- 
strated, independently  and  by  slightly  different  methods,  by  Bonders, 
Hoorweg,  Oliver  and  Hay,  Jackson,  Prentice,  and  Weiland.  The  author's 
demonstration  with  a  practical  method  of  ascertaining  the  sphero-cylin- 
drical equivalent  in  any  given  case  is  contained  in  the  Transactions  of  the 
American  Ophthalmological  Society,  1886,  p.  268.  A  very  ingenious  in- 
strument for  the  purpose  of  finding  this  and  other  equivalents  is  described 
by  Weiland  in  the  Archives  of  Ophthalmology,  1893,  p.  433. 

Effect  of  Astigmatism  and  Cylindrical  Lenses  on  Nodal  Points  and  Images. 
— Since  the  positions  of  the  principal  points  and  nodal  points  of  the  eye  de- 
pend on  the  curvature  of  its  dioptric  surfaces,  the  greater  curvature  of  these 
surfaces  in  one  meridian  than  in  another  causes  such  points  to  lie  at  different 
depths  in  the  eye  for  the  different  meridians.  The  greatest  interval  between 
the  corresponding  points  of  different  meridians  is  produced  by  differences 
of  curvature  in  the  cornea,  the  common  seat  of  this  anomaly  of  curvature. 


THE   DIOPTRICS  OP   THE   EYE.  499 

In  general,  the  meridian  having  the  shorter  radius  of  curvature — the  greater 
refractive  power — has  its  principal  and  nodal  points  near  the  cornea  and  its 
posterior  nodal  point  farthest  from  the  retina.  On  this  account  images  in 
this  meridian  are  larger,  as  compared  with  the  size  and  distance  of  the  object, 
than  images  formed  in  the  other  principal  meridian.  Thus,  a  square  held 
with  its  sides  parallel  to  the  principal  meridians  of  such  an  eye  gives  a 
retinal  image  rectangular  in  form,  with  the  sides  parallel  to  the  meridian  of 
greatest  curvature  slightly  longer  than  the  other  sides.  A  circle  gives  an 
ellipse  with  its  long  axis  parallel  to  the  meridian  of  greatest  curvature. 
Such  a  distortion  of  the  retinal  images  causes  the  images  of  lines  not  parallel 
to  principal  meridians,  as  the  diagonals  of  the  square  referred  to  above,  to 
depart  somewhat  from  parallelism  to  the  lines  themselves.  This  twisting 
of  the  image  may  be  in  different  directions  in  the  two  eyes,  leading  to  a  lack 
of  perfect  correspondence  of  the  retinal  images  in  the  two  eyes.  It  must 
be  borne  in  mind,  however,  that  this  twisting  is  comparatively  slight,  even 
for  lines  farthest  removed  from  the  directions  of  the  principal  meridians, 
and  that  such  lines  can  never  by  the  unconnected  astigmatic  eye  be  seen 
with  perfect  clearness. 

The  correction  of  astigmatism  by  a  cylindrical  lens  placed  in  front  of 
the  cornea  necessarily  affects  the  position  of  the  nodal  point  in  the  meridian 
parallel  to  the  curve  (perpendicular  to  the  axis)  of  the  lens.  A  convex  lens 
brings  the  nodal  point  forward,  a  concave  lens  carries  it  back  from  the 
cornea.  This  displacement  varies  with  the  strength  of  the  correcting  lens 
and  its  distance  from  the  cornea,  but  it  is  always  greater  than  the  displace- 
ment caused  by  the  astigmatism  corrected.  And  being  suddenly  acquired 
instead  of  congenital  or  slowly  developed,  and  involving  lines  now  clearly 
focussed  on  the  retina,  the  distortion  it  causes  in  the  retinal  images  is  gen- 
erally noticeable  and  annoying,  until  the  visual  function  has  adapted  itself 
to  its  new  conditions. 

Aberration. — The  dioptric  system  of  the  eye  is  not  exempt  from  the 
optical  defect  of  spherical  lenses,  spherical  aberration,  although  this  is  partly 
corrected  by  the  diminished  index  of  refraction  towards  the  periphery  of 
the  lens  and  the  form  of  the  corneal  surface.  The  surface  of  the  cornea 
departs  most  notably  from  a  spherical  form,  in  that  the  periphery  is  con- 
siderably less  curved  than  the  centre.  The  crystalline  lens,  on  the  other 
hand,  is  much  more  convex  near  its  periphery  than  at  its  centre.  In  the 
large  majority  of  eyes  the  increased  curvature  of  the  lens  predominates  over 
the  flattening  of  the  cornea  at  the  periphery  of  the  pupil,  so  that  in  this 
portion  the  refraction  of  the  eye  is  more  myopic  or  less  hyperopic  than  at 
the  centre.  In  some  eyes,  however,  the  opposite  is  the  case.  (See  article 
on  Skiascopy.) 

Chromatic  aberration,  or  the  separation  of  colors  by  reason  of  the  differ- 
ent refrangibility  of  light  of  different  colors,  although  fairly  corrected  for 
lens  surfaces  of  such  strong  curvature,  occurs  to  some  extent  in  the  eye.  It 
may  be  demonstrated  by  looking  in  a  darkened  room  at  a  point  of  light, 


500  THE    DIOPTRICS   OF   THE   EYE. 

through  glass  that  intercepts  the  middle  portion  of  the  spectrum,  permitting 
only  the  extreme  rays — the  red  and  blue  or  violet — to  pass.  With  the  focus 
slightly  in  front  of  the  retina  (myopia),  there  will  be  seen  a  red  centre  sur- 
rounded by  a  blue  diffusion  circle,  and  with  the  focus  slightly  back  of  the 
retina  (hyperopia),  a  blue  centre  with  a  red  diffusion  circle  about  it.  With 
astigmatism,  one  may  in  this  experiment  get  the  anterior  focal  line  for  red 
and  the  posterior  focal  line  for  blue  both  on  the  retina  at  the  same  time, 
forming  a  cross,  with  one  line  (the  horizontal  in  astigmatism  with  the  rule) 
red  and  the  other  blue. 

Iwegular  Astigmatism. — Optical  theory  can  deal  only  with  the  dioptrics 
of  regular  surfaces,  regular  astigmatism  and  aberration  being  anomalies 
dependent  on  such  surfaces.  But  in  the  eye  the  surfaces  developed  by  the 
processes  of  nutrition  always  lack  something  of  perfect  regularity,  so  that 
their  action  varies  from  perfect  focussing  in  ways  peculiar  to  the  individual 
eye!  This  may  be  by  unequal  curves  in  different  directions,  as  when  the 
greatest  and  least  curvatures  are  not  perpendicular  to  each  other,  and  the 
error,  therefore,  cannot  be  entirely  corrected  by  cylindrical  lenses ;  or  it  may 
be  from  unequal  curves  in  different  parts  of  the  surface,  as  in  the  cornea 
after  interstitial  keratitis  or  phlyctenular  disease;  or  it  may  be  due  to 
irregularities  in  the  index  of  refraction,  as  in  the  crystalline  lens  before  it 
becomes  opaque.  All  errors  of  refraction  from  such  causes  are  grouped 
under  the  head  of  irregular  astigmatism.  They  cannot  be  corrected,  except 
that  the  diffusion  of  light  caused  by  them  on  the  retina  may  be  reduced  to 
the  minimum  by  narrowing  the  pupil  or  placing  an  opaque  disk  with  a 
narrow  opening  before  the  eye, — stenopaic  spectacles. 

The  Visual  Zone. — In  the  vicinity  of  the  visual  axis  regular  astig- 
matism is,  as  a  rule,  the  least ;  aberration  is  in  most  eyes  well  corrected,  and 
irregular  astigmatism  very  slight,  so  that  the  light  entering  through  this 
part  of  the  dioptric  surfaces  and  media  may  be  very  perfectly  focussed  on 
the  retina.  This  part  I  have  called  the  visual  zone.  The  essential  thing 
about  it  is  to  remember  that  it  does  not  include  the  whole  of  the  dioptric 
surfaces  of  the  eye,  but  is  always  surrounded  by  a  region  not  capable  of 
accurately  focussing  light,  which  may  be  called  the  extra-visual  zone.  The 
extent  of  the  visual  zone  varies  greatly  in  different  eyes.  In  some  its 
boundary  is  exposed  only  by  considerable  dilatation  of  the  pupil,  in  others 
the  extra-visual  zone  encroaches  on  the  pupil  even  when  contracted  by  strong 
light  and  with  accommodation.  The  visual  zone  alone  affords  distinct 
vision.  The  extra-visual  zone  aids  by  the  admission  of  more  light  when, 
on  account  of  the  feeble  illumination,  vision  would  be  at  best  imperfect. 
Its  imperfections  are  illustrated  by  the  comparatively  poor  vision  obtained 
by  optical  iridectomy  or  through  the  margin  of  a  dislocated  crystalline  lens. 

Centring  of  the  Dioptric  Surfaces. — It  was  assumed  for  the  sche- 
matic eye  that  the  dioptric  surfaces  constituted  a  centred  system  ;  but  in 
reality  they  do  not,  and  if  we  take  as  the  optic  axis  the  straight  line  passing 
through  the  centre  of  the  corneal  surface, — the  centre  of  rotation  of  the  eye- 


THE   DIOPTRICS   OF   THE   EYE.  501 

ball  and  its  posterior  pole, — it  will  generally  be  found  that  the  centres  of 
curvature  for  the  different  dioptric  surfaces  do  not  lie  on  this  line,  but  depart 
from  it  in  different  directions  and  to  various  distances.  It  will  also  be  fouud 
that  this  axis  does  not  cut  the  retina  at  the  fovea  centralis.  It  is  therefore 
necessary  to  recognize  certain  other  lines  and  their  relations.  These  are 
shown  in  Fig.  42,  with  their  divergence  exaggerated.  A  A  is  the  optic  axis, 


FIG.  42. 


on  which  N  is  the  nodal  point  and  It  the  centre  of  rotation,  situated  six 
millimetres  behind  the  nodal  point  and  nine  millimetres  in  front  of  the 
retina.  0  is  the  object  looked  at,  M  the  centre  of  the  macula,  and  OM  the 
visual  line.  EE  is  the  long  axis  of  the  ellipsoidal  anterior  surface  of  the 
cornea,  and  OR  is  the  line  of  fixation  from  the  centre  of  rotation  to  the 
object.  The  angle  between  the  axis  of  the  corueal  ellipse  EE  and  the  visual 
line  OM  is  called  the  angle  alpha,  a  ;  and  the  angle  between  the  optic  axis 
AA  and  the  line  of  fixation  OM  is  called  the  angle  gamma,  ?. 

The  angle  gamma  averages  5°,  but  varies  in  different  eyes,  being  usually 
greater  in  hyperopia — as  much  as  10° — and  less  in  myopia,  or  even  negative, 
the  optic  axis  piercing  the  retina  to  the  temporal  side  of  the  macula.  The 
angle  alpha  is  usually  slight,  but  may  be  larger  than  the  angle  gamma. 
These  angles  are  of  practical  importance  in  connection  with  anomalies  of 
the  motor  apparatus. 

ACCOMMODATION. 

For  any  given  dioptric  system,  rays,  to  be  focussed  at  a  certain  point, 
must  have  a  certain  divergence  or  convergence ;  they  must  be  diverging 
from  or  converging  towards  a  certain  other  point,  its  conjugate.  But  to  the 
eye  come  rays  varying  from  extreme  convergence  to  parallelism.  It  needs 
to  focus  on  a  fixed  point — the  centre  of  the  retina — rays  coming  from  objects 
at  various  distances.  This  it  can  do  only  by  changes  in  its  dioptric  system. 
We  have  so  far  discussed  this  system  as  a  fixed  system.  Its  focussing  power 
as  a  fixed  system,  compared  with  the  position  of  the  retina,  is  called  its 
refraction.  The  power  to  vary  its  dioptric  action  to  adapt  it  to  focus  on  the 
retina  rays  coming  from  different  distances  is  called  the  power  of  accom- 
modation. 

The  accommodation  of  the  eye  is  accomplished  by  change  of  shape  in 
the  crystalline  lens  under  the  influence  of  muscular  action  on  the  part  of 
the  ciliary  muscle.  The  crystalline  lens  in  early  life  is  extremely  flexible 
and  elastic,  and,  stripped  of  its  capsule,  almost  globular  in  shape.  In  the 


502 


THE    DIOPTRICS   OF    THE    EYE. 


FlQ.  43. 


living  eye,  under  the  elastic  tension  of  its  capsule  it  is  considerably  flat- 
tened. This  tension  of  the  capsule  is  greatest  and  the  lens  flattest  when  the 
ciliary  muscle  is  relaxed.  Contraction  of  the  ciliary  muscle  takes  the  ten- 
sion of  the  capsule  off  the  lens,  which  then,  in  proportion  to  its  elasticity, 
becomes  more  convex.  These  changes  are  illustrated  by  Fig.  43,  the 

continuous  lines  showing  the  outlines  of  the  cor- 
nea, iris,  ciliary  region,  and  lens  when  the  eye  is 
at  rest,  and  the  dotted  lines  the  altered  outlines 
of  the  ciliary  muscle,  lens,  and  iris  during  accom- 
modation. It  will  be  noted  that  on  contraction 
of  the  ciliary  muscle  the  posterior  surface  of  the 
lens  becomes  slightly  more  convex,  the  anterior 
surface  becomes  much  more  convex,  and  the  ante- 
rior surface  of  the  lens  is  moved  forward,  push- 
ing the  iris  with  it.  Optically,  the  effect  is 
chiefly  that  of  the  addition  of  a  convex  meniscus 
to  the  anterior  surface  of  the  crystalline,  and  its 
amount  is  measured  by  the  strength  of  a  convex 
lens  which  placed  at  the  surface  of  the  cornea 
would  produce  the  same  optical  effect. 

With  differing  degrees  of  contraction  of  the 
ciliary  muscle,  different  degrees  of  increased  con- 
vexity and  increased  refractive  power  are  obtained. 
When  the  muscle  is  exerted  to  the  utmost,  the 

greatest  increase  of  convexity  possible  to  that  eye  is  produced,  and  the  change 
of  refractive  power  it  causes  is  the  total  accommodation.  With  extreme  exer- 
tion of  the  ciliary  muscle  in  children,  the  anterior  surface  of  the  crystalline 
lens  becomes  about  as  convex  as  the  posterior  surface,  each  having  a  radius 
of  curvature  of  five  millimetres  or  a  little  over  ;  the  lens  becomes  more  than 
four  millimetres  thick,  and  its  anterior  pole  but  little  over  three  millimetres 
behind  the  anterior  surface  of  the  cornea. 

With  variations  of  accommodation  the  eye  is  adapted  to  rays  of  different 
degrees  of  divergence,  coming  from  various  distances.  When  the  total 
accommodation  is  brought  into  play,  it  is  adapted  for  the  most  divergent 
rays  that  can  be  brought  to  a  focus  on  the  retina,  rays  coming  from  the 
nearest  point  to  the  eye  from  which  rays  can  be  focussed  on  the  retina. 
This  point  is  called  the  near  point  of  distinct  vision,  or  punctum  proximum. 
When  the  ciliary  muscle  is  relaxed,  and  the  lens  as  flat  as  it  can  become, 
the  eye  is  adapted  for  the  least  divergent  rays  that  it  can  focus  on  the  retina, 
and  this  point  is  called  the  far  point  of  distinct  vision,  or  punctum  remotum. 
The  Region  of  Accommodation. — The  far  point  is  a  focus  conjugate  to 
the  retina  when  the  accommodation  is  in  abeyance,  the  one  referred  to  in 
discussing  hyperopia  and  myopia.  The  near  point  is  the  focus  conjugate  to 
the  retina  with  the  total  accommodation  exerted.  By  appropriate  effort  of 
the  ciliary  muscle,  any  intermediate  point  may  be  made  conjugate  to  the 


THE   DIOPTRICS   OF   THE   EYE.  503 

retina  and  rays  from  it  focussed  on  the  retina.  This  intermediate  space  in 
which  distinct  vision  is  possible  is  the  region  of  accommodation.  The  loca- 
tion of  this  region  varies  with  the  refraction  of  the  eye,  as  is  illustrated  in 
Fig.  44,  in  which  the  heavy  line  indicates  the  region  of  accommodation, 


FIG.  44. 

R 


IX' 


R  being  the  far  point  and  P  the  near  point.  The  upper  diagram  repre- 
sents emmetropia,  with  the  region  starting  at  an  infinite  distance,  where  the 
far  point  of  the  emmetropic  eye  is  situated,  and  extending  to  the  near  point, 
situated  at  the  focal  distance  of  a  lens  equal  to  the  total  accommodation  in 
front  of  the  eye.  The  middle  diagram  represents  hyperopia,  with  the  far 
point  behind  the  eye,  towards  which  the  rays  must  converge  to  be  focussed 
on  the  retina  without  accommodation.  Thence  the  region  of  accommodation 
stretches  back  to  an  infinite  distance,  a  sort  of  negative  or  virtual  region  of 
accommodation  quite  useless  to  its  possessor,  except  that  it  represents  the 
correction  of  the  hyperopia  that  must  be  accomplished  before  distinct  vision 
can  begin,  even  at  a  distance.  Beginning  again  in  front,  the  region  extends 
from  an  infinite  distance  to  the  near  point,  which  is  situated  in  front  of  the 
eye  at  the  focal  distance  of  a  lens  equal  to  the  total  accommodation  minus 
the  hyperopia.  The  lower  diagram  represents  myopia,  in  which  the  region 
of  accommodation  starts  at  the  far  point  in  front  of  the  eye,  at  the  focal  dis- 
tance of  the  myopia,  and  extends  to  the  near  point  at  the  focal  distance  of  a 
lens  equal  to  the  myopia  plus  the  accommodation.  It  should  be  noticed  that 
the  emmetropic  eye  has  the  most  extended  region  of  useful  accommodation. 
Accommodation  in  Astigmatism. — Since  in  astigmatism  the  focal  lines 
are  separated  by  a  certain  interval,  but  one  of  them  can  fall  on  the  retina  at 
one  time.  The  state  of  the  eye  as  to  accommodation  may,  however,  deter- 
mine which  shall  fall  on  the  retina,  or  may  by  rapid  variation  cause  first 
one  and  then  the  other  to  fall  on  the  retina  in  quick  succession.  So  that  in 
both  directions  lines  may  be  seen  distinctly  at  so  brief  an  interval  as  to 
allow  their  combination  into  a  distinct  mental  image.  In  this  way  astig- 
matism of  low  degree  may  not  prevent  distinct  vision.  It  has  been  sup- 
posed that  sometimes  an  unequal  contraction  of  different  parts  of  the  ciliary 
muscle  caused  unequal  increase  of  convexity  in  different  meridians  of  the 
lens,  and  thus  effected  a  true  correction  of  astigmatism.  But  this  is  not 
proved.  The  cases  supposed  to  illustrate  it  have  not  been  observed  with 


504 


THE    DIOPTRICS    OF   THE    EYE. 


sufficient  care  to  exclude  error  from  the  influence  of  conditions  that  might 
be  present  in  the  extra-visual  zone. 

Loss  of  Accommodation  with  Age :  Presbyopia. — The  change  effected  in 
the  shape  of  the  crystalline  lens  by  the  contraction  of  the  ciliary  muscle 
depends  both  on  the  power  of  the  muscle  and  on  the  elasticity  of  the  lens. 
From  infancy  there  is  a  progressive  increase  in  the  rigidity  of  the  lens,  so 
that  the  same  exertion  of  power  on  the  part  of  the  muscle  produces  pro- 
gressively less  change  in  its  shape.  To  this  is  added  late  in  life  weakness 
or  even  atrophy  of  the  muscle.  Hence  there  is  a  progressive  decline  in 
the  power  of  accommodation  from  the  earliest  age  at  which  it  has  been 
carefully  tested  in  any  large  number  of  persons  until  it  is  entirely  lost. 
About  the  average  rate  of  such  decline  is  shown  in  the  following  table,  in 
which  the  first  column  indicates  the  age,  the  second  the  dioptres  of  accom- 
modative power,  the  third  the  near  point  for  an  emmetropic  eye  in  millimetres, 
and  the  fourth  the  distance  of  the  same  point  in  inches. 


Age. 
10  ... 

Dioptres. 
...    14 

Millimetres. 
71 

Inches. 
2.81 

Age. 
45  .    . 

Dioptres. 
.    .    .        3.5 

Millimetres. 
286 

Inches. 
11.25 

15 

.    12 

83 

3  28 

50 

.    .    .       2.5 

400 

15.75 

20 

.    10 

100 

3  94 

55  .    . 

1.5 

667 

26.25 

25  ... 

...      8.5 

118 

4  63 

60  .    . 

.    .    .         .75 

1333 

52.49 

30      .    . 

7 

143 

5.63 

65  .. 

.    .    .         .25 

4000 

157.48 

35  ... 

.    .      5.5 

182 

7.16 

70  .    . 

.    .    .       0. 

oo 

CO 

40. 

4.5 

222 

8.75 

The  above  table  gives  the  total  accommodation  and  the  near  point 
obtained  by  the  maximum  exertion  of  which  the  ciliary  muscle  is  capable. 
Such  exertion  can  be  put  forth  for  only  a  short  time,  and  cannot  be  sustained 
for  any  continuous  work.  For  continuous  work  only  a  fraction  of  the  total 
accommodation  is  available.  What  this  fraction  is  varies  with  different 
persons  and  at  different  ages.  On  the  average,  about  one-half  the  total 
accommodation  can  be  used  at  the  age  of  thirty  and  about  two-thirds  of  it 
after  forty-five.  When  this  fraction  is  no  longer  sufficient  to  give  clear 
vision  for  the  work  required  of  the  eyes,  the  failure  of  accommodation  by 
age  has  progressed  so  far  as  to  be  a  source  of  annoyance,  pain,  and  danger 
to  the  eye,  and  is  called  presbyopia. 

The  advent  of  presbyopia  will  be  earlier  where  a  certain  fixed  portion 
of  the  accommodation  is  consumed  in  correcting  hyperopia ;  but  it  will  be 
later  if  myopia  renders  less  accommodation  necessary  for  near  work,  and 
when  the  myopia  is  sufficient  to  permit  the  doing  of  near  work  without  any 
use  of  accommodation,  presbyopia  will  not  occur. 

The  effect  of  accommodation  on  the  cardinal  points  of  the  eye  is  com- 
paratively slight,  since  the  change  is  mainly  in  the  anterior  surface  of  the 
lens.  The  principal  points  retreat  from  the  cornea,  but  the  nodal  points 
slightly  approach  it.  This  places  the  posterior  nodal  point  farther  in  front 
of  the  retina,  and  causes  some  enlargement  of  retinal  images.  Such  enlarge- 
ment is,  however,  much  less  than  that  produced  by  a  convex  lens  placed 
in  front  of  the  cornea,  which  would  equally  alter  the  refractive  power. 


THE  PERCEPTION  OF  LIGHT. 

BY  J.  McKEEN  CATTELL,  M.A.,  PH.D., 

Professor  of  Experimental  Psychology,  Columbia  College,  New  York. 


EXACT  science  consists  of  measurements  and  the  relations  of  quantities. 
Physiology  and  psychology  are  far  from  this  goal,  but  every  movement 
should  be  in  its  direction.  We  may,  therefore,  with  advantage  follow  in 
the  path  of  physical  science,  and  apply  to  the  perception  of  light  the  three 
units  required  for  measuring  space,  time,  and  energy.  The  field  of  vision, 
the  acuity  of  vision,  binocular  vision,  and  other  subjects  related  to  the 
perception  of  space  are  reviewed  elsewhere  in  this  work.  We  have,  con- 
sequently, in  this  place  only  to  consider  the  measurement  of  intensity  and 
of  time.1 

PART  I.— INTENSITY. 

I. — THE   THRESHOLD. 

Lights,  sounds,  and  other  physical  stimuli  may  be  so  faint  that  they 
cannot  be  perceived.  The  intensity  of  the  stimulus  which  just  calls  forth 
a  sensation  has  been  aptly  called  the  threshold.  The  fact  of  the  threshold 
may  be  partly  due  to  inertia  of  the  sense  organs  and  dispersion  in  the 
paths  of  conduction,  which  might  prevent  the  motion  from  arriving  at 
those  parts  of  the  brain  immediately  concerned  with  consciousness.  But 
the  stimulation  may,  indeed,  be  carried  on  to  the  brain  and  be  given  in 
consciousness,  but  so  faintly  that  under  ordinary  circumstances  it  escapes 
attention.  In  such  a  case  we  have  to  do  with  subconscious  mental  processes, 
— changes  in  consciousness  which  are  not  noticed,  but  which  yet  affect  the 
course  of  mental  life. 

1  The  most  important  general  works  on  the  perception  of  light  are  the  following : 

Purkinje,  J.  E.,  Beobachtungen  and  Versuche  zur  Physiologic  der  Sinne,  Part  i.r 
Prag,  1823;  Partii.,  Berlin,  1825. 

Helmholtz,  H.  v.,  Handbuch  der  physiologischen  Optik,  Leipzig,  1867.  A  second 
revised  edition  is  now  (1895)  in  course  of  publication. 

Aubert,  Hermann,  Physiologic  der  Netzhaut,  Breslau,  1865. 

Aubert,  Grundziige  der  physiologischen  Optik,  Leipzig,  1876.  Reprinted  from  the 
Handbuch  der  gesammten  Augenheilkunde,  edited  by  A.  Graefe  and  Th.  Saemix  h 

Fick,  A.,  Die  Lehre  von  der  Lichtempfindung,  in  vol.  iii.  of  Hermann's  Handbuch 
der  Physiologic,  Leipzig,  1879. 

Rood,  Ogden  N.,  Modern  Chromatics,  New  York,  1879. 

Wundt,  W.,  Grundziige  der  physiologischen  Psychologic,  4th  ed.,  Leipzig,  1893. 

505 


506  THE   PERCEPTION    OF    LIGHT. 

In  the  case  of  vision  we  cannot  obtain  complete  absence  of  physical 
light,  nor  can  we  avoid  the  idio-retinal  light ;  the  threshold  is,  therefore, 
the  least  light  which  can  be  distinguished  in  the  field  of  vision  when  it  is 
made  as  dark  as  possible.  Aubert,1  by  observing  the  temperature  at  which 
metals  begin  to  glow,  estimated  the  just  visible  light  at  -§fa  of  that  of 
the  full  moon  reflected  from  white  paper.  Konig  and  Brodhun 2  have  made 
more  exact  measurements.  As  unit  of  intensity  they  used  the  light  of 
melting  platinum  reflected  from  a  surface  covered  with  magnesium  oxide. 
The  area  of  the  platinum  was  0.1  centimetre  square,  and  it  was  at  a  dis- 
tance of  one  metre  from  the  reflecting  surface,  which  was  seen  through  the 
aperture  of  a  diaphragm  one  millimetre  square.  The  threshold  for  white 
light  was  about  0.0007  of  this  unit,  and  for  colored  lights  varied  between 
0.11  (red,  A  =  670  MM)  and  0.00012  (violet,  /I  =  430  MM).  The  threshold  for 
color  has  also  been  determined  by  Abney 3  and  by  Ferry.4  Langley 5  has 
recently  made  a  further  advance  in  our  knowledge  of  the  subject  by  de- 
termining the  actual  energy  (as  heat)  of  the  light  which  just  excites  a 
sensation.  He  determined  the  amount  of  light  required  to  read  the  figures 
of  a  table  of  logarithms  with  the  several  colors.  The  energy  (expressed  as 
heat)  was  5  o  o  0*0  0  0  0  calorie,  and  only  a  small  part  of  this  energy  would 
affect  the  retina.  Langley  calculates  that  in  the  case  of  one  observer,  and 
for  green  light,  only  the  reciprocal  of  310,000,000  ergs  was  required  to 
call  forth  a  sensation  of  light.  Expressed  in  terms  of  horse-power,  this 
would  be  0.00000000000000000075  horse-power.  The  relative  sensitive- 
ness of  the  eyes  of  four  observers  for  a  constant  amount  of  energy  of 
varying  wave-length  is  shown  in  the  accompanying  curves  (Fig.  1)  which 
the  writer  has  drawn  from  Langley's  table.  Along  the  base-line  are  the 
colors  together  with  the  wave-length  in  micro-millimetres  and  Fraunhofer's 
lines.  The  relative  sensitiveness  for  the  different  colors  and  the  four 
observers  is  shown  by  the  height  of  the  curve  above  the  base-line. 

These  curves  show  that  the  sensitiveness  of  one  eye  may  be  ten  times 
as  great  as  that  of  another,  although  the  color-vision  of  both  would  be 
regarded  as  normal.  Such  a  difference  in  the  sharpness  of  hearing  would 
be  detected  by  the  ordinary  tests  of  the  clinic,  whereas  it  would  escape  the 
methods  of  the  ophthalmologist.  For  three  of  the  observers  a  given 
amount  of  energy  produces  the  greatest  effect  in  blue-green,  and  for  one 
(Langley)  in  yellow-green.  Langley  notes  that  the  younger  observers 
were  relatively  more  sensitive  to  the  more  refrangible  rays.  And  the 


1  Loc.  cit. 

2  Experimentelle  Untersuchungen  iiber  die  psychophysische  Fundamentalformel  in 
Bezug  auf  den  Gesichtssinn,  Sitzungsber.  d.  Akad.  d.  Wiss.  zu  Berlin,  1889,  ii.  641-644, 
June  27,  1889. 

3  Colour  Measurement  and  Mixture,  London  and  New  York,  1891. 

*  Persistence  of  Vision,  Am.  Jour,  of  Sci.,  Ser.  3,  xliv.  192-207,  September,  1892. 
5  Energy  and  Vision,  Am.  Jour,  of  Sci.,  Ser.  3,  xxxvi.  369-379,  1888 ;   also  Mem. 
Am.   Nat.  Acad.  Sci.,  vol.  v.,  1888. 


THE    PERCEPTION   OF   LIGHT. 


.507 


same  observation  has  been  made  by  Ferry,1  independently  and  in  experi- 
ments of  a  different  sort,  If  this  be  the  case,  it  will  have  an  important 
bearing  on  theories  of  color-vision  and  development  of  the  eye.  The 
range  of  color-vision  certainly  requires  investigation.  It  seems  not  un- 
likely that  with  increasing  age  the  eye,  like  the  ear,  may  lose  the  power  of 
perceiving  the  quicker  vibrations. 

In  these  cases  only  light  was  seen,  while  the  color  could  not  be  recog- 
nized.    It  is  a  common  experience  (as  in  twilight)  that  we  can  distinguish 

FIG.  1. 


lights  and  shapes  when  we  cannot  distinguish  colors.  Charpentier 2  found 
the  threshold  for  red  to  be  about  twice  as  great  as  that  for  light  without  re- 
gard to  color,  and  eighty  times  as  great  for  violet  as  for  red.  The  fact  that 
all  colors  appear  gray  when  the  intensity  is  small  and  white  when  it  is 
very  great  does  not  seem  to  be  accounted  for  in  a  satisfactory  manner  by 
the  Young-Helmholtz  theory  of  color- vision.  There  seems  to  be  no  reason 
why  a  color  which  works  chiefly  on  one  sort  of  fibres  or  cells  when  of 
moderate  intensity  should  affect  the  three  sorts  equally  when  faint  or  intense. 
This  is  one  of  a  number  of  facts  which  make  it  necessary  to  assume  that 
the  visual  mechanism  is  sensitive  to  gray  and  white  independently  of  color. 
It  seems  natural  that  in  the  course  of  evolution  the  organism  should  have 
become  sensitive  to  changes  of  light  and  darkness  before  the  visual  mech- 
anism became  fitted  to  perceive  differences  in  color. 

The  threshold  becomes  greater  as  the  area  of  stimulation  is  made 
smaller  and  as  the  time  of  stimulation  is  made  less.  The  threshold  is 
smaller  for  moving  objects.  Thus,  if  the  eyelids  be  closed  and  the  hand 
held  between  them  and  the  sky,  the  hand  can  be  seen  when  it  is  moved, 

1  Loc.  cit. 

2  La  perception  des  couleurs  et  la  perception  des  formes,  Compt.  rend,  de  1'Acad. 
des  Sciences,  xcvi.  858-860,  1880;  La  perception  des  couleurs  et  la  perception  des  dif- 
ferences de  clarte,  Compt.  rend.,  xcvi.  1079-1081,  1883;  Nouvelles  recherches  analytiques 
sur  les  fonctions  visuelles,  Arch.  d'Ophthal.,  iv.  291-323,  1884. 


508  THE   PERCEPTION   OF   LIGHT. 

but  not  otherwise.  So,  also,  we  can  see  a  moving  object  farther  away  from 
the  fovea  centralis  than  an  object  at  rest.  It  is  a  general  psychological  law 
that  we  perceive  changes  rather  than  constant  conditions.  Nor  are  changes 
noticed  which  are  very  gradual.  If  the  temperature  of  a  plate  on  which  a 
frog  is  sitting  be  raised  very  gradually,  it  will  not  move  away,  but  will  be 
burned  up.  Modern  psychology  has  to  a  considerable  extent  confirmed  the 
principle  of  Hobbes,  "Semper  idem  sentire  ac,  non  sentire  ad  idem  revertunt." 

The  light  which  can  just  be  seen  varies  with  the  sensitiveness  of 
the  retina  and  the  size  of  the  pupil.  We  all  know  that  on  first  going 
into  the  dark  we  may  be  able  to  see  nothing,  whereas  in  a  few  minutes 
objects  may  become  visible.  Aubert1  has  studied  the  relation  between 
the  threshold  and  the  time  of  adaptation.  On  going  into  a  dark  room 
the  sensitiveness  increases  at  first  rapidly  and  then  more  slowly  ;  after 
ten  minutes  it  is  about  twenty-five  times  as  great,  and  after  two  hours 
about  thirty-five  times  as  great,  as  at  first. 

The  threshold  deserves  special  attention  in  this  place,  owing  to  its  pos- 
sible importance  in  clinical  ophthalmology.  The  threshold  of  hearing  is 
the  first  determination  made  by  the  aurist,  and  the  corresponding  test  may 
prove  equally  useful  in  the  diagnosis  of  diseases  of  the  eye.  For  example, 
variations  in  the  perception  of  color  at  the  threshold  are  found  which  do 
not  amount  to  color-blindness,  and  which  would  not  be  detected  by  the 
ordinary  tests  for  color-blindness.  Yet  such  differences  may  indicate  im- 
portant variations  in  the  condition  of  the  eye  and  of  the  nervous  system. 

II. — THE   PERCEPTION   OF   SMALL    DIFFERENCES. 

As  a  light  may  be  so  faint  that  it  cannot  be  noticed,  so  the  difference 
between  two  lights  may  be  so  small  that  no  difference  can  be  distinguished. 
Thus,  the  stars  cannot  be  seen  in  the  daytime.  The  light  of  the  stars  is- 
not  less  than  at  night,  but  the  difference  between  their  light  and  the  light 
of  the  sky  is  so  small  that  no  difference  can  be  perceived.  The  least  dif- 
ference which  can  be  noticed  is  an  important  physiological  constant.  It 
can  be  determined  with  greater  ease  than  the  just  noticeable  light,  and, 
being  a  delicate  test  of  the  condition  of  the  eye,  will  prove  useful  in  diag- 
nosis, serving  to  indicate  small  changes  in  progression  or  recovery. 

Bouguer2  was  the  first  to  measure  the  just  noticeable  difference.  He 
found  that  a  shadow  could  be  distinguished  from  its  background  when  the 
difference  in  the  two  lights  was  about  one  sixty-fourth.  This  method  is 
illustrated  in  Fig.  2. 

The  white  screen  SSf  is  illumined  by  the  candle  at  L.  A  second  candle 
of  the  same  sort,  I,  is  placed  at  a  greater  distance  from  the  screen.  It  will 


1  Loc.  cit. 

1  Essai  d'optique,  sur  la  gradation  de  la  lumiere,  Paris,  1729  ;  TraitS  d'optique  sur  la 
gradation  de  la  lumiere,  Paris,  1760 ;  Opus  conversurn  in  Latinum  a  Joachimo  Richten- 
burg,  Vindobonae,  1762. 


THE    PERCEPTION   OF    LIGHT. 


509 


cast  a  shadow  of  the  bar  B  at  Sh,  but  when  the  candle  is  removed  to  a 
sufficient  distance  the  shadow  cannot  be  distinguished  from  the  background. 
When  L  was  (say)  one  foot  distant  and  I  eight  feet  distant,  the  shadow 
could  not  be  seen,  according  to  Bouguer,  and,  the  illumination  varying 
inversely  as  the  square  of  the  distance,  the  just  noticeable  difference  was 

FIG.  2. 


about  one  sixty-fourth.  With  a  brighter  light  the  relation  was  not  altered. 
The  same  or  analogous  methods  have  been  used  by  Lambert,1  Arago,2 
Rumford,3  Volkmann,4  Aubert,5  and  Cammerer.6  Of  these  researches  that 
by  Aubert  seems  to  be  the  most  valuable.  He  altered  the  intensity  of 
illuminatiou,  and  found  that  with  the  brightest  light,  which  was  equal  to 

FIG.  3. 


seven  hundred  and  ten  times  the  light  of  a  candle  at  a  distance  of  two 
metres  from  the  illumined  surface,  the  just  noticeable  difference  was  y^j, 
whereas  with  the  faintest  light,  which  was  ^T^T  °^  tne  candle,  the  just 
noticeable  difference  was  one-third. 

1  Photometria,  sive  de  mensura  et  gradibus  luminis,  colorum  et  umbrae,  Augustas 
Vindelicorum,  1760. 

2  Astronomic  populaire,  i.  192-194,  Paris  and  Leipzig,  1854. 

3  An  Account  of  a  Method  of  Measuring  the  Comparative  Intensities  of  the  Light 
•emitted  by  Luminous  Bodies,  Trans,  of  the  Roy.  Soc.  London,  Ixxxiv.  67-106,  1794. 

*  Physiologische  Untersuchungen  im  Gebiete  der  Optik,  erstes  Heft,  Leipzig,  1863. 

6  Loc.  cit. 

6  Zehender's  Klinische  Monatsblatter  fiir  Augenheilkunde,  Jahrg.  xv.,  56  ff. 


510  THE    PERCEPTION   OF    LIGHT. 

Owing  to  the  variation  in  the  intensity  and  outline  of  the  shadow  due 
to  flickering  of  the  flames,  and  also  for  other  reasons,  such  as  the  difficulty 
of  avoiding  diffused  light,  the  just  noticeable  difference  cannot  be  exactly 
determined  by  this  method.  Masson1  used  for  the  purpose  revolving 
disks.  On  these  blackened  sectors  may  be  painted,  as  shown  in  A,  Fig.  3. 
When  the  disks  are  rapidly  revolved,  the  blackened  sector  fuses  with  the 
white  surface,  making  a  gray  ring,  as  shown  in  B.  Masson  found  that  the 
ring  could  only  just  be  distinguished  when  the  black  was  about  yfg-  of  the 
circle.  This  difference  varied  with  different  observers,  but  not  with  differ- 
ent illuminations,  v.  Helmholtz  used  disks  such  as  are  illustrated  in  Fig. 
4.  If  the  brightness  of  the  whole  be  taken  as  1,  then  the  brightness  b  of 
a  gray  circle  would  be  expressed  by  the  equation 


in  which  d  is  the  width  of  the  line  and  r  its  distance  from  the  centre  of  the 

FIG.  4. 

FIG.  5. 


circle.  By  determining  how  far  from  the  centre  the  gray  circles  are  visi- 
ble, v.  Helmholtz  found  the  just  noticeable  difference  to  vary  from  y£T  to 
TrT,  the  relative  accuracy  of  discrimination  being  greatest  in  a  moderate 
light,  v.  Helmholtz  states  that  the  rate  of  rotation  was  barely  sufficient 
to  cause  fusion,  and  he  does  not  seem  to  have  allowed  for  the  light  reflected 
from  the  black  nor  for  the  length  of  line  which  would  make  the  gray  circle 
not  uniform.  Kraepelin2  used  similar  disks  in  a  more  careful  manner, 
and  altered  the  intensity  of  the  light  by  means  of  smoked  glasses.  He 
found  the  just  noticeable  difference  to  be  about  y^  with  the  strongest  light, 
and  jfo  with  the  weakest.  Aubert3  used  rotating  disks  of  a  different  sort. 
Following  Maxwell,  he  made  black  and  white  wheels  with  a  slit  as  a  radius, 
and  pasted  narrow  white  sectors  on  the  black  wheels.  The  wheels  may  be 
placed  on  the  rotating  machine  so  as  to  overlap,  as  in  Fig.  5,  and  may  be 

1  Etudes  de  photometric  electrique,  Ann.  de  Chim.  et  de  Phys.,  Ser.  3,  xiv.  129-195, 
1845;  also  Pogg.  Ann.  d.  Phys.  u.  Chem.,  Ixiii.  158-165,  1844. 

2  Zur  Frage  der  Giiltigkeit  des  "Weber'schen  Gesetzes  bei  Lichtempfindungen,  Philos. 
Stud.,  ii.  306-326,  651-654,  1885. 

3  Loc.  cit. 


THE   PERCEPTION   OF 


511 


shifted  so  as  to  show  more  or  less  of  the  black.  By  increasing  the  amount 
of  black  until  the  gray  could  be  just  seen,  Aubert  found  the  just  notice- 
able difference  to  vary  from  y^-j  to  j^-g-  of  the  light,  being  smallest  when 
the  light  was  from  a  clear  sky,  and  larger  when  the  sky  was  clouded  or 
when  the  disks  were  illumined  by  direct  sunlight.  Schirmer '  has  recently 
made  experiments  with  revolving  disks,  combining  the  methods  of  v. 
Helmholtz 2  and  Aubert,  and  paying  especial  attention  to  adaptation.  He 
finds  the  just  noticeable  difference  (about  ^}r)  to  remain  the  same  when 
the  intensity  is  varied  from  one  to  one  thousand  metre-candles,  and  thinks 
the  variations  of  other  observers  are  due  to  the  lack  of  proper  adaptation, 
and  that  the  relative  constancy  of  the  just  noticeable  difference  may  be  ex- 
plained by  physiological  conditions  of  adaptation.  Revolving  disks  as  used 
by  Schirmer  would  probably  prove  the  most  convenient  method  for  testing 
sensitiveness  for  light  in  the  clinic.  Merkel 3  compared  lights  given  in  suc- 
cession and  obtained  a  much  larger  difference  as  just  noticeable, — from  one- 
tenth  to  one-half  of  the  stimulus.  The  intensities  were  adjusted  by  altering 
the  distance  of  a  lamp  from  a  ground- glass  screen. 

The  paper  by  Konig  and  Brodhun,4  noticed  in  the  preceding  section, 
was  primarily  concerned  with  determining  the  just  noticeable  difference. 
This  paper  is  of  special  value  because  colors  of  the  spectrum  were  used 
and  a  large  range  of  intensities  was  investigated.  The  research  was  car- 
ried out  in  v.  Helmholtz's  laboratory,  and  full  details  will  be  found  in  the 
second  edition  of  his  "  Hand- 

buch."     The  just  noticeable  Fio.  6. 

difference  for  white  light  was 
found  to  be  the  smallest  part 
of  the  stimulus  (one-sixtieth) 
when  the  latter  was  about  ten 
thousand  of  the  units  used 
(see  above),  and  to  remain 
nearly  the  same  between  one 
thousand  and  fifty  thousand. 
For  greater  and  less  inten- 
sities the  just  noticeable  dif- 
ference was  relatively  larger, 

being  more  than  half  the  stimulus  when  the  intensity  was  near  the  thresh- 
old, v.  Helmholtz 5  accounts  for  the  relative  increase  of  the  just  noticeable 
difference  with  fainter  lights  by  the  interference  of  the  idio-retinal  light. 
With  strong  intensities  the  just  noticeable  difference  was  not  affected  by 

1  Ueber  die  Giiltigkeit  des  Weber'schen  Gesetzes  fur  den  Lichtsinn,  Arch.  f.  Ophth., 
xxxvi.  (4)  121-149,  1890. 

2  Loc.  cit. 

3  Die  AbhJingigteit  zwischen  Reiz  und  Empflndung.   Erste  Abtheilung,  Philos.  Stud., 

iv.  541-594,  1888. 

*  Loc.  cit.  6  Loc"  cit 


o.r, 


o.i; 


0.001       0.01        0.1 


100      1,000    10,000    100,000    1,000,000 


512  THE    PERCEPTION    OF    LIGHT. 

the  color  of  the  light ;  with  weak  intensities,  however,  the  sensitiveness  was 
greater  towards  the  blue  end  of  the  spectrum.  The  relations  are  shown 
in  the  accompanying  curve,  in  which  the  abscissae  are  proportional  to  the 
logarithms  of  the  intensities  and  the  ordinates  are  proportional  to  the  frac- 
tion of  the  light  which  was  just  noticeable.  The  branch  /  includes  all  the 
colors,  the  branch  //  the  red-yellow  half  of  the  spectrum,  and  the  branch 
III  the  blue  half. 

Results  conflicting  with  these  of  Konig  and  Brodhun  and  with  each 
other  were  previously  obtained  by  Lamansky 1  and  by  Dobrowolsky,2  who 
also  worked  under  the  direction  of  v.  Helmholtz.  According  to  Lamansky, 
the  sensitiveness  is  greatest  for  yellow  and  green.  According  to  Dobrowol- 
sky,  the  sensitiveness  is  greatest  for  violet,  being  nearly  twenty  times  as 
great  as  for  red.  v.  Helmholtz  in  the  second  edition  of  his  "  Handbuch" 
does  not  mention  the  experiments  by  Lamansky  and  by  Dobrowolsky,  so 
he  probably  considers  them  superseded  by  Konig  and  Brodhun's  research. 
Muller-Lyer3  has  published  two  papers  on  the  just  noticeable  difference 
which  are  of  special  importance,  because  they  consider  the  relation  of  inten- 
sity to  the  area  of  the  field  and  because  they  were  prepared  with  a  view 
to  practical  application  in  ophthalmology.  He  used  differently  illumined 
disks,  and  found  that  as  the  intensity  increases  the  difference  which  can 
be  distinguished  becomes  larger,  but  more  slowly  than  in  direct  propor- 
tion to  the  stimulus.  The  departure  was  greater  for  the  fovea  than  for  the 
peripheral  parts  of  the  retina. 

The  most  recent  experiments  concerned  with  the  discrimination  of 
lights  were  carried  out  by  Fullerton  in  conjunction  with  the  writer.4  In 
these  experiments  the  times  of  exposure,  areas,  and  other  conditions  were 
kept  constant.  The  arrangement  of  apparatus  is  shown  in  the  figure. 

The  observer  was  placed 

FIG.  7.  in  a  separate  compartment  at 

O  and  saw  the  light  at  S. 
A  pendulum  at  P  allowed 
the  light  to  appear  at  S  for 
"*"***^^  one  second,  cut  it  off  for  one 

LlV-^j  $     second,  and  then  allowed  it 

to    appear    again    for    one 
second.    While  the  light  was 
cut    off    the    intensity    was 
altered  by  shifting  the  lamp  L,  and  the  observer  was  required  to  decide 

1  Ueber  die  Grenzen  der  Empfindlichkeit  des  Auges  fur  Spectralfarben ,  Arch.  f. 
Ophth.,  xvii.  (1)  123-134,  1871 ;  also  Pogg.  Ann.  d.  Phys.  u.  Chem.,  cxliii.  633-643,  1871. 

1  Beitrage  zur  physiologischen  Optik.  II.  Ueber  Empfindlichkeit  des  Auges  gegen 
verschiedene  Spectralfarben,  Arch.  f.  Ophth.,  xviii.  (1)  66-74,  1872. 

8  Ueber  die  Abhangigkeit  der  relativen  Unterschiedsempfindlichkeit  von  Intensitat 
wnd  Extension  des  Reizes,  Arch.  f.  Anat.  u.  Physiol.  (Physiol.  Abtheilung),  Supplement. 
Band,  91-140,  1889. 

*  On  the  Perception  of  Small  Differences,  134-149,  Philadelphia,  1892. 


THE   PERCEPTION   OF   LIGHT.  513 

which  of  the  lights  was  the  more  intense  and  assign  the  confidence  felt  in 
his  decision.  Nine  observers  were  tested,  and  their  accuracy  of  discrimina- 
tion was  found  to  vary  considerably,  the  difference  which  could  be  correctly 
distinguished  seventy-five  per  cent,  of  the  time  varying  between  about 
one-tenth  and  one-fifth  of  the  light.  These  results  show  that  tests  made  in 
the  clinic  would  indicate  individual  differences  in  sensitiveness,  and  would 
probably  give  early  indications  of  certain  diseases  of  the  eye  and  nervous 
system. 

III. — THE   COMPARISON   OF   MAGNITUDES. 

The  sensitiveness  of  the  visual  mechanism  and  the  accuracy  of  dis- 
crimination may  be  tested  not  only  by  measuring  the  least  difference  which 
can  be  perceived,  but  also  by  determining  the  accuracy  with  which  different 
intensities  can  be  estimated  and  compared.  In  the  latter  case,  however, 
the  judgment  of  the  observer  plays  a  more  important  part,  and  the  results 
will  be  found  less  accordant  and  less  useful  in  the  clinic.  The  first  attempts 
to  estimate  differences  in  intensity  were  in  the  classifications  of  the  magni- 
tudes of  the  stars.  These  intensities  have  since  been  determined  by  photo- 
metric methods,  and  it  is  thus  possible  to  compare  the  estimated  difference 
with  the  objective  difference.  A  detailed  discussion  of  the  results  is  given 
by  Miiller1  and  by  Jastrow.2  Plateau  determined  by  direct  experiment 
the  accuracy  with  which  a  shade  of  gray  could  be  adjusted  midway  between 
a  lighter  and  a  darker  shade,  and  Delboeuf  made  similar  and  more  careful 
determinations  with  revolving  wheels.  Revolving  wheels,  with  due  allow- 
ance for  contrast  and  other  disturbing  factors,  have  more  recently  been  used 
in  Wuudt's  laboratory  by  Lehmann 3  and  by  Neiglick.4  Breton  5  arranged 
shades  of  gray  so  as  to  make  a  series  of  equal  differences  in  intensity.  Eb- 
binghaus 6  carried  out  similar  experiments  with  greater  care  and  exactness, 
and  Leuba 7  has  recently  classified  artificial  stars.  Merkel  determined  the 
accuracy  with  which  the  intensity  of  lights  can  be  doubled.  These  experi- 
ments on  the  comparison  of  magnitudes  have  been  carried  out  by  so  many 
different  methods,  and  the  number  of  observers  has  been  so  small,  that  it 
is  impossible  to  learn  from  them  how  different  observers  vary,  or  whether 
variation  would  be  related  to  any  special  condition  of  the  eye  or  of  the 
nervous  system.  The  experiments  have  been  made  with  a  view  to 

1  Zur  Grundlegung  der  Psychophysik,  Berlin,  1878. 

2  The  Psycho-physic  Law  and  Star  Magnitudes,  Am.  Journ.  of  Psych.,  i.  112-127, 
1887. 

3  Ueber  die  Anwendung  der  Methode  der  mittleren  Abstufungen  auf  den  Lichtsinn, 
Philos.  Stud.,  iii.  497-533. 

*  Zur  Psychophysik  des  Lichtsinns,  Philos.  Stud.,  iv.  28-111. 

6  Sur  la  loi  de  Fechner,  Les  Mondes  (Cosmos),  Ser.  2,  xxxviii.  63-69. 

6  Die  Gesetzmiissigkeit  der  Helligkeit,  Sitz.-ber.  d.  Akad.  d.  Wissen.  zu  Berlin,  1887, 
995-1009. 

7  A  New  Instrument  for  Weber's  Law,  with  Indications  of  a  Law  of  Sense  Memory, 
Am.  Jour,  of  Psych.,  v.  370-384,  1893. 

VOL.  I.— 33 


514  THE   PERCEPTION   OF   LIGHT. 

studying  the  laws  of  Weber1  and  of  Fechner,2  which  we  have  next  to 
consider. 

iv.  —  WEBER'S  LAW. 

In  describing  experiments  on  the  just  noticeable  difference  it  has  been 
stated  that  the  addition  of  a  certain  part,  say  y^,  of  the  light,  could  be 
distinguished,  not  that  a  fixed  difference,  say  -j-^  of  the  light  of  a  candle, 
could  be  distinguished.  Weber's  law  states  as  a  general  proposition  that 
the  least  difference  which  can  be  distinguished  is  a  proportional  part  of  the 
intensity  of  the  stimulus.  Thus,  if  in  a  room  lit  up  by  a  hundred  candles 
the  introduction  of  an  additional  candle  made  the  illumination  just  percep- 
tibly greater,  when  the  room  was  lit  up  by  one  thousand  candles  the  in- 
troduction of  an  additional  candle  would  not  be  perceived,  but  ten  candles 
would  be  required  to  make  a  difference  which  could  just  be  distinguished. 
This  relation  was  first  noticed  by  Bouguer,3  but  its  statement  is  usually 
called  Weber's  law,  as  Weber  4  extended  its  application  to  different  senses. 
The  experimental  study  of  Weber's  law,  and  the  attempt  to  apply  it  to  the 
measurement  of  the  intensity  of  sensation,  have  received  much  attention  from 
physiologists,  physicists,  and  psychologists.  Indeed,  it  would  be  difficult 
to  mention  another  subject  so  limited  in  range  which  has  been  so  largely 
contributed  to  by  men  eminent  in  different  departments  of  science.  In  this 
place  we  are  concerned  with  vision  only,  but  it  may  be  worth  while  to  give 
for  comparison  results  concerning  other  senses  obtained  by  various  observers. 
The  fraction  gives  approximately  the  part  of  the  stimulus  which  could  be 
correctly  distinguished  seventy-five  per  cent,  of  the  time.  This  is  the  prob- 
able error  of  mathematics.  Where  writers  have  determined  the  difference 
they  think  they  can  just  notice  we  have  no  definite  standard  for  comparison, 
but  we  may  assume  perhaps  that  they  would  distinguish  such  a  difference 
correctly  about  nine  times  out  of  ten  trials,  and  deduce  the  probable  error 
from  this  relation.  Extreme  values,  such  as  are  obtained  with  very  weak 
stimuli,  are  omitted. 


Simultaneous  lights  .................... 

Successive  lights    .....................  5  -  A 

Sounds     .........................  l~i 

Pressures  on  skin   .....................  i  ~  yV 

Lifted  weights    ......................  tS~A 

Force  of  movement      ...................  i  -  ?V 

Time  of  movement   ....................  i  ~  ^7 

Extent  of  movement    ...................  ^~T!TT 

Length  of  lines  (by  the  eye)  ................  TTTTTTJ 

Temperature  .......................  i-iV°  **• 

1  Annotationes  de  Pulsu,  Kesorptione,  Auditu  et  Tactu,  Lipsiae,  1834  ;  Der  Tastsinn 
und  das  Gemeingefiihl,  in  Wagner's  Handworterbuch  der  Physiologic,  iii.  2,  Braunschweig, 
1846  ;  Annotationes  Anatomicae  et  Physiologicae,  Lipsiae,  1851. 

s  Zend  Avesta,  Leipzig,  1860;  In  Sachen  der  Psychophysik,  1877;  Elemente  der 
Psychophysik,  Leipzig,  1860  ;  Revision  der  Hauptpunkte  der  Psychophysik,  Leipzig,  1882. 

3  Loc.  cit.  *  Loc.  cit. 


THE   PERCEPTION   OF   LIGHT.  515 

It  follows  from  the  above  that  sight  is  the  most  delicate  of  the  senses 
when  the  lights  are  given  side  by  side  and  can  be  compared,  but  that 
when  the  lights  are  successive  (as  must  be  the  case  with  sounds,  move- 
ments, etc.)  the  accuracy  of  perception  seems  to  be  no  greater  than  for 
other  senses. 

If  Weber's  law  do  in  fact  obtain,  we  can  conveniently  compare  different 
senses  and  different  observers.  If,  however,  the  just  noticeable  difference 
vary  with  the  intensity  of  the  stimulus,  it  is  difficult  to  decide  when  stimuli 
of  different  sorts  are  equal  in  intensity,  and  consequently  different  observers 
must  be  tested  with  stimuli  of  the  same  intensity.  It  has  always  been 
found  that  Weber's  law  fails  near  the  threshold,  the  just  noticeable  differ- 
ence becoming  a  larger  part  of  the  stimulus,  but  this  may  be  explained  by 
the  interference  of  faint  stimuli  which  continually  affect  the  sense  organ, 
such  as  the  idio-retinal  light  of  the  eye,  noises  in  the  air,  etc.  With  very 
strong  stimuli  Weber's  law  also  fails,  but  this  may  be  due  to  injury  of  the 
sense  organ,  which  would  interfere  with  its  sensitiveness.  Whether  or  not 
Weber's  law  holds  for  intermediate  intensities  is  still  an  open  question,  but 
the  weight  of  testimony  seems  to  incline  to  the  conclusion  that  its  validity 
is  at  most  only  approximate. 

The  difference  in  intensity  which  can  just  be  distinguished  is  a  func- 
tion not  only  of  the  absolute  brightness  of  the  lights  but  also  of  their  area 
and  time-relations.  These  factors  have  not  been  properly  distinguished  in 
most  researches  on  the  accuracy  of  perception,  and  the  varying  results  are 
thus  to  a  considerable  extent  explained. 

v. — FECHNEE'S  LAW. 

The  results  of  the  experiments  on  the  just  noticeable  difference  have 
been  explained  by  Fechner  in  such  a  manner  as  to  make  it  possible  to  measure 
the  intensity  of  sensation  and  to  determine  a  correlation  between  mental 
and  physical  changes.     Fechner  assumes  that  the  just  noticeable  difference 
is  an  equal  mental  magnitude  for  every 
intensity  of  the  stimulus,  whence  it 
follows  that  as  the  intensity  of  the 
stimulus   is  increased  it  has   a   rela- 
tively decreasing  effect  in  consciousness. 
This  relation  is  shown  in  the  accom- 
panying curve  (Fig.  8).     The  subdi- 
visions of  the  horizontal  axis  represent 
equal  increments  in  the  intensity  of 
the  stimulus,  and  the  subdivisions  of 

the  vertical  axis  represent  the  series  of  differences  which  can  just  be  per- 
ceived, and  these  are  assumed  to  be  equal  increments  to  the  intensity  of 
sensation.  When  the  stimulus  is  very  weak  there  is,  as  we  have  seen,  no 
appreciable  sensation,  and  this  is  shown  in  the  figure  as  the  curve  crosses  the 
horizontal  axis  at  A.  With  this  stimulus  the  sensation  crosses  the  thresh- 


A  B  C  D 


516  THE   PEECEPTION   OF   LIGHT. 

old  of  consciousness.  When  the  stimulus  is  increased  by  the  amount  AB 
and  is  equal  to  OB,  there  is  an  increase  in  intensity  of  sensation,  OM, 
which  can  just  -be  noticed.  When,  however,  the  stimulus  is  further  in- 
creased by  BC,  which  is  equal  to  AB,  no  distinct  change  in  consciousness 
occurs,  but  the  stimulus  must  be  increased  by  an  amount  greater  than  AB, 
say  BD,  in  order  that  the  change  in  sensation,  MN,  may  be  just  noticeable. 
And  thus  as  the  intensity  of  the  stimulus  is  greater  the  increment  which 
can  just  be  noticed  continually  increases.  Fechner  deduces  the  relation 
between  sensation  and  stimulus  mathematically  as  follows  :  If  the  just 
noticeable  difference  be  a  proportional  part  of  the  stimulus,  then 

(1)   N=C^ 

in  which  N  is  the  just  noticeable  difference  and  is  assumed  by  Fechner  to 
be  a  constant  mental  magnitude  for  every  value  of  the  stimulus.  S  is  the 
intensity  of  the  stimulus,  s  the  increment  in  the  stimulus  which  can  just  be 
noticed,  and  C  a  constant  which  might  vary  for  different  senses,  individuals, 
etc.  Supposing  the  above  equation  to  hold  for  very  small  changes,  it  may 
be  written 


and  by  integration 


(2)  dN=  C 


The  constant  A  may  be  determined  at  the  threshold,  where  N=0.     In 
this  case 

(4)  Clog  a  +  4=0. 

(5)  A=  —Clog  a. 

If  we  take  the  intensity  of  the  stimulus  at  the  threshold  as  unity,  a  =  1,  we 
may  write  (3) 

(Q)N=  Clogs; 

that  is,  the  intensity  of  sensation  is  equal  to  the  logarithm  of  the  intensity 
of  the  stimulus  multiplied  by  a  constant. 

Fechner's  deduction  is  open  to  the  criticism  (in  addition  to  the  question 
of  the  validity  of  Weber's  law)  that  the  just  noticeable  difference  is  not  a 
unit  which  may  be  used  to  measure  sensation.  On  the  supposition  that  it 
is  correct,  we  may  ask,  Why  should  the  sensation  increase  as  the  logarithm 
of  the  stimulus?  Three  answers  have  been  given  to  this  question.  Fech- 
ner himself  holds  that  his  law  expresses  an  ultimate  relation  between  mind 
and  matter.  Others  (e.g.,  Miiller,1  Bernstein,2  Ward3)  maintain  that  the 
sensation,  indeed,  varies  directly  as  the  brain-changes  correlated  with  it,  but 
that  these  increase  as  the  logarithm  of  the  stimulus.  Wundt  argues  that 
we  are  concerned  with  an  estimation  which  is  subject  to  the  so-called  "  law 

1  Loc.  cit. 

2  Untersuchungen   iiber  den    Erregungsvorgang   im   Nerven-  und   Muskel-Systeme, 
Heidelberg,  1871. 

3  An  Attempt  to  interpret  Fechner's  Law,  Mind,  i.  452-466,  1876. 


THE    PERCEPTION    OF    LIGHT.  517 

of  relativity  :"  a  pound  would  not  be  a  considerable  addition  to  the  weight 
of  a  man,  but  would  greatly  alter  the  weight  of  a  pigeon.  Bernoulli,1  as 
long  ago  as  1730,  pointed  out  that  the  value  of  money  is  relative  (e.g.,  to 
a  man  having  an  income  of  $1000,  $1  will  have  as  much  value  as  $10 
to  a  man  having  an  income  of  $10,000),  and  deduced  a  formula  exactly 
analogous  to  Fechner's, — namely,  the  worth  of  money  to  the  individual 
increases  as  the  logarithm  of  its  amount. 

VI. — METHODS    FOR   STUDYING   THE   ACCURACY   OF   PERCEPTION. 

It  is  desirable  to  consider  in  this  place  the  methods  which  have  been 
developed  in  researches  on  the  relation  between  intensity  of  stimulus  and 
intensity  of  sensation.  These  methods  are  to  a  certain  extent  analogous  to 
those  required  for  the  adjustment  of  observations  in  the  physical  sciences. 
There  is,  however,  this  important  difference  :  physical  science  aims  at  elimi- 
nating errors  of  observation;  psychology  aims  at  studying  their  nature. 
In  ophthalmology  and  many  other  branches  of  medicine  the  physician  has 
the  task  both  of  the  physicist  and  of  the  psychologist.  On  the  one  hand, 
he  must  determine  from  different  and  conflicting  observations  the  true  state 
of  the  case ;  on  the  other  hand,  it  is  the  error  or  anomaly  of  the  patient 
which  must  be  studied  in  order  that  it  may  be  cured  or  corrected.  It  must 
be  admitted  that  physicians  have  hitherto  depended  chiefly  on  insight  and 
experience  rather  than  on  measurement :  medicine  has  been  an  art,  not 
a  science.  But  considerable  changes  have  recently  taken  place,  and  the 
student  with  the  help  of  scientific  methods  can  often  make  a  diagnosis  as 
correctly  as  the  older  physician  with  years  of  experience. 

Methods  for  studying  the  perception  of  small  differences  may  be  of  two 
sorts :  they  may  seek  to  determine  the  accuracy  with  which  an  observer 
can  estimate  a  difference,  or  they  may  seek  to  determine  his  error  of  obser- 
vation. The  former  method  depends  on  the  judgment  of  the  observer,  and 
lacks  an  objective  criterion.  Most  of  the  experiments  on  the  intensity  of 
lights  considered  above  were  made  by  this  method,  and  it  is  the  one  the 
physician  mostly  uses,  as  when  he  asks  his  patient,  Do  you  feel  better  to- 
day than  yesterday  ?  Does  it  hurt  when  I  prick  you  with  this  pin  ?  But 
the  physician  depends  as  little  as  possible  on  the  answer  of  the  patient ;  he 
seeks  to  observe  for  himself  whether  the  patient  appears  better  or  worse, 
whether  he  shows  signs  of  pain  Avhen  pricked.  The  error  of  observation 
is  now  transferred  to  the  physician,  and  is  lessened  by  his  experience,  skill, 
and  insight.  Still,  the  physician  prefers  an  objective  standard,  as  the  use  of 
a  thermometer,  or  the  action  of  the  patient  when  he  is  in  fact  pricked  and 
when  there  is  only  a  pretence  to  prick.  Great  advances  have  been  made  in 
the  objective  study  of  the  eye  with  the  aid  of  the  ophthalmoscope,  the  op- 
tometer,  the  skiascope,  etc.,  but  in  the  correction  of  defects  of  refraction 
and  in  the  diagnosis  of  many  diseases  the  perceptions  of  the  patient  and  the 

1  Of.  Todhunter's  History  of  the  Theory  of  Probability,  Cambridge  and  London,  1865. 


518  THE   PERCEPTION   OF   LIGHT. 

account  he  gives  of  them  are  important  data  for  the  physician.  It  is 
desirable,  therefore,  to  know  the  best  methods  for  obtaining  these  data,  and 
the  reliance  which  can  be  placed  on  them. 

Two  objective  methods  for  the  study  of  the  perception  of  small  differ- 
ences may  be  distinguished :  (1)  the  method  of  average  error,  and  (2)  the 
method  of  right  and  wrong  cases.  In  using  the  method  of  average  error 
the  observer  is  given  a  stimulus — say  a  light  of  a  certain  intensity — and  is 
required  to  adjust  a  second  light  until  it  appears  the  same.  The  second 
light  will  usually  be  too  great  or  too  small,  and  the  error  of  the  observer 
is  determined.  The  experiment  is  repeated  a  number  of  times,  and  the 
average  error  measures  the  accuracy  of  discrimination.  The  reliability  of 
the  result  increases  as  the  square  root  of  the  number  of  trials.  In  using 
the  method  of  right  and  wrong  cases  two  lights  nearly  alike  in  intensity 
are  shown  to  the  observer,  and  he  is  required  to  say  which  appears  the 
brighter.  He  will  be  sometimes  right  and  sometimes  wrong  in  his  judgment, 
and  from  the  ratio  of  right  to  wrong  cases  the  accuracy  of  discrimination 
may  be  determined. 

The  method  of  right  and  wrong  cases  is  extensively  used  in  ophthal- 
mology, but  not  with  as  great  exactness  as  is  usual  in  psychological  research. 
The  methods  of  the  laboratory  must  of  necessity  be  simplified  in  the  clinic, 
but  the  careful  scientific  study  of  conditions  in  a  medical  course  would 
make  clinical  examination  quicker  as  well  as  more  exact.  Thus,  when  it 
is  found  that  a  patient  can  just  read  the  test  letters  under  the  conditions 
employed,  it  may  be  asked  what  is  meant  by  "just  read."  Probably  not 
that  the  patient  will  never  make  a  mistake,  for  if  the  trial  be  continued 
mistakes  will  occur  even  with  letters  which  can  be  distinctly  seen.  The 
physician  doubtless  fixes  some  standard  in  his  mind,  say  that  the  answer  of 
the  patient  shall  be  correct  nine  times  out  of  ten,  but  he  rarely  records  the 
results  of  the  separate  trials,  and  supposing  two  mistakes  to  be  made  in  ten 
trials  he  would  scarcely  know  how  to  prescribe  glasses  which  would  enable 
the  observer  to  make  one  mistake  only  in  ten  trials.  Yet  the  theory  of 
probability  makes  this  possible.  When  the  method  of  right  and  wrong 
cases  is  used  scientifically  the  theory  of  probability  also  enables  the  physi- 
cian to  know  how  nearly  his  correction  is  exact ;  he  can  assign  its  probable 
error, — that  is,  the  limits  within  which  it  is  likely  that  the  prescription 
is  exactly  correct. 

In  conclusion,  two  precautions  in  method  may  be  mentioned  which 
psychological  research  has  emphasized.  In  the  first  place,  we  can  perceive 
a  thing  better  when  we  know  what  it  is.  The  memory  image  is  added  to 
the  immediate  perception,  and  in  our  daily  life  we  can  scarcely  distinguish 
the  part  played  by  each.  In  the  second  place,  it  is  necessary  to  recognize 
unconscious  memory.  A  series  of  letters  can  be  learned  more  quickly  or 
seen  more  readily  when  it  has  once  been  used.  The  patient  should,  there- 
fore, always  be  ignorant  of  the  test  used,  and  the  same  series  of  impressions 
should  not  be  repeated. 


THE   PERCEPTION   OF   LIGHT.  519 

Psychological  methods  are  discussed  by  Fechner,1  Miiller,2  Wundt,* 
Fuller-ton  and  Cattell,4  and  others.  The  psychological  aspects  of  percep- 
tion and  memory  are  treated  in  a  full  and  interesting  manner  by  James.8 

VII. — INTENSITY   AND   COLOR. 

The  different  colors  of  the  sun's  spectrum  do  not  appear  of  the  same 
intensity.  As  Newton  remarked,  "  The  most  luminous  of  the  prismatic 
colors  are  the  yellow  and  orange,  .  .  .  and  next  to  these  in  strength  are  the 
red  and  green."  The  varying  intensity  of  the  several  colors  was  first  inves- 
tigated by  Fraunhofer,6  and  later  by  Vierordt.7  Fraunhofer  compared  the 
colors  of  the  sun's  spectrum  directly  with  colorless  light.  According  to  his 
results,  if  the  intensity  of  yellow  be  placed  at  1000,  the  intensity  of  the  other 
colors  will  be  red  (B)  32,  orange  (C)  94,  green  (E)  480,  blue  (G)  31,  and 
violet  (H)  5.6.  Vierordt  determined  the  amount  of  white  light  which  could 
be  mixed  with  the  several  colors  without  producing  a  noticeable  decrease  in 
saturation.  By  this  method  he  obtained  as  coefficients  of  intensity,  red 
22,  orange  128,  yellow  1000,  green  370,  blue  8,  violet  0.7.  For  the 
spectrum  of  a  gas  flame  the  intensities  of  the  orange  and  red  were  greater 
and  of  the  green  and  blue '  less.  Rood  has  recently  made  an  important 
advance  in  chromophotometry  by  comparing  the  intensities  of  colors  by 
means  of  the  flickering  of  revolving  wheels.  This  method  obviates  the 
difficulties  in  the  way  of  the  comparison  of  disparate  sensations. 

The  direct  comparison  of  the  intensity  of  different  colors  is  difficult, 
and  observers  differ  greatly  in  the  confidence  with  which  they  make  such 
comparisons.  Careful  and  important  experiments  on  the  subject  have  been 
made  during  the  past  few  years  in  v.  Helmholtz's  laboratory  by  Konig, 
Brodhun,  and  Dieterici.  These  are,  however,  largely  concerned  with  work- 
ing out  three  possible  fundamental  colors,  and  only  partly  fall  within  the 
limits  of  this  article.  Brodhun,8  who  is  color-blind  for  red,  seems  able  to 
compare  the  brightness  of  different  colors  with  less  variation  than  observers 
with  normal  color-vision. 

When  the  objective  intensity  is  altered,  different  colors  do  not  maintain 
the  same  relations  of  brightness.  It  was  noticed  by  Purkinje  that  if  red 
and  blue  be  taken,  which  seem  to  be  of  about  the  same  intensity,  and  the 
illumination  of  each  be  reduced  equally,  the  blue  can  be  seen  the  longer. 
In  general  the  less  refrangible  colors  appear  relatively  brighter  in  a  strong 
light  and  the  more  refrangible  colors  brighter  in  a  faint  light. 

1  Loc.  cit.  2  Loc.  cit.  8  Loc.  cit.  *  Loc.  cit. 

5  The  Principles  of  Psychology,  2  vols.,  New  York,  1890. 

6  Bestimmung    des    Brechungs-   und    Farbenzerstreuungs-Vermogens   verschiedener 
Glasarten  in  Bezug  auf  die  Vervollkommnung  achromatischer  Fernrohre  (Denkschriften 
der  Bayrischen  Akademie,  Munchen,  193,  1815.) 

7  Anwendung  des  Spectralapparutes  zur  Messung  und  Vergleichung  der  Starke  des 
farbigen  Lichtes,  Tubingen,  1871. 

8  Beitrage  zur  Farbenlehre,  Inaug.  Diss.,  Berlin,  1887. 


520  THE    PERCEPTION   OF    LIGHT. 

Konig l  has  recently  investigated  Purkinje's  phenomenon  and  the  bright- 
ness of  colors  of  the  spectrum  with  different  absolute  intensities.  He  used 
eight  different  intensities,  the  strongest  of  which  was  262144  times  as  great 
as  the  weakest.  He  found  Purkinje's  phenomenon  to  be  much  more  pro- 
nounced for  weak  than  for  strong  intensities.  A  further  complication  fol- 
lows from  the  fact  (demonstrated  by  Van  der  Weyde  and  by  Brodhun)  that 
if  a  spectrum  color  be  matched  by  a  mixture  of  two  colors  and  the  intensity 
altered,  the  colors  will  be  no  longer  alike.  It  had  previously  been  dis- 
covered by  Preyer  and  Konig  that  the  position  of  the  neutral  point  of 
color-blind  observers  alters  with  the  intensity.  Ebbinghaus 2  has  further 
found  that  grays  of  the  same  intensity,  made  by  combining  different  pairs 
of  complementary  colors,  do  not  remain  of  the  same  intensity  when  the 
illumination  of  both  grays  is  altered  equally. 

The  alterations  in  color  due  to  changing  intensity  greatly  affect  the 
appearance  of  natural  objects.  In  a  general  way  increasing  the  intensity 
makes  colors  more  yellowish,  decreasing  the  intensity  makes  colors  more 
bluish.  Thus,  grass  in  the  sunlight  looks  yellowish-green,  while  the  part 
of  the  same  plot  of  grass  on  which  the  shadow  falls  looks  bluish-green. 
The  general  effect  of  a  sunny  day  is  yellowish,  and  that  of  a  clouded  day, 
or  of  twilight,  bluish.  A  moonlight  scene  is  still  more  distinctly  bluish, 
and  painters  use  blue  tones  to  represent  such  a  scene.  We  can  obtain  the 
effects  of  a  cloudy  day  by  looking  through  a  bluish  glass  on  a  sunny  day, 
and  the  converse  effects  by  using  a  yellowish  glass.  A  pure  gray  looks 
bluish  when  compared  with  white.  Rood3  found  it  necessary  to  add 
seventeen  per  cent,  of  indigo  to  white  in  order  to  obtain  the  color-tone 
of  gray  made  by  adding  fifty  per  cent,  of  black. 

When  the  illumination  is  very  intense  or  very  faint,  colors  disappear 
altogether,  or  perhaps  it  should  be  said  that  when  intensely  illumined  they 
become  a  yellowish-white,  and  when  faintly  illumined  a  bluish-gray.  We 
have  already  considered  the  threshold  for  color.  Before  colors  disappear 
their  tone  is  altered.  Thus,  if  the  light  of  the  sun's  spectrum  be  gradually 
diminished,  the  colors  will  disappear,  except  red,  green,  and  violet-blue. 
These  colors  then  become  red-brown,  olive-brown,  and  blue-gray,  and 
finally  disappear,  the  entire  spectrum  becoming  gray.  When  the  intensity 
is  very  great,  violet  is  the  first  of  the  colors  to  become  white,  blue  becomes 
violet,  and  green  yellowish,  before  the  colors  disappear.  Red  is  said  to 
remain  yellowish  with  the  greatest  intensity.  In  this  gray  spectrum  the 
maximum  of  intensity  is  at  a  wave-length  of  535  w. 

According  to  v.  Helmholtz,  these  relations  of  color  and  intensity  are 
brought  into  harmony  with  the  theory  of  three  fundamental  colors  and 

1  Ueber  den  Helligkeitswert  der  Spektralfarben,  Beitrage  zur  Psychologic  und  Physi- 
ologie  der  Sinnesorgane,  H.  v.  Helmholtz  als   Festgruss,  etc.,   Hamburg  and   Leipzig, 
1891,  311-388. 

2  Theorie  des  Farbensinnes,  Zeitsch.  fur  Psychol.,  v.  145-238,  May,  1893. 

3  Loc.  cit. 


THE   PERCEPTION   OF   LIGHT.  521 

nerve-fibres  by  assuming  that  the  relative  effects  of  the  components  of  the 
colors  on  the  three  sets  of  fibres  vary  with  the  intensity.  It  seems,  how- 
ever, to  the  writer  that  the  facts  are  not  explained  by  the  theory,  but  rather 
that  unlikely  hypotheses  are  made  in  order  that  the  facts  may  not  be  sub- 
versive of  the  theory.  Hering's  theory,  which  in  general  is  more  satisfac- 
tory from  the  point  of  view  of  psychology,  fails  to  account  for  the  intricate 
relations  of  intensity  and  color. 

VIII. — INTENSITY  AND   SHARPNESS   OP  SIGHT. 

If  Weber's  law  were  exactly  correct,  differences  would  be  equally  ap- 
parent whatever  the  intensity  of  the  light,  and  a  printed  page  could  be 
read  equally  well  in  any  illumination.  Weber's  law  does  not  hold  for  very 
faint  nor  for  very  strong  intensities,  and  a  printed  page  cannot  be  read  as 
well  in  twilight  or  in  direct  sunlight  as  in  ordinary  daylight.  The  relation 
of  intensity  of  illumination  to  sharpness  of  sight  is  important  in  practical 
ophthalmology,  as  it  is  necessary  to  know  what  illumination  is  the  most 
favorable  for  reading  test-types,  etc.,  and  what  corrections  must  be  made 
for  other  illuminations.  It  would  also  be  of  great  importance  to  determine 
what  intensity  of  light  is  most  suitable  and  advantageous  to  the  eyes  in 
reading  and  in  other  tasks  requiring  near  fixation.  The  writer  believes 
that  polarized  light  may  be  irritating  and  destructive  to  the  skin  and  the 
eyes,  but  this  has  not  been  investigated.  The  relation  between  intensity 
and  sharpness  of  sight  has  been  studied  by  several  observers,  of  whom  it 
is  necessary  to  mention  here  only  Mac4  de  Lepinay  and  W.  Nicati l  and 
Uhthoff.2  The  research  by  the  latter  was  carried  out  with  great  care  in  v. 
Helmholtz's  laboratory.  He  used  the  symbols  proposed  by  Snellen  for  testing 
sharpness  of  sight,  and  also  a  wire  grating.  The  sharpness  of  sight  in- 
creased from  0.13  to  2.37  as  the  illumination  increased,  at  first  rapidly  and 
then  more  slowly,  but  the  maximum  sharpness  could  not  be  obtained  with 
the  light  used.  The  sharpness  of  sight  differs  for  the  different  colors,  but 
this  is  due  chiefly  to  the  inherent  difference  in  brightness  of  the  colors. 
When  a  light  is  used  sufficient  to  give  the  maximum  sharpness  of  sight, 
this  seems  to  be  the  same  for  the  several  colors. 

IX. — INTENSITY   AND  THE   FIELD   OF  VISION. 

The  apparent  intensity  of  light  and  color  varies  with  the  size  of  the 
object,  the  part  of  the  retina  affected,  and  the  surrounding  field.  Lights 

1  Kecherches  sur  la  comparaison  photometrique  des  diverses  parties  d'un  meme 
spectre,  Ann.  de  Chem.  et  de  Phys.,  Ser.  6,  xxiv.  289-337,  1881;  deuxieme  Memoire, 
Ser.  5,  xxx.  146-214,  1883. 

1  Ueber  das  Abhangigkeitsverhaltniss  der  Sehscharfe  von  der  Beleuchtungsintensitat, 
Arch.  f.  Ophth  ,  xxxii.  (1)  171-204,  1886 ;  Ueber  das  Abhangigkeitsverhaltniss  der 
Sehscharfe  von  der  Intensitat  so  wie  von  der  Wellenlange  im  Spektrum,  xxxvi.  (1)  33-61, 
1890;  Ueber  die  kleinsten  wahrnehmbaren  Gesiohtswinkel  in  den  verschiedenen  Teilen 
des  Spektrums,  Zeitsch.  f.  Psych.,  i.  155-160,  1890. 


522  THE   PERCEPTION   OF   LIGHT. 

and  colors  when  very  intense  can  be  seen  even  though  they  have  no 
appreciable  area,  as  in  the  case  of  the  stars,  whose  colors  we  can  dis- 
tinguish. With  moderate  intensities,  however,  colors  must  have  a  certain 
area  in  order  to  be  seen  as  light,  and  a  certain  greater  area  in  order  that  the 
color  may  be  recognized.  The  fact  that  colors  must  have  a  certain  area  in 
order  that  they  may  be  seen  was  noticed  by  Plateau,1  and  the  relations  have 
been  investigated  by  Aubert  and  v.  Wittich.2  In  bright  but  diffused 
daylight  the  angle  the  surface  subtends  is  from  39"  to  IV  46".  On  a 
black  ground  the  bright  colors,  yellow  and  orange,  are  seen  first  as  "  clouds 
of  light."  On  a  white  ground  the  different  colors  are  seen  at  nearly  the 
same  angle  as  dark  spots.  When  a  color  is  very  small  in  area  its  tone 
varies,  as  is  the  case  when  the  intensity  is  faint. 

The  difference  in  intensity  which  can  just  be  distinguished  depends 
on  the  area  of  the  surfaces.  Aubert3  studied  the  relation  with  revolving 
wheels,  the  area  of  stimulation  being  altered  by  increasing  the  distance 
from  which  the  wheels  were  viewed.  Increasing  the  distance  does  not  alter 
the  brightness  of  the  lights,  excepting  so  far  as  the  rays  may  be  absorbed 
by  the  air.  With  a  moderate  intensity  Aubert  found  the  just  noticeable 
difference  increased  from  one  seventy-second  to  one-eleventh  as  the  angle 
decreased  from  7°  to  6°  22',  and  with  a  faint  light  and  a  small  angle  the 
one  ring  had  to  be  nine  times  as  bright  as  the  other  in  order  that  a 
difference  might  be  perceived. 

Not  only  is  the  apparent  intensity  of  lights  altered  by  the  area,  but 
the  apparent  area  is  altered  by  the  intensity.  A  brightly  illumined  object 

appears  larger  than  an  object 
of  the  same  size  less  brightly 
illumined,  a  phenomenon  to 
which  the  name  irradiation 
has  been  given.  Thus,  in  the 
figure  the  white  square  on  a 
black  ground  appears  larger 
than  the  black  square  on  the 
white  ground.  In  the  same 
manner  a  bright  light  partly 
covered  by  a  black  screen 
seems  to  dip  into  the  screen,  as  shown  in  the  figure. 

Irradiation  seems  to  be  chiefly  due  to  imperfect  accommodation,  and  is 
much  more  marked  when  the  object  is  not  exactly  fixed.  The  diffusion 
circles  of  the  white  cover  part  of  the  black  field,  and  the  white  is  thus 

1  Ueber  einige  Eigenschaften  der  vom  Lichte  auf  das  Gesichtsorgan  hervorgebrachten 
Eindriicke,  Pogg.  Ann.  d.  Phys.  u.  Chem.,  xx.  304-332,  1830. 

2  Ueber  die  geringsten  Ausdehnungen  welche  man  farbigen  Objecten  geben  kann, 
sie  noch  in  ihrer  specifischen   Farbe  wahrzunehmen,  Konigsberger  medicinische  Jahr- 
biicher,  iv.  23,  1864. 

3  Op.  cit. 


THE    PERCEPTION    OF    LIGHT.  523 

enlarged.     It  is  true  the  white  near  the  edge  is  darkened  by  the  diffusion 
circles  from  the. black,  but  the  gray  zone  is  attributed  to  the  white,  in 
accordance  with  "Weber's  law.     Plateau  and  others  have  explained  irra- 
diation by  assuming  that  the  neighboring  ele- 
ments of  the  retina  are  sympathetically  affected  FlG*  10- 
by  the  light  which  does  not  directly  fall  upon 
them ;  but  this  supposition  does  not  seem  re- 
quired for  the  explanation  of  the  phenomenon. 

The  astronomers  have  noticed  that  a  faint 
star  can  be  seen  best  in  indirect  vision,  but  this 
may  be  because  the  fovea  centralis  is  fatigued 
by  light  reflected  from  the  sides  of  the  tele- 
scope. Aubert  could  not  notice  any  appre- 
ciable difference  in  the  intensity  of  light  seen 
in  direct  and  in  indirect  vision.  Schadow,1 
Kirschmann,2  and  others  have  published  in- 
vestigations on  the  relative  intensity  of  lights 

in  direct  and  in  indirect  vision,  with  conflicting  results.  It  seems  proba- 
ble, however,  that  the  peripheral  portions  of  the  retina  are  the  more  sen- 
sitive. The  appearance  of  a  color  differs  greatly  according  to  the  part  of 
the  retina  stimulated,  the  outer  zones  being  altogether  color-blind.  The 
perimeter  is  now  universally  used  in  the  clinic,  and  is  the  subject  of 
special  articles  in  this  work. 

The  apparent  intensity  of  light  and  color  is  greatly  influenced  by  the 
surroundings.  Thus,  a  gray  surface  on  white  appears  much  darker  than  on 
black.  These  effects  of  contrast  are  partly  due  to  movements  of  the  eyes 
and  fatigue,  and  will  be  noticed  below.  The  intensity  of  light  is  somewhat 
greater  when  it  affects  both  eyes  than  when  it  affects  one  only.  Jurin's 
result  that  the  difference  in  intensity  is  about  one-thirteenth  has  been  con- 
firmed by  later  observers.  An  apparently  conflicting  result  is  obtained  in 
a  curious  experiment  suggested  by  Fechner.  If  we  look  at  a  white  surface 
with  one  eye  and  then  open  the  other  with  a  smoked  glass  before  it,  the 
field  becomes  darkened,  although  the  total  amount  of  light  has  been  in- 
creased. This  experiment  is  related  to  the  conflict  of  the  fields  of  vision, 
which  subject  is  treated  elsewhere  in  this  work. 

PART  II.— TIME. 

X. — THE   THRESHOLD   AND   THE   MAXIMUM   SENSATION. 

When  the  area  of  a  light  is  very  small,  or  when  the  intensity  is  very 
faint,  it  cannot  be  seen.  It  also  holds  that  a  light  cannot  be  seen  when  its 
duration  is  very  short.  We  find,  therefore,  a  threshold  of  size,  a  threshold 

1  Die  Lichtempfindlichkeit  der  peripheren  Netzhauttheile  im  Verhaltniss  zu  deren 
Raum-  und  Farbensinn,  Arch.  f.  d.  ges.  Physiol.,  xix.  439-461.  1879. 

2  Die  Farbenempfindung  im  indirecten  Sehen,  Philos.  Studien,  viii.  692-614,  1893. 


524  THE    PERCEPTION   OF    LIGHT. 

of  intensity,  and  a  threshold  of  time.  It  is  of  especial  interest  to  physio- 
logical and  psychological  science  that  these  qualities  are  correlated  at  the 
threshold.  Thus,  the  threshold  of  intensity  depends  on  the  area  and  time 
of  stimulation,  and  a  measurable  decrease  in  intensity  is  compensated  by 
a  measurable  increase  in  area  or  time.  We  are,  consequently,  developing  a 
mechanics  of  the  nervous  system  and  of  sensation. 

The  fact  of  the  threshold  of  time  was  first  indicated  by  Plateau  and  by 
Fick.1  If  the  time  of  exposure  of  a  white  light  be  gradually  shortened, 
the  white  will  appear  gray  and  will  finally  disappear.  The  threshold  for 
white  light  has  not  been  exactly  measured,  but  investigations  with  colors 
have  been  carried  out  by  several  observers, — in  the  first  instance  by  v. 
Vierordt 2  and  his  pupils.  They  used  a  pendulum  myograph  which  carries 
a  screen  with  a  vertical  opening,  the  time  of  exposure  being  known  from 
the  rate  at  which  the  pendulum  swings  and  the  width  of  the  opening.  In 
order  that  the  light  reflected  from  pigments  when  the  time  of  exposure 
was  2.90  a  (ff  =  .001  second),  it  was  necessary  that  the  illumination  should 
be  from  one  hundred  and  seventy-three  to  five  hundred  and  thirteen  times 
as  great  as  when  the  time  was  not  limited.  As  the  time  was  lengthened 
the  intensity  decreased,  but  no  definite  relation  is  deduced  by  the  writers. 
Kunkel3  in  Fick's  laboratory  made  similar  experiments  with  colors  of  the 
spectrum.  He  found  the  threshold  to  be  different  with  different  colors,  and 
the  appearance  of  the  colors  to  vary  with  the  intensity  and  time  of  exposure. 
Thus,  green  and  even  yellow  may  appear  blue.  Such  changes  in  color  should 
be  compared  with  those  occurring  when  the  intensity  is  faint  and  when  the 
area  is  small.  Baxt4  in  v.  Helmholtz's  laboratory  determined  the  interval 
which  must  elapse  after  an  impression  has  been  in  view  a  short  time  (4.8  <r) 
in  order  that  it  may  be  obliterated  by  a  following  bright  light.  This  time 
varies  with  the  impression,  being  50  a  for  a  group  of  letters.  The  process 
in  this  case  is  complex,  but  the  time  may,  perhaps,  give  the  difference  in 
the  time  of  perception  of  the  impression  used  and  of  a  bright  light.  The 
writer6  has  published  experiments  measuring  the  time  threshold  for  colors, 
letters,  and  words.  For  colors  (pigments  compared  with  Chevreul's  tables 
and  reflecting  daylight  from  the  clear  sky)  the  average  times  for  seven 
observers  were  red  1.28,  orange  0.82,  yellow  0.96,  green  1.42,  blue  1.21,  and 

1  Ueber  den  zeitlichen  Verlauf  der  Erregung  in  der  Netzhaut,  Arch.  f.  Anat.  u. 
Physiol.,  1863,  739-764. 

2  Das  Pendel  als  Messapparat  der  Dauer  der  Gesichtseindriicke,  Arch.  f.  d.  ges.  Phys., 
ii.  121-127,  1869.     Bruckhardt  and  C.  Faber,  Versuche  iiber  die  zu  einer  Farbenempfin- 
dung  erforderliche  kleinste  Zeit,  Arch.  f.  d.  ges.  Physiol.,  ii.  127-142,  1869. 

3  Ueber  die  Abhangigkeitder  Farbenempflndung  von  der  Zeit,  Arch.  f.  d.  ges.  Physiol., 
ix.  197-220,  1874. 

*  Ueber  die  Zeit,  welche  nOtig  ist,  damit  ein  Gesichtseindruck  zum  Bewusstsein 
kommt  und  iiber  die  Grosse  (Extension)  der  bewussten  Wahrnehmung  bei  einem  Gesichts- 
eindrucke  von  gegebener  Dauer,  Arch.  f.  d.  ges.  Physiol.,  iv.  325-336,  1871. 

5  Ueber  die  Tragheit  der  Netzhaut  und  der  Sehcentrums,  Philos.  Stud.,  iii.  94-127, 
1885  ;  Abridged,  Brain,  viii.  295-312,  1885-86. 


THE   PERCEPTION   OF   LIGHT. 


625 


PIG.  11. 


violet  2.23  <r,  the  difference  being  probably  influenced  by  the  intensities  of 
the  colors.  The  times  tended  to  increase  in  arithmetical  progression  as  the 
intensity  of  the  light  decreased  in  geometrical  progression.  The  times  for 
single  letters  and  for  words  were  about  the  same,  but  the  times  for  different 
types  and  for  the  different  letters  of  the  same  type  were  not  the  same. 
The  relative  legibility  of  the  several  letters  was  thus  determined,  and  is 
shown  for  capital  small  pica  letters  in  the  accompanying  curve.  The  ordi- 
nates  represent  the  percentage  of  times  each  letter  was  correctly  read  in  two 
hundred  and  seventy  trials.  E,  the  letter  most  frequently  used,  is  thus  the 
most  illegible.  The  relative  legibility 
of  the  letters  is  of  practical  importance, 
as  the  more  illegible  letters  could  be 
modified  so  as  to  make  them  more 
easily  read.  It  is  also  of  interest  in 
this  place,  owing  to  the  use  made  of 
letters  in  determining  defects  of  ac- 
commodation. The  letters  selected  for 
this  purpose  should  be  as  nearly  as 
possible  equally  legible,  and  it  is  prob- 
able that  it  would  be  an  advantage  to 
use  geometrical  forms  such  as  those  pro- 
posed by  Snellen  or  the  series  planned 
by  Jastrow  and  Oliver  for  the  Colum- 
bian Exposition.  With  Snellen's  test- 
types  the  time  of  exposure  required  in  order  that  they  should  be  correctly 
seen  was  D  =  4,  0.65  ;  D  =  1.75,  0.82  ;  D  =  1.25,  1.23  j  D  =  0.8,  1.57 ; 
D  =  0.5,  3.50  <r.  It  is  not  unlikely  that  more  accurate  results  could  be 
obtained  in  clinical  work  if  the  time  of  exposure  were  measured. 

When  the  time  of  exposure  is  made  greater  than  the  threshold,  the 
intensity  of  the  sensation  increases,  and  when  the  time  is  in  the  neighbor- 
hood of  one-fifth  second  the  sensation  reaches  a  maximum.  This  can  be 
seen  by  cutting  a  narrow  opening  (five  millimetres  by  five  centimetres)  in  a 
black  screen  and  jerking  the  screen  over  small  pieces  of  colored  paper. 
When  the  movement  is  quick  and  the  time  of  exposure  very  short  (the 
threshold),  no  color  can  be  seen ;  as  the  time  is  increased  the  color  becomes 
brighter  and  brighter,  and  when  the  time  is  one-tenth  to  one-half  second 
the  color  seems  brighter  than  the  same  color  does  in  ordinary  vision.  A 
landscape  looked  at  in  this  way  appears  unusually  brilliant.  Briicke,1  with 
revolving  wheels,  found  the  maximum  sensation  to  occur  when  the  time  of 
stimulation  was  189  a  for  intermittent  lights.  More  extended  measurements 
with  white  lights  have  been  made  by  Exner,2  and  with  colored  light  by 

1  Ueber  den  Nutzeffect  intermittierender  Netzhautreizungen,  Sitzungsber.  d.  Wiener 
Akad  ,  Math.-Natur.,  Ixix.  (2)  128-153,  1864. 

2  Ueber  die  zu  einer  Gesichtswahrnehmung  nothige  Zeit,  Sitzungsber.  d.  Wiener 
Akad.,  Math.-Natur.,  Iviii.  (2)  601-632,  1868. 


526  THE    PERCEPTION   OF   LIGHT. 

Kunkel.1  The  former  found  the  times  to  decrease  in  arithmetical  progres- 
sion as  the  intensity  of  the  light  is  increased  in  geometrical  progression. 
The  latter  found  the  times  for  colors  reduced  to  about  the  same  intensity 
to  be  red  57,  green  133,  and  blue  92  a. 

XI. — FATIGUE. 

When  the  time  of  exposure  is  still  further  increased,  the  sensation 
begins  to  fade,  and  finally  loses  its  quality  and  may  disappear  altogether. 
The  effects  of  fatigue  may  be  seen  by  placing  a  small  black  cross,  as  shown 
in  Fig.  12,  on  a  sheet  of  white  paper.  If  after  looking  at  the  cross  for 

ten  seconds  it  be  jerked  away  by 
the  thread,  a  very  white  cross  will 
be  seen  on  the  sheet  of  paper.  The 
eye  has  become  fatigued  for  the 
white  excepting  the  part  covered 
by  the  cross,  which  consequently 
appears  the  brighter.  The  same 
experiment  may  be  made  for  colors 
by  placing  the  black  cross  on  a 
sheet  of  colored  paper.  The  part 

which  had  been  covered  by  the  cross  will  look  much  more  intense  and 
saturated  than  the  rest.  In  making  such  experiments  a  halo  is  usually  seen 
about  the  cross,  this  being  due  to  involuntary  movements  of  the  eyes. 
According  to  Fechner,2  a  bright  white  light  (as  white  paper  in  the  sunlight) 
does  not  simply  become  less  bright,  but  passes  through  a  series  of  colors. 
The  white  at  first  looks  yellow,  then  blue-green  or  blue,  and  finally  red- 
violet  or  red.  These  changes  in  color  are  thought  by  Fechner  to  be  due  to 
some  of  the  components  of  white  light  producing  fatigue  sooner  than  the 
others. 

Hess 3  has  recently  made  a  complete  study  of  the  appearance  of  spec- 
trum colors  after  the  eye  had  previously  been  fatigued  for  certain  colors. 
He  used  nine  points  in  the  spectrum  and  two  combinations  of  red  and 
violet,  and  was  able  to  obtain  quantitative  results.  He  thinks  the  alter- 
ations do  not  accord  with  the  requirements  of  v.  Helmholtz's  color  triangle. 
If  a  small  colored  bit  be  placed  on  a  sheet  of  gray  paper  it  can  be 
looked  at  until  the  color  disappears  altogether.  As  first  shown  by  Maria 
Bokowa,4  fatigue  amounting  to  color-blindness  may  be  brought  about  by 

1  Ueber  die  Abhangigkeit  der  Farbenempfindung  von  der  Zeit,  Archiv.  f.  d.  ges. 
Physiol.,  ix.  197,  1874. 

2  Ueber  die  subjectiven  Nachbilder  und  Nebenbilder,  Pogg.  Ann.  d.  Phys.  u  Chem., 
1.  193-221,  427-470,  1840. 

3  Ueber  die  Tonanderungen,  der  Spectralfarben  durch  Ermiidung  der  Netzhaut  mit 
homogenem  Lichte,  Arch.  f.  Ophth.,  xxxvi.  (1)  1-32,  1890. 

*  Ein  Verfahren  kunstliche  Farbenblindheit  hervorzubriugen,  Zeitsch.  f.  ration.  Med. 
3.  Keihe,  xvii.  161-165,  1863. 


THE    PERCEPTION   OF   LIGHT.  527 

wearing  spectacles  with  colored  glasses,  all  side-light  being  cut  off.  Indeed, 
the  same  fact  is  illustrated  by  comparing  our  sensation  on  going  from  the 
daylight  into  a  room  lit  up  by  gas  or  lamps  with  that  which  we  have  in  the 
same  room  in  the  evening.  In  the  former  case  the  light  seems  reddish,  in 
the  latter  we  notice  no  color.  Quantitative  determinations  of  fatigue  have 
been  obtained  by  C.  F.  Miiller 1  in  Fick's  laboratory,  by  Exner,  and  by 
Schon.2  According  to  Miiller,  if  the  intensity  of  the  original  sensation 
be  1,  the  intensity  after  three  seconds  will  be  0.72,  after  five  seconds  0.66, 
after  ten  seconds  0.49,  after  fifteen  seconds  0.46,  after  twenty  seconds  0.43, 
after  twenty-five  seconds  0.37,  and  after  thirty  seconds  0.35.  Fatigue  con- 
sequently follows  most  rapidly  at  first,  and  more  slowly  afterwards,  the 
apparent  intensity  waning  to  half  in  about  ten  seconds.  Fullerton  and 
the  writer  have  found  that  when  two  lights  are  viewed  in  succession  the 
second  is  apt  to  appear  the  fainter,  the  constant  error  being  on  the  average 
one-twelfth  of  the  light.  Schon  used  colors  of  the  spectrum  and  obtained 
results  corresponding  to  Miiller's.  After  three  seconds  red  decreased  to 
0.59,  green  to  0.52,  and  blue  to  0.37.  The  visual  mechanism  is  most 
sensitive  when  we  first  awake  in  the  morning.  According  to  Miiller,  the 
sensitiveness  decreases  during  the  day,  and  objects  appear  only  half  as 
bright  in  the  evening  as  in  the  early  morning.  If  this  were  the  case, 
the  time  of  day  should  be  considered  in  making  tests  for  the  sharpness  of 
vision.  Recent  experiments  by  Fick  and  Griiber,  however,  show  that  fatigue 
reaches  its  maximum  in  three-quarters  of  an  hour  or  less  after  awakening, 
and  that  so  long  as  the  light  is  kept  constant  no  further  decrease  in 
sensitiveness  occurs  in  the  course  of  the  day.  These  writers  hold  that 
the  sensitiveness  of  the  retina  is  restored  by  movements  of  the  eyelids  and 
of  accommodation ;  but  this  view  seems  to  be  refuted  by  Heriiig. 

The  relation  of  the  sensa- 
tion to  the  time  of  stimulation 
is  shown  in  the  accompanying 
curve.      The  abscissae  repre- 
sent the  time  of  stimulation, 
and  the  ordinates  the  intensity 
of  the   corresponding   sensa- 
tion.     The  curve  is  divided 
in  two,  the  right-hand  half 
being   drawn    in    a   different 
scale  as  regards  time.      The 
curve  shows  that  a  short  time  elapses  before  there  is  any  sensation,  the 
sensation  then  increases  with  the  time,  at  first  rapidly  and  then  more  slowly, 
to  a  maximum,  and  afterwards  begins  to  wane,  and  more  slowly  as  the  time 
becomes  longer. 

1  Versuche  iiber  den  Verlauf  der  Netzhautermiidung,  Inaug.  Diss.,  Zurich,  1866. 
1  Einfluss  der  Ermudung  auf  die  Farbenempfindung,  Arch.  f.  Ophth.,  xx.  (2)  273- 
286,  1874. 


528  THE   PERCEPTION    OF   LIGHT. 

The  threshold  and  the  maximum  sensation  are  accounted  for  by  the 
principles  of  inertia.  It  requires  a  certain  amount  of  energy  to  set  the 
visual  mechanism  in  motion,  and  a  certain  further  amount  of  energy  to 
excite  it  fully.  The  correlation  between  intensity,  extensity,  and  time  may 
thus  be  understood,  for  we  are  concerned  with  the  total  amount  of  energy 
which  may  be  contributed  chiefly  by  greater  force,  increased  size,  or  longer 
time.  Thus,  with  the  intense  light  of  the  electric  spark  objects  may  be 
seen,  although  the  time  of  exposure  be  yery  short,  whereas  a  faint  star 
can  be  seen  only  after  looking  for  some  time  at  the  point  in  the  heavens 
at  which  it  will  appear.  The  waning  of  the  sensation  as  the  stimulation 
is  continued  is  accounted  for  in  a  satisfactory  manner  by  fatigue  and  ex- 
haustion. We  can  understand  that  the  substance  in  the  retina  (supposing 
light  to  start  a  nervous  impulse  by  chemical  action)  may  become  decom- 
posed during  stimulation  faster  than  it  can  be  built  up  by  the  cells,  as  a 
photographic  film  becomes  less  sensitive  as  it  is  exposed  to  the  light.  The 
exhaustion  may  be  also  of  brain-centres.  The  waning  of  the  sensation  as 
the  time  of  stimulation  continues  is  somewhat  analogous  to  the  relations  of 
intensity  reviewed  in  the  first  part  of  this  article.  We  saw  that  the  sensa- 
tion seemed  to  increase  more  slowly  than  the  energy  of  the  stimulus. 

The  times  which  we  have  been  considering  would  undoubtedly  vary 
with  the  condition  of  the  eye  and  of  the  nervous  system,  but,  unfortu- 
nately, we  do  not  know  anything  at  present  about  the  relations.  The 
time  of  exposure  required  in  order  that  a  color  may  be  recognized  would 
be  an  accurate  test  and  measure  of  color-blindness.  The  rate  of  fatigue 
and  restoration  would  probably  be  affected  by  disease  of  the  eye  and  of  the 
nervous  system,  and  might  prove  surprisingly  useful  in  diagnosis. 

XII. — POSITIVE    AFTER-IMAGES. 

The  same  principles  of  inertia  which  demand  a  certain  time  to  set  the 
nervous  mechanism  in  motion  would  lead  us  to  expect  that  this  motion 
will  continue  after  the  stimulation  ceases.  We  in  fact  find  that  a  rocket 
or  a  meteor  is  seen  as  a  line  of  light,  due  to  the  impression  persisting  after 
the  stimulation  is  no  longer  present.  Aristotle,  who  has  in  so  many  cases 
anticipated  the  results  of  modern  science,  seems  to  have  first  described 
after-images,  and  with  great  exactness.  The  next  reference  to  the  subject 
seems  to  be  by  Augustine  ;  then,  after  a  long  interval,  Pieresc  (1634)  de- 
scribed the  positive  and  negative  after-image  of  the  window  cross-bar,  and 
the  subject  attracted  the  attention  of  Mariotte,  Newton,  Buffon,  Darwin, 
Goethe,  and  others.  Our  present  knowledge  of  the  subject  we  owe  largely  to 
Plateau,1  Fechner,2  Seguin,3  Aubert,4  and  v.  Helmholtz.6  In  spite  of  the 

1  Loc.  cit.  2  Loc.  cit. 

*  Kecherches  sur  les  couleurs  accidentelles,  Ann.  de  Chem.  et  de  Phys.,  Ser.  3,  xli. 
413-431,  1854;    Memoires  sur  les   couleurs  accidentelles,   Compt.    rend,   de   1'Acad.  des 
Sciences,  xxxiii.  642-644,  1851;  xxxiv.  767,  768,  1852;  xxxv.  476,  1852. 

*  Loc.  cit.  5  Loc.  cit. 


THE    PERCEPTION    OF    LIGHT.  ')'1\\ 

large  amount  of  work  which  has  been  devoted  to  the  investigation  of  after- 
images —  v.  Helmholtz  gives  twenty-four  references,  which  number  might 
readily  be  doubled  —  the  results  are  far  from  being  accordant  or  complete. 
After-images  are,  indeed,  peculiarly  difficult  to  investigate,  because  a  great 
deal  of  practice  is  required  before  they  can  be  properly  observed,  whereas 
the  eyes  are  apt  to  be  injured  by  such  observations.  Fechner's  experi- 
ments entailed  the  loss  of  his  eyesight,  and  several  other  observers  have 
injured  their  eyes  in  studying  after-images  and  entoptic  phenomena.  Those 
undertaking  the  investigation  of  these  subjects  should  continue  their  work 
for  a  series  of  years  and  make  only  a  few  observations  at  one  time. 

In  order  to  observe  a  positive  after-image,  a  bright  object,  as  the  sun  or 
the  globe  of  a  lamp,  should  be  looked  at  for  a  short  time,  say  one-half 
second.  This  can  be  done  conveniently  by  holding  a  black  screen  (pierced 
in  oj*der  to  secure  a  point  of  fixation)  before  the  eyes  and  uncovering  the 
object  for  a  moment.  The  eyes  should  be  rested  (closed  and  covered)  for 
from  one  to  five  minutes  previously,  so  that  traces  of  previous  after-images 
may  disappear  and  nothing  be  left  in  the  field  of  vision  excepting  the  light 
chaos  or  "  own  light"  of  the  eye.  Care  should  be  taken  to  avoid  movements 
of  the  eye  or  body.  Observers  often  find  it  difficult  to  notice  after-images  at 
first,  but  my  students  have  always  been  able  to  see  them  after  a  little  practice. 

The  positive  after-image  maintains  the  same  relations  of  light  and 
shade  as  the  original  objects,  and  the  colors  may  be  the  same  at  first,  but 
these  quickly  change.  When  the  eyes  are  covered  after  exposure  to  a 
bright  object,  nothing  is  seen  at  first  by  most  observers,  but  after  a  couple 
of  seconds  the  after-image  appears  in  the  light  chaos  of  the  field  of  vision. 
v.  Helmholtz  holds  that  the  delay  in  the  appearance  of  the  after-image  is 
due  to  movements  of  the  eyes  and  body  ;  but  this  can  scarcely  be  the  case, 
as  the  after-image  does  not  appear  at  once,  even  when  no  appreciable  move- 
ments are  made,  and  does  appear  in  due  time,  although  movements  be  pur- 
posely made.  Indeed,  according  to  recent  observations  by  Hess,  the  effects 
of  the  stimulus  disappear  almost  immediately  and  give  place  to  a  negative 
after-image  (difficult  to  notice),  and  this  is  followed  by  the  positive  after- 
image, which  we  are  here  considering,  v.  Vintschgau  and  Lustig  have 
also  measured  the  latent  period  of  an  after-image.1 

In  the  after-image  details  can  often  be  noticed  which,  owing  to  lack  of 
time  or  to  intense  brightness,  were  not  seen  in  the  original  object.  Thus, 
the  twigs  of  a  tree  may  be  imperceptible  when  they  are  between  the  eyes 
and  the  sun,  but  may  become  apparent  in  the  after-image.  As  the  inin^' 
fades,  the  relations  of  light  and  shade  change,  the  brighter  parts  lasting  the 
longer.  The  further  course  of  the  positive  after-image  becomes  complicated 
with  the  results  of  fatigue  or  exhaustion,  and  its  consideration  must  be 
postponed  until  negative  after-images  have  been  noticed. 


1 


Zeitmessende  Beobnchtungen  iiber  die  Wahrnehmnng  des  sich  entwickelnden  p.-i- 
tiven  Nachbildes  eines  electrischen  Funkens,  Arch.  f.  d.  ges.  Phys.,  xxxiii.  494-512,  1884. 
VOL.  I.—  34 


530  THE    PERCEPTION    OF    LIGHT. 

XIII. — NEGATIVE   AND   COMPLEMENTARY   AFTER-IMAGES. 

When  an  object  is  not  very  bright  and  is  looked  at  for  several  seconds 
or  longer,  the  positive  after-image  can  be  perceived  with  difficulty  or  not  at 
all,  but  the  stimulation  leaves  effects  which  are  called  negative  after-images. 
In  these,  as  in  the  negative  plate  of  a  photograph,  the  relations  of  light 
and  shade  are  reversed,  so  that  a  bright  object  on  a  dark  background  be- 
comes a  dark  object  on  a  bright  background.  Such  negative  after-images 
we  have  already  met  in  the  section  on  fatigue.  They  can  also  be  seen  in 
the  field  of  vision  when  the  eyes  are  closed,  or  can  be  projected  on  any 
surface. 

The  color  of  the  negative  after-image  is  in  normal  cases  complementary 
to  the  color  of  the  original  object.  It  is  a  curious  and  important  fact  that  in 
the  case  of  red  the  after-image  may  be  positive  and  complementary.  Fech- 
ner1  has  made  the  most  careful  study  of  the  color  of  negative  after-images, 
considering  the  background  on  which  the  object  is  exhibited  and  the  color 
of  the  field  on  which  it  is  projected. 

While  the  color  of  the  negative  after-image  may  be  said  to  be  comple- 
mentary to  the  color  of  the  original  light,  it  is  not  established  that  the 
relation  is  exact,  and  cases  have  been  recorded 2  in  which  it  does  not  hold 
at  all.  Hilbert3  has  recently  described  his  own  case,  in  which  the  color 
of  the  after-image  is  entirely  altered  when  he  is  fatigued.  It  is  possible 
that  the  nature  and  course  of  after-images  may  prove  unexpectedly  useful 
in  the  diagnosis  of  certain  diseases  of  the  eye  and  of  the  nervous  system. 

The  complementary  color  of  negative  after-images  is  accounted  for  in  a 
general  way  by  fatigue :  the  eye  has  become  exhausted  for  the  color  at 
which  it  has  been  looking,  and  the  complementary  components  of  white 
light  produce  relatively  greater  effects.  When  the  after-image  is  projected 
on  a  colored  field  complementary  to  the  original  light,  the  color  (even  of 
the  sun's  spectrum)  appears  brighter  and  more  saturated  than  otherwise. 
Colors  more  intense  and  beautiful  than  can  be  imagined  may  be  seen  by 
looking  for  one-fourth  second  at  a  part  of  the  spectrum  early  in  the  morn- 
ing after  the  night's  rest,  the  eyes  having  previously  been  exposed  for  one 
minute  to  the  complementary  color. 

The  longer  the  time  of  fixation  the  longer  does  the  negative  after- 
image last;  indeed,  Purkinje  goes  so  far  as  to  state  that  there  is  an  exact 
proportion,  each  additional  second  of  fixation  (of  a  candle)  increasing  the 
duration  of  the  after-image  twenty  seconds.  Aubert  found  that  when  the 
sun  was  regarded  for  three  seconds  the  after-image  lasted  two-thirds  of  a 
minute;  when  the  time  of  regard  was  five  seconds  the  image  lasted  about 
five  minutes ;  when  eight  seconds,  it  was  about  ten  minutes.  The  after- 
image also  lasts  the  longer  the  brighter  the  light  of  the  original  object. 
In  the  writer's  laboratory  exact  measurements  are  being  made  of  the 

1  Loc.  cit.  2  Cf.  Aubert,  Physiol.  Optik,  p.  562. 

3  Zur  Kenntniss  des  successiven  Kontrastes,  Zeitsch.  f.  Psychol.,  iv.  74-77,  1892. 


THE   PERCEPTION   OF   LIGHT.  531 

duration  of  after-images  regarded  as  a  function  of  the  time,  intensity,  and 
area  of  stimulation.  The  average  duration  of  the  after-image  increased 
from  eight  to  fourteen  seconds  as  the  time  of  exposure  was  increased  from 
three  to  twenty  seconds,  and  from  eight  to  sixty  seconds  as  the  intensity 
of  the  light  was  increased  three-hundred-and-twenty-fold. 

The  apparent  size  of  the  after-image  depends  on  the  distance  of  the  field 
on  which  it  is  projected :  thus,  the  after-image  of  a  surface  one  centimetre 
square,  regarded  at  a  distance  of  thirty  centimetres,  when  projected  on  a 
wall  appears  enlarged  in  proportion  to  the  distance  of  the  wall.  This  is, 
of  course,  explained  by  the  fact  that  a  small  object  near  by  stimulates  the 
same  area  on  the  retina  as  a  larger  object  farther  away.  If  the  after-image 
from  a  surface  one  centimetre  square  cover  as  much  space  on  a  distant  wall 
as  a  picture,  we  judge  it  to  be  as  large  as  the  picture.  The  shape  of  an 
after-image,  as  of  a  cross  or  a  circle,  is  distorted  when  it  is  projected  on  an 
oblique  surface.  Thus,  the  after-image  of  a  cross  A  (Fig.  14)  will  appear 


as  B  when  projected  on  the  wall  above  and  to  the  left  of  the  observer ;  to 
the  right  it  will  appear  as  C.  This  is  the  converse  of  the  fact  that  in  our 
daily  life  a  distorted  image  in  this  position  would  be  the  result  of  a  true 
cross,  and  would  be  interpreted  as  a  true  cross. 

After-images  are  not  usually  observed,  it  being  a  psychological  law  that 
we  attend  only  to  those  things  which  interest  us.  Sometimes  people  notice 
after-images  or  entoptic  phenomena  late  in  life  for  the  first  time,  and  fancy 
that  something  is  wrong  with  their  eyesight,  and  may  even  consult  a  phy- 
sician. The  physician  should,  consequently,  be  prepared  to  explain  how 
far  these  phenomena  are  normal.  While  after-images  are  not  distinctly 
distinguished,  yet.  they  must  greatly  alter  the  appearance  of  the  external 
world.  Thus,  moving  objects  are  fused,  and  produce  a  total  result  which 
consciousness  is  unable  to  analyze.  Not  only  so,  but,  our  eyes  being  con- 
stantly in  motion,  the  appearance  of  objects  is  greatly  altered  by  what  we 
have  previously  looked  at.  We,  to  a  large  degree,  correct  for  these  altera- 
tions ;  we  need  to  perceive  the  same  objective  things  as  the  same,  and  do  so 
to  a  considerable  extent  in  spite  of  subjective  differences.  But  to  all  of  us 
a  piece  of  gray  paper  on  a  white  surface  looks  darker  than  the  same  gray 
on  black  paper.  This  is  partly  due  to  movements  of  the  eyes ;  the  parts 
of  the  retina  which  had  been  exhausted  by  white  are  not  so  effectively 
stimulated  by  the  gray  as  those  parts  which  had  been  rested  by  looking  at 


532  THE    PERCEPTION   OF    LIGHT. 

black.  In  the  same  manner  red  looks  much  brighter  on  a  green  surface 
than  on  an  orange  surface.  These  results  persist  even  when  care  is  taken 
not  to  move  the  eyes,  and  are  termed  "  contrast."  But  the  phenomena  are 
evidently  not  explained  by  naming  them.  Hering  maintains  that  the  ex- 
citation spreads  in  the  retina,  so  that  not  only  the  parts  stimulated  by  white 
but  also  the  neighboring  parts  are  exhausted,  v.  Helmholtz,  on  the  other 
hand,  maintains  that  the  alterations  are  due  to  mistaken  judgment,  our 
perceptions  being  relative.  Thus,  a  medium-sized  man  may  seem  tall  in 
the  company  of  a  short  man  and  short  in  the  company  of  a  tall  man.  The 
effects  of  contrast  are  probably  determined  by  various  causes,  some  of 
which  we  do  not  understand.  The  subject  is  very  important  for  the  artist, 
especially  for  the  painter,  who  must  depend  on  contrast  to  secure  light  and 
color.  Thus,  a  painted  scene  by  sunlight  is  far  less  bright  and  a  scene  by 
moonlight  far  brighter  than  nature,  the  effects  being  secured  by  proper  atten- 
tion to  contrast.  The  subject  of  contrast  is  treated  elsewhere  in  this  work. 

XIV. — THE   COMPLICATION   OF   POSITIVE    AND    NEGATIVE   AFTER-IMAGES. 

The  course  of  the  after-image  is  not  so  simple  and  regular  as  might 
be  supposed  from  reading  the  accounts  usually  given  in  the  text-books  of 
psychology  and  physiology.  Nor  is  the  theoretical  explanation  offered  by 
the  Young-Helmholtz  theory  as  satisfactory  as  is  commonly  supposed. 
When  a  bright  object  is  looked  at  for  an  instant  the  positive  after-image  is 
seen  as  above  described,  and  may  not  be  followed  by  other  images,  and 
when  a  rather  faint  color  (as  a  pigment  in  diffused  daylight)  is  looked  at 
for  some  time  the  negative  and  complementary  after-image  is  seen  without 
further  complications.  But  very  striking  results  may  follow  the  fixation 
of  a  white  light.  These  have  been  described  by  Plateau,  Fechner,  Seguin,  v. 
Helmholtz,  and  Aubert.  If  bright  white  light  be  looked  at  for  a  moment, 
according  to  v.  Helmholtz,  there  will  be  a  white  after-image  which  passes 
quickly  through  a  green-blue  to  a  brilliant  indigo-blue  and  then  into  violet 
or  rose.  Then  follows  a  gray-orange,  the  after-image  usually  disappearing 
or  becoming  negative.  In  the  latter  case  the  orange  may  be  followed  by  a 
dim  yellow-green.  The  order  and  nature  of  the  colors  vary  according  to 
the  time  of  exposure  and  the  intensity  of  the  light.  Admitting  light  or 
projecting  the  image  on  a  brighter  field  advances  the  course  of  the  image} 
and,  conversely,  decreasing  the  light  brings  it  back  to  an  earlier  stage.  If 
the  sun  be  viewed  for  an  instant,  similar  results  follow,  but  there  are  con- 
centric rings  of  color  which  proceed  from  the  outside  towards  the  centre. 
Owing  to  imperfect  fixation  and  accommodation  (and,  it  may  be,  to  the 
spreading  of  stimulation  on  the  retina),  the  image  extends  beyond  the  true 
disk  of  the  sun,  and,  the  outer  circles  being  less  intense,  the  after-image 
proceeds  more  rapidly  through  its  phases. 

The  descriptions  of  Fechner  and  v.  Helmholtz  seem  largely  to  ignore 
oscillations  of  the  after-image,  ascribing  such  as  occur  to  accidental  move- 
ments, pressures,  and  changes  of  illumination.  The  oscillations  have,  how- 


THE    PERCEPTION   OF   LIGHT.  533 

ever,  been  properly  described  by  Purkinje,  Plateau,  and  Aubert,  and  are  to 
the  writer  the  most  striking  of  all  the  phenomena.  These  authors  have 
noted  four  or  five  oscillations  from  positive  to  negative,  the  after-image 
lasting  in  all  about  five  minutes.  The  writer  has  made  the  unexpected 
observation  that  an  after-image  may  last  indefinitely,  the  oscillations  from 
positive  to  negative  being  innumerable.  The  writer  obtained  (after  resting 
the  eyes  five  minutes  and  exposing  them  for  one  minute)  an  after-image  of 
the  clear  sky  and  the  bars  of  a  window,  which  can  be  seen  at  the  present 
writing,  after  an  interval  of  eight  months.  During  the  first  hour  the  oscil- 
lations occurred  continually,  at  first  at  intervals  of  about  ten  seconds,  the 
panes  and  bars  displaying  brilliant  and  beautiful  colors,  mostly  greens  and 
purples.  In  the  course  of  the  first  month  the  after-image  became  gradually 
less  distinct.  On  closing  the  eyes  it  always  appeared  positive,  becoming 
negative  after  a  few  seconds,  and  passing  through  a  series  of  oscillations 
which  could  be  continued  indefinitely  by  altering  the  illumination.  Since 
that  time  the  after-image  has  become  continually  less  distinct.  On  closing 
the  eyes  it  always  appears  positive,  becoming  negative  only  so  far  as  lines 
of  light  appear  along  the  dark  bars.  The  colors  of  the  panes  are  dim 
yellow  and  violet.  When  projected  on  a  bright  surface,  as  the  sky,  the 
after-image  is  negative.  This  result  has  been  partially  confirmed  by  a 
second  observer,  in  whose  case  the  after-image  lasted  three  weeks,  after 
which  it  could  not  be  distinctly  observed.1 

The  theory  of  Fechner  and  v.  Helmholtz  that  positive  after-images 
are  due  to  persistence  of  motion  in  the  retina  and  that  negative  and  com- 
plementary after-images  are  due  to  exhaustion  offers  a  r6ugh-and -ready 
explanation  of  the  more  evident  phenomena,  but  is  inadequate  in  many 
ways.  The  recurrence  of  the  positive  after-image  several  times,  as 
observed  by  Purkiuje,  Plateau,  and  Aubert,  and  by  all  the  students  of  the 
writer,  and  its  duration  for  many  months  in  the  observation  given  above, 
cannot  be  due  to  simple  persistence  of  the  original  stimulation.  According 
to  v.  Helmholtz,  the  color  of  negative  after-images  is  due  to  exhaustion  of 
the  eye  for  the  original  color  and  the  consequent  prepqnderance  of  other 
colors  when  the  eye  is  stimulated  by  white  light.  But  this  is  contradicted 
by  the  great  brilliancy  of  colors  and  the  fact  that  they  are  seen  best 
when  all  external  light  is  excluded.  The  theory  of  Bering  accounts 
better  for  the  facts,  and  is  more  adequate  from  the  point  of  view  of 
psychology.  But  it  is  not  easy  to  believe  that  a  process  of  assimilation 
is  accompanied  by  a  sensation,  especially  one  that  differs  only  in  quality 
from  the  accompanying  process  of  dissimilation.  Theories  of  color- 
vision  have  recently  been  elaborated  by  Wundt,  Donders,  Franklin,  and 
Ebbinghaus,  and  it  is  certainly  time  to  stop  teaching  the  Young-Helni- 
holtz  theory  in  elementary  text-books  of  physics  and  physiology.  To 

1  Since  writing  the  above  1  have  read  in  a  letter  by  Newton  (Brewster's  Life)  a  descrip- 
tion of  a  persistent  after-image  of  the  sun. 


534 


THE   PERCEPTION   OF   LIGHT. 


FlQ.  15. 


the  psychologist  it  is  gratifying  to  know  that  the  facts  are  obtained  by 
introspection  and  psychological  experiment,  the  physiological  theories  being 
based  on  these  facts  and  not  on  any  knowledge  of  processes  in  the  eye  and 
the  brain.  The  psychological  facts  are  established,  whereas  the  physio- 
logical theories  are  in  dispute. 

XV. — INTERMITTENT  STIMULATION. 

One  of  the  most  important  consequences  of  after-images  and  the  in- 
ertia of  the  nervous  mechanism  is  the  fusion  of  stimuli  which  follow  each 
other  in  rapid  succession.  Thus,  in  the  case  of  the  disks  which  have 
already  been  referred  to  and  illustrated  (Fig.  3)  the  sectors  fuse  and  give 
a  uniform  sensation.  The  phenomenon  can  be  illustrated  by  the  accom- 
panying figure.  Suppose  the  eye  to  be  stimulated  by  the  lights  OA,  BC, 

DE,  etc.,  there  being  alter- 
nate pauses  AS,  CD,  EF, 
etc.  Time  is  thus  repre- 
sented by  the  horizontal  line, 
the  abscissa  (each  subdivision 
being  say  0.01  second),  and 
intensity  of  sensation  by  the 
vertical  lines  (the  ordinates), 
the  light  being  such  as  would 
give  an  intensity  of  sensation 
OX.  If  there  were  no  inertia 
of  the  visual  mechanism,  one 
would  have  for  0.01  second  a 
~~  sensation  equal  to  OX,  then 
for  0.01  second  a  complete 

absence  of  sensation,  then  another  sensation  for  0.01  second,  etc.  As  a 
matter  of  fact,  however,  in  the  first  0.01  second  the  light  only  partly  pro- 
duces its  effect  (as  explained  in  Section  X.),  and  one  has  a  sensation  whose 
intensity  is  represented  by  Aa.  Then  when  the  light  ceases  for  0.01  second 
the  motion  of  the  visual  mechanism  does  not  subside  entirely,  but  gradually, 
the  sensation  after  0.01  second  being  Bb.  A  new  stimulation  carries  the 
sensation  to  Cc,  which  in  the  pause  falls  to  Dd,  and  so  on,  until  the  amount 
of  fall  during  each  pause  is  equal  to  the  amount  of  rise  during  each  stimu- 
lation. If  the  times  be  longer  than  about  0.01  second,  there  will  be  a 
flickering  sensation,  the  rise  and  fall  of  sensation  being  given  in  conscious- 
ness. If  the  times  are  shorter  than  about  0.01  second,  the  intermittent 
stimulation  gives  a  uniform  sensation.  This  would  follow  from  the  facts 
discussed  under  the  perception  of  small  differences  (Section  II.).  If  the 
time  be  so  short  that  the  fall  in  sensation  is  less  than  would  occur  were  the 
light  decreased  by  say  one-hundredth  of  its  amount,  no  difference  will  be 
perceived. 

Alternate  stimulations  and  pauses  can  be  given  most  conveniently  by 


OABCDEFGHIJK 


THE    PERCEPTION   OF    LIGHT.  535 

means  of  revolving  wheels.  The  pause  can  be  secured  by  painting  one-half 
or  other  sector  of  the  wheel  black,  or  (as  all  pigments  reflect  some  light)  the 
pause  can  be  made  complete  by  revolving  a  sector  before  a  dark  box.  The 
intervals  at  which  fusion  occurs  become  longer  as  the  light  is  taken  fainter. 
This  is  probably  due  to  the  fact  that  the  just  perceptible  difference  is  a 
larger  part  of  a  faint  light,  v.  Helmholtz  found  that  ten  revolutions  per 
second  were  sufficient  to  cause  fusion  in  the  light  of  the  full  moon,  and 
twenty-four  revolutions  per  second  in  strong  lamplight.  Others  (Plateau, 
Aubert,  and  the  present  writer)  find  a  more  rapid  rate  of  revolution  neces- 
sary—about fifty  per  second — in  ordinary  diffused  daylight,  the  duration 
of  the  stimulation  and  pause  together  being  thus  about  0.02  second.  Ferry 
has  found  that  the  duration  decreases  from  about  0.04  to  about  0.02  second 
as  the  intensity  of  the  light  is  increased  twenty-four-fold,  and  deduces  the 
law,  "  the  duration  of  retinal  impression  is  inversely  proportional  to  the 
logarithm  of  the  luminosity."  Plateau  and  Emsmann  found  the  period 
at  which  fusion  occurs  to  be  dependent  on  the  color ;  but  Ferry  has  shown 
that  the  difference  in  time  is  probably  due  to  the  varying  intensities  of  the 
colors. 

When  complete  fusion  occurs,  and  the  sensation  is  uniform,  the  inten- 
sity is  the  same  as  would  occur  were  the  same  amount  of  light  spread  uni- 
formly over  the  disk.     This  was  first  stated  by  Talbot  (1834),  and  is  known 
as  Talbot's  law.     It  has  been  verified  experimentally  by  Plateau,  Fick, 
and  v.  Helmholtz.     After  fusion  occurs,  increasing  the  rate  of  stimulation 
does  not  alter  the  intensity  of  illumination.     This  can  readily  be  demon- 
strated by  a  disk  such  as  is  shown  in  the  figure.     When  this  is  revolved 
fifty   times   per   second,  the    inner   circle, 
where  there  are  fifty  interruptions,  and  the 
outer  circle,  where  there  are  one  hundred 
interruptions,  appear  of  the  same  intensity. 
Before  the  disk  revolves  rapidly  enough  to 
secure  complete  fusion  it  is  brighter  than 
when  complete  fusion  occurs,  the  sensation 
having  time  to  arrive  at  a  maximum  (cf. 
Section  X.).     Revolving  wheels  are  one  of 
the  most  convenient  methods  for  measuring 
the  intensity  of  lights  and  also  for  mingling 
colors.     The  fusion  of  spectrum  colors  has 
been  studied  by  Nichols  and  Ferry,  who 

revolved  dark  sectors  before  the  colors,  and  spectrum  colors  could  be  mixed 
by  means  of  revolving  prisms  or  mirrors. 

The  fusion  of  sensation  gives  the  impression  of  movement  in  the  in- 
strument (known  as  a  toy)  called  the  stroboscope,  or  "  wheel  of  life."  By 
various  methods  objects  in  the  position  they  would  have  at  intervals  in  the 
course  of  movements  are  shown  successively,  and  the  impressions  fuse  and 
give  the  appearance  of  natural  movements.  Lissajous's  curves,  showing 


536  THE    PERCEPTION   OF   LIGHT. 

the  vibrations  of  tuning-forks,  also  depend  on  this  principle.  Conversely, 
in  daily  life  an  object  in  motion  makes  a  total  impression,  and  we  are 
unable  to  distinguish  the  separate  positions.  Instantaneous  photographs 
(as  those  by  Muy bridge)  show  the  positions  of  animals  when  moving,  and 
these  are  such  as  no  one  could  have  perceived  by  looking  at  the  animal. 

XVI. — THE   REACTION-TIME   ON    LIGHT   AND   THE    TIME   OF   PERCEPTION. 

In  treating  the  time-phenomena  of  vision  we  have  so  far  considered  the 
relation  between  the  time  of  stimulation  and  the  nature  of  sensation,  and 
the  continuation  of  sensation  after  the  stimulation  ceases.  We  should  not, 
however,  neglect  the  time  required  to  convert  the  physical  energy  into  a 
nervous  impulse,  the  time  required  to  transmit  the  nervous  impulse  along 
the  optic  nerve  to  the  brain,  and  the  time  taken  up  in  cerebral  changes  and 
in  consciousness  before  the  impression  is  perceived.  These  times  have  often 
been  confused  with  the  time  threshold,  the  time  of  stimulation  for  maximum 
sensation,  and  with  one  another.  Yet  the  processes  are  entirely  different, 
and  the  times  are  not  only  different  but  independent.  Thus,  in  a  general 
way,  the  time  a  light  must  work  on  the  retina  in  order  to  excite  a  sensation 
may  be  0.001  second,  the  time  it  must  work  to  call  forth  the  maximum  sen- 
sation may  be  0.02  second,  the  impulse  may  travel  along  the  optic  nerve  at 
the  rate  of  sixty  metres  per  second,  and  the  time  of  perception  may  be  0.05 
to  0.2  second.  The  first  three  times  depend  largely  on  the  intensity  of  the 
light ;  the  last  time  depends  chiefly  on  the  complexity  of  the  impression, 
whereas  the  rate  in  the  nerve  is  probably  independent  of  either  intensity  or 
complexity. 

Science  has  not  succeeded  in  determining  separately  the  times  we  are 
now  considering,  but  a  complex  time  which  includes  them  may  be  measured 
with  great  accuracy.  This  is  the  reaction-time,  the  time  elapsing  after  a 
light  or  other  stimulus  has  occurred  until  a  movement  is  made.  The 
process  requires  0.1  to  0.2  second.  Its  time,  the  factors  of  which  it  is 
composed,  and  its  alterations  under  various  conditions  have  been  studied 
by  many  observers.  The  best  general  treatment  of  the  subject  is  given  by 
Jastrow1  and  Wundt.2 

During  the  time  of  the  reaction  the  light  is  converted  into  a  nervous 
impulse,  the  impulse  travels  to  the  brain,  in  the  brain  changes  occur  which 
release  a  predetermined  movement,  the  motor  impulse  travels  (say)  along 
the  spinal  cord  and  motor  nerve  to  the  hand,  and  the  muscle  is  innervated. 
The  time  required  to  convert  the  light  into  a  nervous  impulse  may  be  0.02 
second,  varying,  doubtless,  with  the  intensity  of  the  light.  This  time  is, 
however,  only  an  assumption  from  the  facts  of  intermittent  stimulation, 
already  considered,  and  the  fact  that  the  reaction-time  for  light  is  longer 
than  for  the  so-called  mechanical  senses,  hearing  and  touch,  with  which  this 


1  The  Time-Kelations  of  Mental  Phenomena,  New  York,  1890. 

2  Op.  cit. 


THE   PERCEPTION   OF   LIGHT.  537 

time  wonld  almost  disappear.  The  time  of  transmission  along  the  sensory 
nerve  has  been  studied  (in  the  first  instance  by  v.  Helmholtz)  by  applying 
a  stimulus  to  the  skin  nearer  or  farther  away,  and  determining  the  difference 
in  the  length  of  the  reaction-time,  or  by  comparing  the  time  at  which  the 
impressions  arrive  in  consciousness.  The  difficulties  in  the  way  of  such 
experiments  are  considerable,  and  the  results  of  the  numerous  researches 
which  have  been  made  are  not  accordant.  The  rate  of  transmission  in  the 
motor  nerve  may  be  studied  by  directly  stimulating  the  nerve-trunk  at 
varying  distances  from  the  muscle  and  measuring  the  time  elapsing  before 
a  contraction  occurs.  This  can  be  managed  more  easily  than  experiments 
on  sensory  nerves,  and  the  rates  are  commonly  supposed  to  be  the  same 
in  the  two  cases.  This  rate  is  said  to  be  from  thirty  to  sixty  metres  per 
second.1 

It  was  formerly  thought  that  the  light  was  seen  and  a  movement  willed, 
but  it  is  more  likely  that  the  "  willing"  is  done  beforehand,  and  consists 
in  placing  the  centres  concerned  with  the  special  movement  in  a  state  of 
unstable  equilibrium.  Then  when  the  stimulus  occurs  the  movement  is 
discharged  reflexly  before  we  see  the  light  and  recognize  its  nature.  This 
may  be  understood  by  reference  to  the  accompanying  figure.  The  light 


FIG.  17. 


strikes  the  eye  (E),  and  after  a  certain  time  (taken  up  probably  in  chemical 
changes)  an  impulse  is  sent  along  the  optic  nerve  ( 0)  at  a  given  rate.  It 
reaches  a  brain-centre  (O)  which  it  finds  in  a  state  of  unstable  equilibrium. 
The  impulse  is  now  divided,  changes  proceed  to  the  cortex  (X),  where 
the  nature  of  the  light  is  distinguished,  and  simultaneously  other  changes 
proceed  from  the  centre  along  the  spinal  cord  (S)  and  motor  nerve  (M)  to 
the  hand,  where  innervation  takes  place.  The  time  of  the  reflex  act  of 
blinking  is,  according  to  Exner,  about  0.06  second. 

The  length  of  the  reaction  varies  with  the  intensity  and  area  of  the 
light,  but  the  color  has  no  appreciable  effect.  It  also  varies  with  the  atten- 
tion, fatigue,  etc.,  of  the  observer.  It  is  this  latter  fact  which  is  of  especial 
interest  to  the  physician.  The  variation  in  length  and  regularity  of  the 

1  Charles  S.  Dolley  and  J.  McK.  Cattell,  On  Reaction-Times  and  the  Velocity  of  the 
Nervous  Impulse,  Psych.  Rev.,  vol.  i.  No.  11,  March,  1894,  pp.  159-168. 


538  THE   PERCEPTION   OF   LIGHT. 

reaction  depends  on  the  condition  of  the  eye  and  other  parts  of  the  visual 
mechanism,  and  the  relations  deserve  careful  study,  as  they 'add  to  the 
accuracy  and  range  of  methods  of  diagnosis. 

The  time  required  to  perceive  the  nature  of  an  impression  may  also  be 
determined.  Thus  conditions  may  be  arranged  so  that  the  impression  must 
be  distinguished  before  the  movement  is  made.  The  observer,  for  example, 
does  not  know  what  color  will  be  presented,  and  is  told  to  move  his  hand 
only  in  case  blue  occur.  In  this  case  the  movement  cannot  be  released 
until  the  nature  of  the  impression  is  distinguished,  although  here,  too,  the 
co-ordination  of  the  movement  with  the  impression  may  be  substantially 
automatic.  The  writer  found  the  time  required  to  distinguish  a  white 
light  to  be  about  0.05  second,  a  color  from  other  colors  0.10,  the  picture 
of  an  object  0.11,  a  letter  0.12,  a  word  0.13  second.  It  is  also  possible 
to  measure  the  time  of  movements  in  answer  to  visual  impressions :  thus, 
it  takes  about  0.4  second  to  name  a  word  and  about  0.6  second  to  name 
a  color.  The  difference  in  time  is  due  to  our  greater  practice  in  reading, 
and  does  not  hold  for  the  uneducated.  We  can  also  determine  the  time 
of  processes  which  we  know  only  on  the  side  of  consciousness,  such  as 
judging  which  of  two  lights  is  the  brighter  or  the  time  required  for  one 
idea  to  suggest  another.  Such  determinations  are  important,  as  they  make 
an  exact  science  of  physiology  and  psychology.  They  also  have  practical 
applications  in  medicine,  especially  in  the  diagnosis  and  treatment  of  aphasia, 
insanity,  and  other  diseases  of  the  nervous  system,  and  there  is  reason  to 
believe  that  they  may  prove  useful  in  ophthalmology. 


BINOCULAR  VISION,  CONFLICT  OF  THE 
FIELDS  OF  VISION,  APPARENT  AND 
NATURAL  SIZE  OF  OBJECTS,  ETC.1 

BY   EUGEN    BRODHUN,   M.D., 

Assistant  in  the  Frederick  William  University,  Berlin,  Germany. 

TRANSLATED  BY 

CHRISTINE   LADD  FRANKLIN, 

Baltimore,  Maryland,  Lf.S.A. 


I.    INTRODUCTION  :    PERCEPTION   OF   DEPTH    IN    MONOCULAR   VISION. 

THE  human  eye  is  like  a  photographic  apparatus,  the  lens  correspond- 
ing to  the  objective  and  the  retina  to  the  receiving  plate.  As  the  photo- 
graphic image  is  only  a  superficial  projection  of  the  space  pictured,  so 
also  the  excitations  caused  by  light  upon  the  retina  are  arranged  in  a  two- 
dimensional  order,  and  it  would  therefore  seem  natural  that  we  should 
have  considered  them  as  related  to  a  two-dimensional  form  in  the  outer 
world.  Every  one,  however,  knows  that  we  immediately  refer  our  visual 
sensations  to  space  of  three  dimensions.  Every  concept  got  through  the 
sense  of  sight  is  a  spatial  concept.  The  objects  which  our  visual  sensa- 
tions appear  to  give  us  cognizance  of  are,  even  when  we  look  at  them  with 
one  eye,  perceived  as  not  only  extension  in  height  and  breadth,  as  corre- 
sponds to  the  structure  of  the  eye,  but  also  as  extension  in  depth  :  that  is, 
in  addition  to  height  and  breadth,  we  have  a  clear  conception  that  the 
objects  we  look  at  are  at  different  distances  from  the  eye  of  the  observer. 
This  perception  of  depth  is,  indeed,  less  perfectly  developed  than  the  per- 
ception of  the  other  two  dimensions  :  the  painter  of  pictures  and  of  pano- 
ramas is  highly  successful  in  deceiving  the  eye  and  in  conveying  the 
impression  of  a  scene  in  space  when  in  reality  nothing  but  a  surface  is 

1  Upon  account  of  the  limits  of  space,  only  a  few  references  are  given  to  the  literature 
of  the  subject.  A  full  bibliography  up  to  1866  will  be  found  in  the  first  edition  of  Helm- 
holtz's  Physiologische  Optik.  In  the  new  edition  of  this  work  there  will  be  a  complete 
bibliography  coming  down  to  the  present  time,  prepared  by  Professor  A.  Konig.  The 
chapters  in  Helmholtz's  book  in  which  this  subject  is  discussed  will  be  a  simple  reprint  of 
the  first  edition.  Since  in  this  second  edition  the  paging  of  the  first  is  reproduced,  references 
will  here  be  made  to  the  first  edition.  Among  other  exhaustive  discussions  of  the  subject 
here  treated  may  be  mentioned  "  Der  Raumsinn  und  die  Bewegungen  des  Auge8,"  by 
Hering,  in  Herrmann's  "  Handbuch  der  Physiologic." 

MV 


540  BINOCULAR  VISION. 

presented  to  the  eye,  though  this  impression  remains  less  strong  than  that 
of  an  extended  surface. 

When  we  analyze  the  various  means  by  which  the  idea  of  depth— 
i.e.,  of  varying  distances  of  objects  from  the  beholder — is  attained,  we  find 
that  they  fall  naturally  into  two  classes.  The  first  is  the  class  of  those 
which  arise  from  experience ;  in  these  the  feeling  of  depth  is  not  given 
directly  as  a  sensation,  but  is  obtained  as  an  inference  from  other  sensations. 
It  is  by  means  of  this  class  that  the  painter  produces  the  plastic  effect  of 
his  pictures. 

To  accomplish  this  the  eye  has  to  compare  objects  in  respect  to  their 
size,  and  added  to  this  is  the  experience  which  we  have  stored  up  in  regard 
to  the  relative  sizes  of  known  objects.  The  size  of  an  object  in  sensation 
— that  is,  the  size  of  its  image  upon  the  retina— depends  upon  the  visual 
angle  under  which  it  is  seen.  When,  therefore,  objects  which  we  know  to 
be  of  the  same  size  give  images  of  different  sizes,  we  attribute  to  them  a 
factor  of  greater  distance  in  proportion  as  they  are  seen  to  be  smaller. 
Thus  the  apparent  size  of  some  men'  on  a  tower,  or  on  a  distant  country 
road,  gives  us  information  as  to  the  remoteness  of  the  tower  or  of  the  road. 
It  is  such  well-known  objects  of  definite  size  that  the  painter  makes  use  of 
as  accessories  in  order  to  give  to  the  beholder  a  distinct  impression  of  the 
relative  distances  of  all  the  objects  that  are  represented  in  his  painting.1 

Objects  of  relatively  simple  geometrical  forms  (houses,  rooms,  streets), 
which  we  know  exactly  from  having  constantly  seen  them  from  all  sides, 
produce  at  the  first  glance  an  impression  of  solidity  ;  and  it  is  comparatively 
easy,  in  drawings  of  such  objects,  provided  that  they  are  executed  with 
anything  like  a  correct  perspective,  to  convey  at  once  the  feeling  of  their 
existence  in  space.  Very  instructive  in  this  respect  is  the  Schroder  flight 
of  steps.2  (Fig.  1.)  The  picture  produces  at  once  the  impression  of  a 
j,  ,  flight  of  steps  against  a  wall,  beginning  at 

the  right  and  ending  at  the  left,  this  being  so 
whether  one  looks  at  it  with  the  line  ac  below, 
or,  after  turning  it  through  an  angle  of  180°, 
with  the  line  bd  below.  If  the  first  impression 
be  kept  distinctly  in  mind  while  turning  the 
picture  upside  down,  an  overhanging,  stair-like 


?8teps-  piece  of  masonry  leaning  against  a  wall 

be  seen.  If  now  the  first  impression  be  recalled  to  mind,  the  object  looked 
at  will  be  suddenly  transformed  into  a  flight  of  steps  beginning  at  d. 
Moreover,  while  before  the  impression  predominated  that  the  surface  a  was 

1  The  extreme  importance  of  this  means  for  the  perception  of  varying  depth  is  evi- 
denced by  the  fact  that  mountain-climbers,  especially  in  barren  and  snow-covered  regions, 
find  it  impossible  to  estimate  distances  correctly.      (See  Bonvalot's  "Across  Thibet.") 
"  Here,  in  a  few  weeks,  we  have  lost  the  sense  of  distance  which  we  had  gained  by  the 
experience  of  our  lifetime." — TRANS. 

2  Helmholtz,  Physiologische  Optik,  and  Pogg.  Ann.,  cv.  S.  298. 


BINOCULAR   VISION.  541 

the  nearer  to  the  eye,  the  surface  6  will  now  appear  the  nearer.  After 
some  practice,  either  impression  can  be  produced  at  will. 

Of  great  importance  for  the  perception  of  depth  is  the  distribution  of 
light  and  shade  in  the  field  of  view, — not  only  in  regard  to  the  shadows 
that  are  cast  by  objects,  but  also  in  reference  to  the  different  degrees  of 
illumination  according  as  objects  are  turned  towards  or  away  from  the 
source  of  light.  That  a  well-shaded  picture  gives  a  far  better  impression 
of  depth  than  one  that  is  not  shaded  is  well  known. 

Again,  the  so-called  aerial  perspective  is  of  great  importance  in  deter- 
mining the  feeling  of  space.  The  atmosphere  is  always  more  or  less  full 
of  vapor  and  of  cloud,  and  distant  objects,  therefore,  have  more  or  less  the 
appearance  of  being  veiled.  Hence  we  easily  believe  the  same  mountains 
when  they  are  misty  to  be  jiearer,  and.  therefore  larger,  than  when  they 
are  seen  in  a  clear,  transparent  light.  For  the  same  reason  the  moon  and 
the  sun  when  they  are  near  the  horizon  and  their  rays  consequently  pass 
through  a  greater  thickness  of  atmosphere  appear  to  us  to  be  larger  than 
when  they  are  near  the  zenith.1  They  appear  to  be  especially  large  in  a 
misty  atmosphere. 

The  second  division  of  means  by  which  we  judge  of  distance  depends 
directly  upon  sensation.  The  least  essential  is  the  feeling  of  change  of 
accommodation.  If,  in  the  monocular  vision  of  objects  not  very  far  away, 
we  direct  the  attention  from  a  more  distant  to  a  nearer  object,  we  have 
plainly  the  feeling  of  the  muscular  effort  involved.  This  gives  a  means, 
which  is,  to  be  sure,  very  uncertain,  for  the  estimation  of  the  relative  dis- 
tance of  two  objects.  Wundt 2  has  made  experiments  upon  this  subject  by 
means  of  a  stretched  thread  whose  ends  were  not  visible,  and  which  could 
be  made  to  approach  or  to  recede  from  the  eye,  and  also  by  means  of  two 
such  threads.  He  came  to  the  conclusion  that  the  feeling  of  accommodation 
gives  no  information  in  regard  to  the  absolute  distance  of  an  object,  but  only 
in  regard  to  its  relative  distance,  and  that  the  less  the  absolute  distance  the 
more  certain  is  the  estimation  of  relative  distance.  For  a  distance  of  forty 
centimetres  between  the  eye  and  the  thread,  for  instance,  the  limit  of  per- 
ceptible change  is  four  and  one-half  centimetres.  In  general,  the  latter 


1  The  reason  just  given  is  certainly  at  least  the  principal  one  for  this  phenomenon. 
It  has,  indeed,  been  affirmed  that  the  moon  and  the  sun  appear  larger  in  the  horizon  even 
when  the  atmosphere  is  perfectly  clear.  According  to  Smith  (1755),  this  is  due  to  the  fact 
that  the  vault  of  heaven  does  not  appear  to  us  to  be  in  the  form  of  a  hemisphere,  but  in 
that  of  a  natter  vaulted  surface,  about  the  shape  of  a  watch-glass.  The  moon  in  the 
zenith,  therefore,  seems  to  us  to  be  nearer,  and  since  we  see  it  under  the  same  visual  angle 
.  i  we  take  it  to  be  larger.  Helmholtz,  by  means  of  a  glass  plate  with  plane  parallel  sur- 
faces, used  as  a  mirror,  threw  an  image  of  the  moon  in  the  zenith  upon  the  horizon,  and 
failed  to  detect  any  distinct  change  in  its  size.  On  the  other  hand,  direct  experiments 
which  Strobant  (1884)  carried  out  in  a  room,  in  which  he  compared  the  apparent  distance 
of  bright  points  on  the  ceiling  and  at  an  equal  distance  from  the  observer  on  the  wall,  gave 
the  result  that  those  on  the  wall  appeared  to  be  farther  away. 
1  Theorie  der  Sinneswahrnehmungen. 


542  BINOCULAR  VISION. 

quantity  is  less ;  that  is,  the  capacity  for  forming  a  judgment  is  greater 
when  the  thread  is  moved  farther  away  than  when  it  is  brought  nearer. 

Further,  we  are  able  to  form  a  judgment  as  to  the  different  distances 
of  objects  from  one  another  by  means  of  a  change  in  the  retinal  images 
produced  by  motion, — not  only  the  motion  of  the  objects  relatively  to  each 
other,  but  also,  and  especially,  the  motion  of  the  observer.  When  the  eye  is 
moved,  the  images  of  objects  change  their  relative  position  upon  the  retina, 
the  images  of  the  more  distant  objects  moving  less  rapidly,  and  those  of 
very  distant  objects  (e.g.,  the  stars)  not  moving  at  all.  This  change  in  the 
images  produces  a  feeling  of  space ;  and  when,  in  paintings  and  especially 
in  panoramas,  a  plastic  impression  has  been  produced,  the  observer  must 
not  change  his  position,  or  he  will  lose  this  impression  for  the  moment. 
We  make  use  of  this  power  of  motion  in  the  eye,  in  scientific  experiments, 
to  bring  an  optical  image  into  coincidence  with  some  object  (for  example, 
the  crossed  threads  of  a  telescope). 

When  we  fix  our  eyes  upon  any  object,  the  greater  part  of  the  objects 
which  we  see  at  the  same  time  throw  an  image  upon  both  retinas.  It  is 
true,  however,  that  not  the  whole  field  of  vision  is  seen  binocularly,  the 
outer  part  on  one  or  the  other  side  being  seen  with  the  eye  of  that  side 
only.  A  very  considerable  increase  of  the  field  of  vision  is  secured  by 
this  monocular  part.  According  to  Aubert,1  the  field  of  vision  in  the  hori- 
zontal meridian,  with  the  eye  at  rest, 
Fia2-  for  one  eye,  includes  145°,— 55°  in- 

ward from  the  point  of  fixation  and  90° 
outward  :  the  binocular  field,  therefore, 
amounts  to  180°.  It  is  plain  that  the 
increase  from  145°  to  180°  is  of  great 
importance  for  the  certainty  of  our 
motions  in  space.  Fig.  2  gives  a  repre- 
sentation of  the  field  of  vision  accord- 
ing to  Moser.2  The  monocular  part  is 

Monocular  and  binocular  fields  of  vision. 

shaded,  F  is  the  fixation-point,  and  M 

and  Ml  are  the  blind  spots.    The  middle  unshaded  part  corresponds  to  that 
portion  of  the  field  of  vision  which  is  seen  binocularly. 

II.    PERCEPTION  OP   DEPTH   BY   MEANS   OF   BINOCULAR   VISION  ;    STEREO- 
SCOPIC  VISION. 

When  we  look  at  an  object  with  two  eyes,  the  two  retinas  do  not  both 
receive  exactly  the  same  image,  for  the  two  eyes  have  a  slightly  different 
position  in  space.  If  we  look  at  a  folded  sheet  of  paper  held  so  that  the 
folded  edge  is  in  the  median  plane  (the  plane  perpendicular  to  the  line  join- 
ing the  nodal  points  of  the  two  eyes  at  its  middle  point),  we  see,  when  we 
shut  the  left  eye,  more  of  the  right  half  of  the  sheet  of  paper ;  when  we 

1  Physiologische  Optik.  2  Das  Perimeter  und  seine  Andeutung. 


BINOCULAR   VISION. 


543 


shut  the  right  eye,  we  see  more  of  the  left  half.  We  have  not,  however, 
commonly,  when  we  look  with  two  eyes,  the  sensation  that  we  are  seeing 
double.  On  the  contrary,  the  effect  of  the  images  upon  the  two  retinas  is 
to  produce  a  definite  idea  of  space,  especially  for  near  objects.  When  we 
look  with  one  eye  at  an  object  which  is  unfamiliar  to  us,  and  which  has 
irregular  bounding  surfaces  (as  a  roughly  broken  stone),  we  have  only  a 
very  indefinite  impression  as  to  the  arrangement  of  the  bounding  surfaces, 
but  they  become  immediately  plain  to  us  as  soon  as  we  look  at  it  with  both 
eyes. 

When  we  consider  stereoscopic  pictures  we  recognize  very  plainly  the 
influence  of  vision  with  two  eyes  upon  the  perception  of  depth.  If  we 
make  two  perspective  drawings  of  some  simple  regular  solid  body, — a 
cube,  for  instance, — one  from  the  point  of  view  of  the  right  eye,  the  other 
from  that  of  the  left  eye,  and  if  we  look  at  these  two  drawings,  one 
with  the  right  eye  and  the  other  with  the  left  eye,  in  such  a  way  that  the 
two  retinal  images  fall  upon  exactly  the  same  parts  of  the  retinas  (as  they 
would  do  in  the  case  of  a  real  solid  body),  we  'have  immediately,  and  with 
great  clearness,  the  impression  of  a 

solid  body.    Such  pictures  are  called  FIG.  3. 

stereoscopic  pictures,  and  after  some 
practice  one  can  look  at  them  in  the 
proper  way.  They  should  be  placed 
at  a  distance  apart  that  is  equal  to 
the  distance  between  the  eyes,  and 
in  such  a  way  that  corresponding 
points  are  at  the  same  height,  and 
then  looked  at  with  the  axes  of  the 
eyes  parallel.  This  is  not  easy  at 
first,  because  one  is  accustomed  to 


accommodate  for  a  distant  point  and 
not  for  a  near  one  when  the  axes  of 
the  eyes  are  parallel.  When  this 
is  accomplished,  one  sees  three 
images,  two  with  each  eye,  of  which 
the  two  middle  ones,  by  fusion  with 
each  other,  give  rise  to  the  impres- 
sion of  a  solid  body. 

Stereoscopic    pictures    may    be 
made    either    by   geometrical   con- 
struction or  by  photography,  which 
latter  gives,  of  course,  a  central  pro- 
jection.    If  in  Fig.  3  R  and  L  are  supposed  to  be  the  nodal  points  of  the 
two  eyes,  and  M  the  middle  point  of  the  line  joining  them,  and  if,  further, 
MO  is  a  horizontal  perpendicular  to  RL,  of  the  length  of  distinct  vision, 
and  AB  is  the  projection  of  a  plane  through  0  and  perpendicular  to  OMt 


544  BINOCULAR   VISION. 

then  the  plane  AB  is  the  plane  of  the  picture,  and  to  every  point  of  the 
space  to  be  represented,  as  8  (which  does  not  in  general,  of  course,  lie  in 
the  plane  of  the  paper),  lines  must  be  drawn  from  each  eye.  If  these  lines 
meet  the  plane  of  the  picture  AB  in  the  points  P  and  Q  respectively,  then 
all  points  P  form  one  stereoscopic  picture  and  all  points  Q  the  other.  If 
stereoscopic  pictures  are  to  be  produced  with  exactness  by  photography, 
two  impressions  must  be  taken  at  distances  apart  that  are  equal  to  the 
average  distance  between  the  two  eyes,  and  with  objectives  whose  focal 
lengths  are  exactly  alike  and  about  equal  to  the  distance  of  distinct  vision. 

We  add  some  rules  of  stereoscopic  representation.1  Let  a  perpendicular 
SF  fall  from  8,  in  Fig.  3,  upon  the  line  RL,  cutting  the  plane  of  the  picture 
in  T,  and  at  the  points  P,  Q,  T,  S  let  fall  perpendiculars  upon  the  horizontal 
plane  Pp,  Qq,  Tt,  Ss.z  Let  the  distance  between  the  eyes  RL  be  repre- 
sented by  Qa,  the  distance  of  distinct  vision  by  b,  the  distance  of  the  object 
JS  from  the  plane  of  the  picture  by  ?,  and,  finally,  put  FM  equal  to  a. 

Then 

Ss       SR         ,    Ss       KL 
—  = ,  ana  —  —  —  ; 

and  therefore,  since 

SR      SL 
PR  ~  QL1 

Pp=Qq  =  v; 

that  is,  the  heights  of  corresponding  points  of  the  two  pictures  are  equal. 

We  have  also  PQ  parallel  to  pq,  and  therefore  PQ=pq;  that  is,  the 
distances  apart,  on  the  picture-plane,  of  the  images  of  (1)  a  given  point 
of  the  object  and  (2)  the  projection  of  that  point  upon  the  horizontal 
plane  through  the  eyes,  are  equal.  Therefore  it  is  sufficient  to  determine 
the  distance  apart  of  the  two  images  of  a  point  in  this  horizontal  plane  (the 
plane  of  the  diagram).  We  have 

pq       pjs^ at  _      y 

Qa  =  RS  =  sF  = 

and  hence,  for  the  distance  in  question, 


We  proceed  to  determine  the  value  of  the  stereoscopic  parallax, — that 
is,  the  distance  apart  of  the  corresponding  points  of  two  stereoscopic  draw- 
ings when  the  latter  are  so  placed  that  the  images  of  an  object  at  infinity 
coincide.  In  our  figure  the  images  of  an  infinitely  distant  object  do  not 
coincide,  but  are  at  a  distance  apart ;  that  is,  they  are  equal  to  the  distance 
between  the  two  eyes,  or  Qa.  The  stereoscopic  parallax  is  obtained,  there- 

1  For  further  details  see  Helmholtz,  Physiologische  Optik,  p.  664  ff. 

1  In  the  diagram  the  feet  of  the  perpendiculars  are  enclosed  iu  parentheses. 


BINOCULAR   VISION.  545 

fore,  by  subtracting  from  Qa  the  distance  apart  of  the  two  images  of  an 
arbitrary  point :  thus, 


or,  if  we  write  p  for  6  -J--  ^, — that  is,  for  the  distance  between  the  point  of 
the  object  S  and  a  vertical  plane  through  the  nodal  points  of  the  eyes, — 


Since  Qa  and  b  are  constants,  it  follows  that  the  stereoscopic  parallax 
depends  only  upon  the  distance  between  a  point  of  the  object  and  the  vertical 
plane  through  the  nodal  points  of  the  eyes,  and  is  inversely  proportional  to  this 
distance. 

This  circumstance,  that  the  stereoscopic  parallax  is  the  same  for  all 
objects  equally  far  away,  has  been  utilized  by  O.  N.  Rood  in  the  construc- 
tion of  an  apparatus  by  means  of  which  for  every  perspective  drawing  the 
corresponding  stereoscopic  drawing  can  readily  be  produced.  It  consists 
of  a  frame  which  is  movable  in  one  direction  (the  direction  of  the  stereo- 
scopic parallax),  the  amount  of  motion  being  capable  of  being  measured. 
In  this  framework  is  fastened  a  piece  of  transparent  paper,  and  underneath 
is  the  original  picture,  which  is  to  be  drawn  through,  but  in  such  a  way 
that  with  a  given  position  of  the  frame  all  those  points  are  to  be  drawn 
which  are  in  a  given  plane  of  the  picture.  On  passing  to  another  plane 
of  the  picture  the  frame  is  to  be  shoved  on  by  a  corresponding  amount. 

Let  the  amount  of  the  parallax  belonging  to  two  different  distances  of 
the  object  pl  and  p2  be  el  and  e2  respectively  ;  then  we  have 


PI 
or,  if  we  put 


then 


If  in  this  formula  el  —  e2  represents  the  least  difference  in  distance 
which  is  stereoscopically  perceptible,  then  we  have  expressed  the  fact  that 
the  differences  in  depth  which  are  stereoscopically  just  perceptible  ran/  directly 
as  the  square  of  the  mean  distance  of  the  points. 

Helmholtz  has  made  experiments  to  determine  with  what  exactness 
differences  in  the  two  retinal  images  can  be  detected,  and  he  has  found  that 
a  difference  of  one  minute  of  arc  is  sufficient  to  be  perceived.1  It  should 
be  noticed,  in  passing,  that  this  is  about  the  same  angular  distance  as  that  at 
which  two  bright  points  can  be  distinguished  as  two.  If  we  now  put  in 

1  Physiologische  Optik,  p.  644. 
VOL.  I.—  35 


546  BINOCULAR   VISION. 

the  above  formula  p2  =  o>,  and  call  e  the  angle  at  which,  in  stereoscopic 
vision,  one  point  is  seen  to  be  just  perceptibly  behind  another,  we  have 


The  distance  apart  of  the  eyes  as  thus  estimated  is  sixty-eight  milli- 

metres (Helmholtz)  :  hence 

68 
Pi  =  gin  p  =  (about)  240  metres. 

The  greatest  distance,  therefore,  at  which  one  can  obtain  stereoscopic 
effects  is  two  hundred  and  forty  metres.  It  is  hence  of  no  use  for  stereo- 
scopic purposes  to  take  photographs  of  objects  more  than  two  hundred  and 
forty  metres  away,  if  the  distance  between  the  objectives  is  the  same  as  the 
distance  between  the  eyes. 

A  visual  angle  of  one  minute  corresponds,  at  the  distance  of  distinct 
vision,  to  not  quite  one-tenth  of  a  millimetre  of  length.  If,  therefore,. 
there  are  two  drawings  which  are  nearly  alike,  but  which  differ  from  each 
other  in  any  part  by  so  much  as  one-tenth  of  a  millimetre,  this  difference 
will  be  at  once  apparent  if  they  are  looked  at  stereoscopically.  According 
to  Dove,  this  method  is  made  use  of  to  detect  counterfeit  paper  money  ; 
differences  can  be  determined  between  impressions  which  are  not  made  from 
the  same  die,  though  they  look  exactly  alike  when  compared  in  the  ordi- 
nary way.1  We  do  not  need  to  mention  that  the  difference  to  be  detected 
does  not  consist  in  an  appearance  of  indistinctness,  as  would  be  the  case  if 
one  picture  were  superimposed  upon  the  other  by  optical  means  and  they 
were  then  looked  at  with  one  eye  :  what  happens  is  that  the  irregular  parts 
of  the  picture  seem  to  be  in  a  different  antero-posterior  plane. 

We  shall  now  determine  how,  in  looking  at  stereoscopic  pictures,  the 
apparent  position  of  an  object  depends  upon  the  position  of  its  images.  The 
apparent  position  of  the  object  is  determined  by  p,  by  8s  =  P,  and  by 
FM=  a.  By  a  former  equation  we  have 


and  also 

£«£«§ 

v       b         e 

and  hence 

fl  =  ,&. 

e 

Finally, 


b          tp  p 

and     . 


b  i  \ 

=  -(a  —  a  , 


1  Hirth,  Das  plastische  Sehen,  German  and  French  editions,  contains  a  number  of 
interesting  stereoscopic  objects. 


BINOCULAR   VISION. 


547 


hence 
or,  since 


iq-tp=V-  Qa; 


go  —  op  —  -  a, 
a 


If  we  put  3- —       "  =  x,  where  x  is  the  arithmetical  mean  of  the  distances 

1L> 

of  two  corresponding  points  of  the  image  from  the  median  plane,  we  have 

Qa 

n  r=—  fr.  * 


If,  then,  two  stereoscopic  pictures,  drawn  according  to  the  plan  repre- 
sented in  Fig.  3,  are  brought  nearer  to  the  eye  or  are  removed  farther  from 
the  eye,  6  becomes  respectively  smaller  or  larger ;  £  and  a  are  not  changed, 
but  p  becomes  smaller  or  larger  proportionately  with  6  ;  that  is,  in  the  first 
case  the  depth  of  the  apparent  object  is  too  small,  in  the  second  it  is  too 
great.  This  occurs  when  photographic  stereoscopic  pictures  are  looked  at 
at  a  distance  that  is  not  equal  to  the  distance  between  the  photographic 
plate  and  the  focus  of  the  objective  which  lies  nearest  to  it. 

III.   STEREOSCOPIC  APPARATUS. 

To  look  at  stereoscopic  pictures  with  the  axes  of  the  eyes  parallel  requires 
a  certain  amount  of  practice.  For  the  sake  of  greater  convenience,  apparatus 
has  been  devised  by  means  of  which  the  stereoscopic  effect  can  be  produced 
with  the  axes  of  the  eyes  convergent.  The  earliest  of  these  instruments  is  : 

(a)  The  Mirror-Stereoscope  of  Wheatstone  (invented  in  1833}. — It  con- 
sists, as  is  shown  in  Fig.  4  schematically,  of  two  mirrors  that  are  inclined 
to  each  other  at  an  angle  of 
90°,  ab  and  cd;  o,  oly  are 
the  positions  of  the  two  eyes, 
ef  and  gh  of  the  two  stereo- 
scopic views.  The  mirrors 
form  virtual  images  of  these 
views,  which  fall  exactly  to- 
gether in  ik,  and  of  which  one 
is  seen  with  one  eye  and  the 
other  with  the  other  eye. 
The  distance  of  the  virtual 
images  from  the  eye  must 
be  that  of  distinct  vision. 

(6)  The  Lens-Stereoscope 
of   Brewster    (1843).— This 

instrument  has  the  advantage  that  it  is  far  more  handy  than  Wheatstone's, 
that  the  pictures  lie  side  by  side  and  so  can  be  fixed  upon  a  single  piece  of 


Fio.  4. 


548 


BINOCULAR   VISION. 


Fio.  5. 


card-board,  and  that  a  good  illumination  can  be  more  readily  obtained.    This 
is  the  form  of  stereoscope  that  is  found  in  common  use.    The  arrangement  is 

schematically  represented  in  Fig.  5.  A  and 
B  are  two  prismatic  lenses,  which  are  so 
chosen  that  by  means  of  the  prismatic  devi- 
ation, upon  convergence  for  the  distance  of 
distinct  vision,  two  pictures  which  are  as 
far  apart  as  the  distance  between  the  eyes 
are  seen  single,  while,  by  reason  of  the  lenses, 
the  pictures,  which  are  usually  nearer  than 
for  distinct  vision,  are  seen  distinctly  :  thus 
a  slight  enlargement  of  the  pictures  is  ob- 
tained at  the  same  time.  In  order  to  obtain 
a  perfect  effect,  the  photographic  pictures 
must  have  been  properly  taken  ;  this,  how- 
ever, is  seldom  the  case.  Moreover,  there  is 
the  disadvantage  that  the  pictures,  especially 
on  the  edges,  show  some  color,  unless  achro- 
matic prisms  have  been  chosen,  which  also 
is  seldom  done. 

(c)  Helmholtz's  Stereoscope. — Since,  in  the 
stereoscopic  views  to  be  found  in  the  shops,  corresponding  points  are  often 
not  at  exactly  the  right  distance  apart,  and  the  heights  are  also  sometimes 
not  exactly  the  same,  Helmholtz  has  constructed  a  stereoscope  in  which  both 

Fio.  6. 


Schematic  representation  of  the  lens- 
stereoscope  of  Brewster. 


Schematic  representation  of  Helmholtz's  telestereoscope. 


of  these  faults  can  be  corrected.  This  instrument  is  designed  to  be  used 
with  accommodation  for  infinity  and  with  the  axes  of  the  eyes  parallel ;  the 
prisms,  with  their  disadvantages,  are  therefore  left  out.  Distinctness  and, 
if  it  is  desired,  enlargement  of  the  pictures  are  obtained  by  means  of  a 


BINOCULAR   VISION. 

combination  of  two  convex  lenses  which  can  be  brought 
nearer  together  or  removed  farther  apart.  Each  pair 
of  lenses  can  be  moved,  by  means  of  two  screws,  in  the 
direction  of  and  at  right  angles  with  the  line  joining 
the  eyes.  It  has  the  same  external  appearance  as  that 
of  Brewster.  Several  other  instruments  of  a  similar 
kind  have  been  constructed. 

(d)  Helmholtz's  Telestereoscope. — We  have  seen  that 
for  a  distance  greater  than  two  hundred  and  forty 
metres,  with  the  ordinary  distance  between  the-  eyes, 
a  plastic  effect  cannot  be  obtained,  upon  account  of 
the  difference   between  the  two  retinal  images.      In 
order  to  bring  out  more  clearly  the  dimension  of  depth 
for  greater  distances,  Helmholtz  has  constructed  the 
so-called  telestereoscope,  which  is  schematically  repre- 
sented in  Fig.  6.    The  position  of  the  two  eyes  is  repre- 
sented by  o  and  ol ;  ah,  cd,  ef,  gh,  are  four  plane  mirrors 
which  are  at  an  angle  of  45°  with  the  median  plane 
(i.e.,  the  plane  perpendicular  to  the  line  joining  the 
nodal  points  of  the  eyes  at  its  middle  point)  in  a  way 
which  is  made  plain  by  the  figure.     The  effect  of  the 
mirrors  is  such  that  the  apparent  position  of  the  eyes 
is  at  o2  and  o3,  or  the  effective  distance  between  the  eyes 
is  increased.    The  apparent  size  of  objects  is  not  altered 
by  this  arrangement :  it  is  as  if  one  regarded  an  ac- 
curately reproduced  model  of  a  real  scene,  made  smaller 
in  the  proportion  of  the  actual  to  the  apparent  distance 
between  the  eyes,  but  at  a  correspondingly  less  distance. 
Exactly  the  same  effect  is  produced  by  stereoscopic 
photographs  if  they  are  taken  at  distances  apart  greater 
than  the  distance  between  the  eyes. 

Helmholtz  has  combined  this  arrangement  with 
two  terrestrial  telescopes  by  putting  two  rectangular 
prisms  between  the  second  and  third  lens  of  the  objec- 
tive and  two  plane  mirrors  behind  the  objective.  The 
instrument  is  represented  in  Fig.  7.  The  mirrors  aa 
and  «!«!,  which  must  be  accurately  constructed,  can  be 
so  adjusted  by  screws  that  the  images  of  the  two  tele- 
scopes fall  exactly  together  for  the  observer.  Helm- 
holtz made  use  of  a  sixfold  magnifying  power,  and  of 
an  apparent  distance  sixteen  times  greater  than  the 
actual  distance.  He  therefore  obtained  the  effect  of  a 
scene  at  a  distance  from  the  observer  equal  to  one-six- 
teenth of  the  actual  distance. 

(e)  Zeiss's  Relief  Telescope. — The   instrument  j 


549 


Fio.  7. 


550 


BINOCULAR   VISION. 


described  did  not  come  into  common  use,  but  Zeiss,  of  Jena,  has  recently 
taken  up  the  idea.  As  is  well  known,  the  ordinary  binocular  telescopes 
(opera-glasses,  field-glasses),  which  consist  of  two  so-called  Dutch  telescopes, 
give  only  a  very  feeble  enlargement,  or  if  the  magnifying  power  is  made 
greater  the  field  is  extremely  small.  With  a  four-  to  sixfold  magnifying 
power  the  field  is  only  a  third  of  the  actual  field.  The  terrestrial  telescope 
cannot  be  used  for  this  purpose,  on  account  of  its  length  and  consequent 
unhandiness.  The  astronomical  telescopes  are  shorter,  but  they  are  still 
too  long,  and,  moreover,  they  giye  inverted  images.  Zeiss  has  therefore 
constructed  astronomical  telescopes  after  the  design  of  Porros,  which  pro- 
duce upright  images  by  means  of  reflecting  prisms,  and  combined  them 
with  binocular  instruments.  The  totally  reflecting  prisms,  whose  construc- 
tion we  cannot  further  describe  here,  give  opportunity  for  combining  a 
telestereoscopic  with  the  binocular  effect.  He  makes  both  telestereoscopic 

Fio.  8. 


Schematic  representation  of  Zeiss's  relief  telescope. 

field-glasses,  which  have  only  a  slight  telestereoscopic  effect,  and  relief  tele- 
scopes, in  which  the  apparent  distance  between  the  eyes  is  equal  to  twice 
the  focal  length  of  the  objective.  In  Fig.  8  is  shown,  in  two  sections,  one- 
half  of  the  latter  instrument, — the  half  intended  for  the  right  eye  :  a  is 
a  horizontal  and  6  is  a  vertical  section  through  the  optic  axis. 

(/)  The  Binocular  Ophthalmoscope  and  Otoscope, — It  is  sometimes  de- 
sirable to  produce  the  opposite  effect  to  that  obtained  by  the  telestereoscope. 
This  happens  when  it  is  desired  to  look  at  objects  which  one  cannot  see  with 
both  eyes  at  once  because  the  distance  between  the  eyes  is  too  great,  as,  for 
example,  the  drum  of  the  ear  and  the  fund  us  of  the  eye.  In  this  case  we 
can  make  use  of  an  instrument  the  essential  part  of  which  is  represented  in 
cross-section  in  Fig.  9.  There  are  two  glass  prisms  whose  cross-section  is 


BINOCULAR   VISION. 


551 


Fio.  9. 


FIQ.  10. 


a  parallelogram  with  an  acute  angle  of  45°  ;  two  of  their  sharp  edges  come 
together  in  c,  and  o  and  ot  indicate  the  position  of  the  eyes  of  the  observer. 
The  rays  which  come  from  the 
object  looked  at  are  divided  into 
two  halves,  one  of  which  reaches 
the  right  eye  and  the  other  the 
left  eye.  They  have  a  slight  in- 
clination towards  each  other.  By 
this  means  is  obtained  a  feeble 
but  not  unimportant  stereoscopic 
effect.  These  instruments  are  con- 
structed by  Geraud-Teulon  and 
by  Bottcher. 

(g)   Wheatstone's  Pseudoscope. 
-This  instrument  is  designed  for   Schematic  representation  of  binocular  ophthalmoscope. 

the  inversion  of  the  relief  of  a  natural  object.     It  is  instructive,  as  by  its 

use  stereoscopic  vision  may  be  set  in  opposition  with  the  other  means  of 

plastic  vision.     In  Fig.  10,  P  and  P1  represent 

the  two  prisms  of  this  instrument,  and  o  and  ol  the 

position  of  the  eyes.     Since  total  reflection  takes 

place  from  the  widest  surface  of  these  prisms,  they 

have  the  effect  of  interchanging  right  and  left 

without   changing  the  direction  of  vision.     By 

bringing  two  images  into  superposition  the  relief 

must  appear   inverted.      The  illusion  does  not 

always  succeed,  however ;  the  other  aids  for  the 

production  of  the  dimension  of  depth,  especially 

shadows,  frequently  destroy  the  expected  result. 

Helmholtz   recommends  that  objects  which   are 

to  be  seen  pseudoscopically  should  be  hung  in  the 

middle  of  a  room  with  a  monochrome  background 

on  which  no  shadows  can  be  thrown.     Wooden 

cylinders  look  like  hollow  tubes,  and  cigars  appear 

like  hollow  sheets  of  tobacco.     Righi,  in  1889, 

described  an  apparatus  which  one  can  use  both  as 

pseudoscope  and  as  telestereoscope.     It  consists 

of  two  parallel  mirrors  inclined  at  an  angle  of 

45°  to  the  median  plane ;  that  is,  it  is  one-half 

of   a   Helmholtz    mirror- telestereoscope.      It   is 

held  before  one  eye  only,  while  the  other  looks  in  the  ordinary  way.    When 

held  in  the  same  way  as  the  Helmholtz  instrument,  a  telestereoscopic  effect 

is  produced.      When  the  instrument  is  turned  180°  about  the  axis  of 

vision,  the  effect  is  pseudoscopic. 

(A)    The   Stereoscopic    Microscope. — This    is    a    microscope   with   one 
objective  and  two  eye-pieces.     The  rays  which  proceed  from  the  object  are 


Schematic  representation  of 
Wheatstone's  pseudoscope. 


552 


BINOCULAR   VISION. 


divided  into  two  halves,  which  are  directed  into  two  tubes  by  means  of 
prisms.  Such  microscopes  have  been  constructed  by  Nachet  and  by  Wen- 
ham.  Fig.  11  shows  an  arrangement  of  this  sort.  By  means  of  three 
equilateral  prisms,  a,  6,  and  c,  of  which  one,  a,  is  immediately  over  the 
objective  d,  there  is  produced  a  twofold  total  reflection  and  inversion  of 
the  rays.  Another  arrangement  is  exhibited  in  Fig.  12;  the  same  result 
is  here  accomplished  by  two  totally  reflecting  prisms,  a  and  b.  The  prism 
a  receives  the  right  half  of  the  rays  which  have  come  through  the  objective 
d,  and,  after  reflection  from  the  second  prism,  these  rays  reach  the  left  eye 
of  the  observer.  A  screw  is  sometimes  introduced,  by  means  of  which  the 
prism  a  can  be  moved  over  the  other  half  of  the  objective :  a  pseudoscopic 
eifect  is  then  produced.  Abbe  has  constructed  an  eye-piece  for  the  produc- 

Fio.  11. 


Diagram  illustrating  Nachet's  and  Wenham's 
microscopes. 


Diagram  illustrating  the  stereoscopic  effect  ob- 
tained by  two  totally  reflecting  prisms. 


tion  of  the  stereoscopic  eifect  with  a  microscope  made  for  monocular  use, 
which  can  be  attached  at  once  in  place  of  an  ordinary  eye-piece. 

It  is  to  be  remarked  that  in  the  microscopes  just  described  the  stereoscopic 
effect,  as  Helmholtz  has  shown,  is  brought  about  in  a  peculiar  way,  as  the 
result  of  the  circle  of  diffusion.  Let  a  in  Fig.  1 3  be  a  point  of  an  object 
in  the  plane  of  accommodation  PQ,  and  6  a  point  somewhat  below  a.  A 
sharp  image  of  the  plane  PQ  is  formed  for  the  right  eye  at  P'Q'  by 
means  of  the  objective ;  the  image  of  a  is  therefore  at  a'.  The  image  6 
of  6j  is  then  beneath  the  plane  P'Q',  and  in  this  plane  there  is  a  diffusion 
circle  formed  by  6,  of  which,  however,  upon  account  of  the  shutting  off  of 
half  the  lens,  only  the  right  half  is  visible  to  the  right  eye.  The  centre, 
62,  of  this  half  diffusion  circle,  which  represents  for  this  eye  the  image 
of  6,  is  therefore  somewhat  to  the  right  of  a'.  In  the  same  way,  for  the 
left  eye,  the  image  of  6  is  to  the  left  of  a'.  In  the  plane  P'Q'  there  is 


BINOCULAR   VISION. 


553 


Fio.  13. 


produced,  therefore,  in  effect,  for  the  two  eyes  taken  together,  a  correct 
stereoscopic  image  of  the  point  6. 

IV.   CORRESPONDING   POINTS  OF  THE   RETINA. 

We  have  hitherto  discussed  the  influence  which  binocular  vision  has 
upon  the  perception  of  depth  in  the  visual  space  by  a  consideration  of  the 
total  effect  obtained,  without  analyzing   in 
detail  the   unitary  impression  that  is  pro- 
duced in  spite  of  the  fact  that  we  see  with 
two  eyes  and  from  two  different  positions. 
We  shall  now  take  up  this  latter  problem. 
If  we  gaze  at  an  object  binocularly,  and  then 
push  one  eye  a  little  to  one  side  by  the  fin- 
ger, we  shall  see  two  images  of  the  object. 
From  this  it  follows  that  it  is  not  a  matter 
of  indifference,  for  the  attainment  of  a  single 
image,  upon  what  portion  of  the  retina  the 
object  is  pictured ;  on  the  contrary,  if  the 
object  is  pictured  for  one  eye  on  the  position 
a  of  the  retina,  it  must  be  pictured  for  the 
other  eye  on  a  perfectly  definite  position,  6, 
of  its  retina,  in  order  to  secure  coincidence. 
Portions  of  the  two  retinas  which  work  to- 
gether in  this  way  are  known  as  correspond- 
ing points  (or  identical  points),  and  portions 
which  have  not  this  property  are  called  dis- 
parate points  ;  different  points  will  refer,  then, 
to  different  points  on  a  single  retina.     From 
the  fact  that  we  see  objects  single  which  we 
gaze  upon,  it  follows  that  the  middle  points 
of  the  fovea  are  corresponding  points.     If, 
in  fixing  our  eyes  upon  an  object  that  is 
situated  in  the  median  plane,  we  direct  the  attention  upon  an  object  that  is 
considerably  nearer  or  farther  off"  than  the  object  gazed  at,  we  see  that 
object  not  single,  but  double ;  as  we  say,  there  are  double  images.     These 
images,  therefore,  fall  upon  disparate  portions  of  the  retinas. 

For  the  further  determination  of  corresponding  points  several  different 
methods  have  been  employed,  some  of  which  are  fatiguing  to  the  eye  and 
some  are  applicable  only  in  a  limited  region.  The  best  method  consists 
in  the  use  of  an  apparatus  which  offers  a  special  field  of  vision  to  each 
eye,  while  the  contents  of  these  two  fields  of  vision  are  united  in  con- 
sciousness. Such  an  arrangement  has  been  called  by  Hering  a  haploscope. 
For  the  following  experiments  it  is  sufficient  if  the  instrument  consists  of  a 
vertical  screen  upon  which,  at  a  horizontal  distance  apart  that  is  equal  to 
the  distance  between  the  eyes,  two  points  are  marked,  as,  for  example, /and 


Diagram  illustrating  stereoscopic  effect 
produced  in  binocular  microscope. 


554  BINOCULAR   VISION. 

f  in  Fig.  14.  One  is  to  look  at  this  screen  with  both  body  and  head  held 
upright  (the  primary  position)  and  with  the  visual  lines  (the  lines  which 
connect  the  nodal  points  of  the  eyes  with  their  visual  centres)  placed  hori- 
zontal and  parallel.  The  left  eye  is  to  be  directed  upon  the  left  mark  /, 

and  the  right  eye  is  to  be  fixed  upon  the 
FlQ-  14>  right  mark  /.     If  now  at  /a  vertical  line 

be  drawn  upward  and  one  at  /  drawn 
downward,  the  experimenter  will  see  (since 
/  and  /  throw  their  images  upon  corre- 
sponding points)  a  continuous  1  ine  ggr.  This 
/'  line,  however,  is  not  in  general,  as  one  might 
/  expect,  a  straight  line,  but  the  two  lines 
form  an  obtuse  angle  with  each  other.  The 
inclination  of  the  two  lines  can  be  measured 
if  one  of  the  two  lines — say  fg' — is  made 
movable  about  an  axis,  and  if  the  amount 
9'  of  its  rotation  can  be  read  off  upon  the  arc 
of  a  circle.  This  line  can  then  be  turned 
until  it  appears  to  be  the  exact  continua- 
tion of  the  other.  If  in  this  new  position  of  fg'  both  lines  should  be  pro- 
duced beyond  /  and  /  respectively,  only  a  single  straight  line  would  be 
visible. 

A  line  in  which  the  retina  is  cut  by  any  plane  going  through  the  visual 
line  is  called  a  retinal  meridian.  From  the  above  experiments  it  follows, 
then,  that  there  are  two  retinal  meridians,  nearly  but  not  exactly  vertical, 
which  correspond  with  each  other.  These  are  called  by  Helmholtz  the 
apparent  vertical  meridians.  Hering  terms  them  the  longitudinal  sections, 
and  Ruete  designates  them  the  vertical  lines  of  separation.  For  most  people 
these  lines  diverge  above  and  converge  below.  The  angle  between  them, 
F,  is  between  0°  and  3°.  In  many  cases  the  meridians  seem  not  to  form 
exactly  straight  lines,  but  appear  to  have  a  little  indentation  at  the  place  of 
distinct  vision. 

PIG.  15. 

G : 


/  /'  *' 

Diagram  illustrating  the  action  of  Hering's  haploscope. 

If  now  one  draws  a  line  from/  towards  the  left,/*,  and  a  line  from/ 
towards  the  right,/*7,  and  observes  as  before,  one  will  see  either  a  straight 
line,  ii'  (Fig.  15),  or  an  obtuse  angle  with  its  opening  situated  upward. 
According  to  Helmholtz,  this  line  is  straight  for  those  who  have  normal 
vision,  and  forms  an  obtuse  angle  for  those  who  are  short-sighted.  Even 
in  the  latter  case,  however,  the  departure  from  a  straight  line  is  far  less 
than  it  is  for  the  vertical  lines  (about  ^°).  By  rotating  one  line  as  before, 
the  corresponding  horizontal  meridians  can  be  obtained.  These  are  called 


BINOCULAR   VISION. 


555 


by  Helmholtz  the  retinal  horizons.     Hering  terms  them  cross-sections,  and 
Ruete  designates  them  the  horizontal  divisional  lines. 

Helmholtz  and  Bonders  have  found  that  the  angle  Fis  not  constant  for 
any  given  observer.  Hering  attributes  this  to  the  fact  that  the  eye  involun- 
tarily rolls  slightly  about  the  visual  axis;  if  this  is  avoided,  the  angle  re- 
mains constant.  According  to  Hering,  it  can  easily  be  brought  about  that 
the  middle  cross-sections  coincide  with  the  actual  horizontal  meridians  by 

FIG.  16. 


n'- 


-o 


Diagram  illustrating  the  method  for  determining  corresponding  points  on  the  mean  cross  and 

longitudinal  sections. 

compelling  the  eyes,  if  necessary,  to  roll  slightly.  If  one  fixes  the  eyes  in 
such  a  position  by  means  of  a  horizontal  thread  which  passes  through  the 
field  of  vision  of  both  eyes,  a  constant  value  for  Fcan  be  preserved. 

We  have  now  to  investigate  what  points  on  the  mean  cross  and  longi- 
tudinal sections  are  corresponding  points.  To  this  end  we  introduce  two 
rectangular  (or  nearly  rectangular)  crosses  into  the  haploscope,  with  their 

FIG.  17. 


Diagram  illustrating  the  method  for  determining  corresponding  points  in  the  middle  cross-sections. 

intersections  situated  at /and/'  and  so  placed  that  they  will  fall  together  in 
the  visual  field.  (Fig.  16.)  If  then  there  are  two  horizontal  lines  mn  and 
m'n',  which  are  movable  up  and  down,  and  if  one  of  thorn  (m'n')  is 
moved  until  mn  and  m'n'  fall  together  in  one  straight  line,  it  will  be  found 
that  when  this  is  the  case  the  distances  fin  and  fm'  are  equal.  From  this 
it  follows  that  those  points  in  the  middle  longitudinal  section  which  are 


556 


BINOCULAR   VISION. 


equally  distant  from  the  middle  cross-section  correspond.1     By  a  similar 
arrangement,  which  is  represented  in  Fig.  17,  it  will  be  found  that  in  the 


FIG.  18. 


Diagram  illustrating  the  method  for  determining  corresponding  points  within  any  one  of 

the  quadrants. 

middle  cross-section  those  points  correspond  which  are  at  equal  distances 
from  the  middle  longitudinal  section. 

Finally,  to  determine   corresponding   points  within  any  one   of  the 

FIG.  19. 


0  0' 

Diagram  illustrating  Bering's  mirror-haploscope. 

quadrants  the  haploscope  should  be  arranged  as  in  Fig.  18.     If  the  points 
o  and  o',  as  well  as  the  points  p  and  pf,  are  equally  distant  from  both  the 

1  It  is  still  bettor  if  in  these  figures  the  dotted  arm  of  the  cross  is  omitted.  The  figures 
are  drawn,  for  the  sake  of  simplicity,  for  those  eyes  in  which  the  middle  longitudinal  sec- 
tions are  not  convergent.  Let  the  reader  compare  what  is  said  helow  about  the  fusion  of 
double  images. 


BINOCULAR  VISION. 


557 


vertical  and  the  horizontal  lines  through  /  and  /,  then  in  the  haploscopic 
combination  o  will  coincide  with  o'  and  p  with  p'.  In  order  to  carry  out 
these  and  other  haploscopic  experiments  with  different  degrees  of  con- 
vergence of  the  visual  axes,  the  Hering  mirror-kaploscope  may  be  employed. 
This  is  represented  in  Fig.  19.  0  and  0'  are  the  positions  of  the  nodal 
points  of  the  two  eyes,  EC  and  B'C'  are  two  mirrors,  DE  and  D'E1  are 
the  two  haploscopic  fields  of  vision,  which  are  at  an  angle  of  45°  with  the 


FIG.  20. 


iff' 


Diagram  illustrating  construction  for  finding  corresponding  points  and  corresponding  meridians. 

mirrors,  and  are  so  arranged  that  a  ray  of  light  perpendicular  to  each  field 
at  its  middle  point  ( MN  and  M'N'),  after  reflection  from  a  mirror,  reaches 
the  nodal  point  of  the  corresponding  eye  (O  and  0').  To  this  end,  BC, 
DE,  and  0  on  the  one  hand,  and  B'C',  D'E',  and  0'  on  the  other,  are 
firmly  connected  together.  Each  of  these  systems,  however,  can  be  rotated 
about  the  points  0  and  0'  respectively,  so  that  one  is  in  a  position  to  make 
the  directions  OMN  and  O'M'N'  correspond  with  every  position  of  the 

FIG.  21. 


Haploscopic  figure  of  Helmholtz. 

visual  axes.  For  a  given  position  of  the  visual  axes  the  apparatus  is  the 
same  as  the  Wheatstone  stereoscope. 

In  accordance  with  the  foregoing  propositions,  all  corresponding  points 
and  all  corresponding  meridians  can  easily  be  found  by  construction.  With 
reference  to  the  meridians,  however,  it  must  be  remarked  that,  when  meas- 


558  BINOCULAR   VISION. 

ured  upon  them,  corresponding  points  are  not  equally  distant  from  the 
visual  centres  in  the  case  where  the  middle  longitudinal  sections  are  con- 
vergent. This  is  apparent  from  Fig.  20,  where,  for  the  sake  of  plainness, 
the  convergence  is  exaggerated.  If  o  and  o'  are  the  fixation-points,  bl  and 
b'V  the  middle  longitudinal  sections,  and  ak  and  a'k'  the  middle  cross-sec- 
tions, then  m  and  m'  are  corresponding  points  on  the  corresponding  me- 
ridians om  and  o'm'.  This  is  so,  provided  that  oc  =  o'c',  od  =  o'd',  and 
efH  ak,  e'f'H  a'k',  gJi  //  bl,  and  g'h'  //  b'V.  It  is  evident  that  om  is  not  equal 
to  o'm'.  In  performing  these  last  experiments  care  must  be  taken  that 
the  lines  be  drawn  increasingly  thicker  as  they  pass  farther  away  from  the 
centre.  The  reason  for  this  is  that  the  power  of  discriminating  falls  away 
very  rapidly  in  the  peripheral  portions  of  the  retina. 

For  the  purpose  of  enabling  this  whole  matter  to  be  observed  at  once, 
Helmholtz  has  drawn  the  haploscopic  figure  which  we  have  reproduced  in 
Fig.  21.  In  order  to  avoid  fusion  when  the  lines  do  not  fully  correspond, 
the  left  half  is  drawn  in  black  on  white  and  the  right  in  white  on  black. 
In  haploscopic  combination  with  parallel  visual  axes  (if  necessary,  with  a 
stereoscope)  one  sees  two  coincident  rectangular  gratings.  For  eyes  which 
are  not  at  this  distance  apart,  or  which  have  a  different  convergence  of  the 
apparent  vertical  meridians,  a  different  picture  must,  of  course,  be  drawn. 

V.    THE   HOROPTER. 

On  the  basis  of  the  laws  just  given,  we  are  in  a  position  to  determine 
for  every  position  of  the  eyes  what  points  in  space  will  throw  their  images 
upon  corresponding  points  of  the  retina.  The  entire  collection  of  points 
which  satisfy  this  condition  is  called  the  horopter.  It  is  evident  that,  upon 
the  doctrine  of  corresponding  points  already  explained,  the  horopter  can  be 
determined  mathematically.  This  problem  has  been  solved  by  Helmholtz 
by  analytical  geometry,  and  by  Hering  at  about  the  same  time  by  synthetic 
geometry.  We  shall  not  undertake  to  reproduce  in  this  place  this  general 
investigation,  which  has,  moreover,  hardly  any  physiological  importance ; 
but,  after  a  few  remarks  on  the  most  general  form  of  the  horopter,  we  shall 
take  up  those  special  cases  which  are  of  most  interest. 

In  the  most  general  case  the  horopter  is  a  curve  of  the  third  degree, — a 
curve,  therefore,  which  can  be  cut  by  a  plane  in  only  three  points.  It  may 
be  looked  upon  as  the  intersection  of  two  surfaces  of  the  second  degree. 
Surfaces  of  the  second  degree  intersect  in  general  in  a  curve  of  the  fourth 
degree ;  but  in  this  case  the  curve  of  the  fourth  degree  breaks  into  a  curve 
of  the  first  degree,  a  straight  line  which  is  not  part  of  the  horopter,  and 
the  curve  of  the  third  degree,  already  mentioned,  the  horopter  curve.  This 
curve  has  two  branches,  which  extend  to  infinity ;  they  can  be  conceived  as 
on  the  surface  of  a  cylinder.  Three  points  through  which  the  curve  must 
go  we  can  at  once  determine.  One  of  these  is  the  fixation-point  (we  know 
that  this  falls  upon  corresponding  points  in  the  retina),  and  the  two  others 
are  the  nodal  points  of  the  eyes.  Since  these  latter  points  and  the  parts 


BINOCULAR   VISION.  559 

of  the  curve  lying  near  them  are  not  in  reality  pictured  upon  the  retina,  it 
follows  that  the  mathematically  determined  curve  has  parts  which  are  of 
no  practical  significance,  but  which  simply  form  a  part  of  the  solution  of  the 
question  as  mathematically  formulated.  The  curve  determined  by  mathe- 
matics Helmholtz,  therefore,  calls  the  horopter  curve,  and  that  portion  of  it 
which  has  an  actual  significance  he  designates  simply  as  the  horopter,  or 
the  point-horopter.  Hering  calls  it  the  total  horopter. 

In  those  cases  which  we  are  about  to  consider  more  minutely,  the  curve 
of  the  third  degree  is  composed  of  a  straight  line  and  a  curve  of  the  second 
degree ;  in  one  case  both  of  these  curves  lie  in  one  plane,  and  this  entire 
plane  is  then  the  point-horopter. 

Besides  the  point-horopter,  we  can  consider  also  the  line-horopter  (the 
partial  horopter,  according  to  Hering).  This  is  the  total  collection  of  those 
lines  in  space  which  are  seen  singly,  without  their  separate  points  falling 
necessarily  upon  corresponding  points  of  the  retina.  To  the  image  of  a 
point  of  one  of  these  lines  in  one  eye  may  correspond  the  image  in  the 
other  eye  of  some  other  point  of  the  same  line.  Lines  of  this  kind  whose 
images  fall  upon  corresponding  retinal  sections  that  are  parallel  to  the 
retinal  horizons  form,  according  to  Helmholtz,  the  horizontal  horopter  (the 
transversal  horopter,  according  to  Hering).  Lines  whose  images  are  formed 
in  retinal  sections  parallel  to  the  middle  longitudinal  section  form  the 
vertical  horopter  of  Helmholtz  (the  longitudinal  horopter  of  Hering).  In 
general,  the  line-horopter  is  a  surface  of  the  second  degree. 

For  greater  convenience  in  the  determination  of  the  line-horopter  we 
introduce,  with  Hering,  the  idea  of  longitudinal  and  transversal  planes. 
If  we  construct  a  plane  through  a  visual  axis  and  a  middle  cross-section, 
and  in  this  plane  form  a  perpendicular  to  the  visual  axis  through  the  nodal 
point,  then  the  bundles  of  planes  which  go  through  this  line  are  called  trans- 
versal planes.  If  a  plane  be  drawn  through  the  visual  axis  and  the  middle 
longitudinal  section,  and  in  this  plane  a  perpendicular  to  the  visual  axis 
through  the  .nodal  point  be  made,  then  all  planes  which  go  through  this 
perpendicular  are  called  longitudinal  planes. 

We  shall  now  determine  the  horopter  for  certain  special  cases. 

I.  When  the  Visual  Axes  are  Parallel  and  Symmetrical  with  respect 
to  the  Median  Plane. — The  fixation-point  is  at  infinity.  If  the  middle 
cross-sections  are  exactly  horizontal?  then  the  transversal  planes  which  cut 
the  retinas  in  corresponding  lines  are  coincident.  In  this  case,  therefore,  the 
entire  visual  space  is  the  horizontal  horopter.  If  the  middle  cross-sections, 
however,  form  an  angle  with  each  other,  then  the  corresponding  trans- 
versal planes  will  all  intersect  in  the  median  plane,  and  this  plane  becomes 
the  horizontal  horopter.  If  the  middle  longitudinal  sections  are  exactly 
vertical,  the  corresponding  longitudinal  planes  intersect  in  the  plane  at 
infinity.  This  is  then  the  vertical  horopter.  The  point-horopter  is  the 
intersection  of  these  two  line-horopters,— that  is,  when  the  middle  cross- 
sections  are  horizontal.  It  is  also  the  plane  at  infinity. 


560 


BINOCULAR   VISION. 


FIG.  22. 


Scbeme  representing  a  section 
passing  through  the  retinas  and  made 
perpendicular  to  the  visual  plane. 


In  general,  however,  as  we  know,  the  middle  longitudinal  sections  form 
an  angle  V  with  each  other.  In  this  case  the  longitudinal  planes  intersect 

in  a  plane  that  is  parallel  to  the  visual  plane 
and  at  a  finite  distance  from  it.  This  may  be 
perceived  from  the  schematic  Fig.  22,  which 
represents  a  section  passing  through  the  retinas 
and  made  perpendicular  to  the  visual  plane. 
If  we  represent  the  distance  between  the  cen- 
tres of  the  eyes  by  d,  and  the  distance  of 
this  line-horopter  from  the  visual  plane  by  a, 
then  we  have 

d 

~  2  tan  \  V. 

If  the  middle  cross-sections  are  also  inclined 
to  each  other,  the  point-horopter  is  a  straight 
line  in  the  median  plane,  parallel  to  the  visual 
plane  and  at  a  distance  a  beneath  it.  If  the 
middle  cross-sections  are  parallel  (which  is 

usually  the  case),  the  point-horopter  is  a  plane,  and,  in  fact,  the  plane  be- 
neath the  visual  plane  is  situated  at  the  distance  a. 

According  to  Helmholtz,  the  distance  a  is  for  him  and  for  many  other 
persons  equal  to  the  height  of  the  eyes  above  the  ground.  When,  there- 
fore, as  is  usually  the  case  in  walking,  the  visual  plane  is  horizontal,  or 
nearly  so,  and  the  eyes  are  directed  to  a  distant  point,  the  ground  is  the 
horopter  of  points.  Images  of  objects  on  the  ground,  therefore,  fall  upon 
identical  points  in  the  retina.  Helmholtz  believes  that  the  reason  for  the 
convergence  of  the  middle  longitudinal  sections  is  to  be  found  in  this  cir- 
cumstance. 

For  the  following  cases  we  assume  that  the  middle  cross-sections  are  in 
the  same  plane. 

II.  The  Visual  Lines  are  Symmetrical  with  respect  to  the  Median  Plane. 
— The  fixation-point  is  in  the  median  plane  and  at  a  finite  distance.  We 
assume  that  the  middle  cross-sections  have  remained  in  the  visual  plane  as 
the  eyes  have  moved  from  the  parallel  position  of  the  visual  lines.1 

The  horizontal  horopter  consists,  then,  of  the  visual  plane.  The  reason 
for  this  is  that  the  planes  of  the  middle  cross-sections  lie  wholly  in  the 
horizontal  horopter.  This  is  true  for  the  median  plane,  for  in  that  plane 
all  the  other  corresponding  transversal  planes  intersect. 

The  vertical  horopter  consists  (when  the  middle  longitudinal  sections  are 
parallel)  of  a  cylinder  perpendicular  to  the  visual  plane,  whose  section  by 

1  The  motion  of  the  eyes  would  not  then  take  place  in  strict  accordance  with  Listing's 
law.  A  determination  of  the  horopter  in  which  the  departure  of  the  plane  of  the  middle 
cross-sections  from  the  plane  of  the  visual  lines  has  been  taken  account  of  is  given  by 
Helmholtz,  Physiologische  Optik,  S.  717. 


BINOCULAR   VISION. 


561 


Fio.  23. 


that  plane  is  a  circle  through  the  fixation- point  and  the  nodal  points, — the 
horopter  circle  of  Muller.     This  is  easily  understood  from  Fig.  23,  which 

represents  a  section  through  the 
cylinder  in  the  visual  plane.  C 
and  C'  correspond  to  the  middle 
longitudinal  sections,  D  and  D' 
to  any  two  corresponding  longi- 
tudinal sections,  A  is  the  fixa- 
tion-point, K  and  K'  are  the 
nodal  points.  Then  we  have 
DKC=  D'K'C'=  p.  Hence, 
also,  KAK'=KBK';  that  is, 
B  is  on  the  circle  through 
K,  K',  A.  The  point-horopter 
is,  therefore,  this  circle  and  the 
line  perpendicular  to  it  at  the 
fixation-point.  When  the  mid- 
dle longitudinal  sections  are 
convergent,  the  vertical  horopter 
becomes  a  cone  whose  intersec- 
tion with  the  visual  plane  is  the 
same  circle  of  Muller,  and  whose 
vertex  is  the  point  in  which  the 
axes  of  planes  of  the  longitudinal  sections  intersect.  To  find  the  distance 
of  the  vertex  from  the  visual  plane,  we  must  consider  the  section  through 

FIG.  246. 


Diagram  illustrating  the  horopter  circle  of  Muller. 


Diagram  illustrating  section  through  the  visual  plane. 

the  visual  plane  as  represented  in  Fig.  24a.     In  this  figure  A^and  K'  are 
the  nodal  points,  KAE=  P  is  half  the  angle  of  convergence,  KF=  |  d  is 

VOL.  1—36 


562  BINOCULAR   VISION. 

half  the  distance  between  the  eyes,  and  KE  is  perpendicular  to  KA,  the 
visual  line,  and  therefore  parallel  to  the  retina  at  the  visual  centre.  Then 

FKE  is  also  equal  to  p,  and  hence  KE= Construct  now  through 

2  cos  p 

KE  a  plane  perpendicular  to  the  visual  line.  (Fig.  246.)  In  the  figure 
KG  represents  the  section  of  this  plane  with  the  plane  of  the  middle  longi- 
tudinal section,  and  EG  its  section  with  the  median  plane.  KGE  is  there- 
fore equal  to  £  V,  and  hence  for  the  required  distance  we  have 


It  follows  that  the  poiut-horopter  is  the  circle  through  the  nodal  points  and 
the  fixation-point,  and  also  the  straight  line  through  the  fixation-point  and 
the  vertex  of  the  cone. 

III.  The  fixation- Point  is  in  the  Horizontal  Plane  and  the  Convergence 
is  Unsymmetrical. — The  vertical  horopter  is  a  hyperboloid  whose  section 
with  the  visual  plane  is  the  Miiller  circle.  The  horizontal  horopter  consist? 
of  two  planes;  one  of  these  is  the  visual  plane,  and  the  other  is  that  plane 
perpendicular  to  it  which  goes  through  the  intersection  A  of  the  Miiller 
circle  with  the  median  plane,  and  through  one  end  of  the  diameter  of  this 
circle  which  goes  through  the  fixation-point.  The  point-horopter  is  the 
Miiller  circle  and  a  straight  line  which  is  inclined  to  the  visual  plane  and 
which  goes  through  the  just-described  point  A.  (For  further  details  see 
Helmh<  /tz,  "  Physiologische  Optik,"  S.  718.) 

W'  have  hitherto  considered  only  the  horopters  of  vertical  and  of  hori- 
zontal lines.  There  are  other  line-horopters,  among  which  we  may  men- 
tion especially  the  horopter  of  meridians.  It  consists  of  those  straight  lines 
whose  images  fall  in  corresponding  meridians, — that  is,  retinal  sections 
which  go  through  the  intersection  of  the  middle  longitudinal  and  cross 
sections,  and  which  are  inclined  to  them. 

What  we  have  just  given  is  the  mathematical  determination  of  the 
horopter  based  upon  the  experimental  determination  of  certain  identical 
points.  The  entire  horopter  (both  for  lines  and  for  points)  can,  of  course, 
be  determined  experimentally  by  seeking  those  lines  and  points  which  are 
seen  single  while  a  given  point  is  constantly  fixed  upon.  For  this  purpose 
brilliant  and  strongly  illuminated  metal  wires  are  made  use  of,  or  small 
metal  balls  (heads  of  pins,  for  instance)  are  employed.  Even  candle  flames 
can  be  used.  In  this  way  the  empirical  horopter  is  obtained ;  but  much 
practice  in  the  recognition  of  double  images  is  a  necessary  prerequisite. 

VI.    MEANING   OF   THE   HOROPTER   VISION   WITH    DISPARATE    POINTS. 

According  to  what  has  been  said,  only  those  points  in  space  appear 
single  whose  images  fall  upon  identical  points  of  the  two  retinas.  Since, 
for  a  definite  fixation-point,  only  a  single  row  of  points  lie  in  the  horopter, 
only  these  points  ought  to  be  seen  single.  To  these  must  be  added  the  lines 


BINOCULAR  VISION.  563 

which  may  happen  to  lie  in  line-horopters.  It  would  follow,  therefore,  that, 
in  general,  chiefly  double  images  ought  to  be  seen  in  the  visual  field,  and 
very  few  single  images.  It  is  plain,  however,  that  this  does  not  in  the  least 
agree  with  experience.  So  far  are  double  images  from  forming  the  chief 
content  of  our  visual  field,  that  the  layman,  in  general,  knows  nothing  of 
them,  or  at  most  only  after  his  attention  has  been  directed  to  them  by 
experiment.  What  is  the  reason  of  this  ?  We  cannot  assume  that  one  of 
the  double  images  does  not  usually  enter  consciousness  at  all,  for  then  we 
should  not  be  able  to  explain  the  very  considerable  influence  which  vision 
with  two  eyes  has  upon  the  perception  of  depth.  We  need  especially  to 
consider  here  that  we  direct  our  attention  only  to  those  objects  which 
are  at  the  fixation-point  or  are  near  it;  to  those  objects,  that  is,  which 
are  in  the  horopter.  If  another  object  engages  our  attention,  we  direct 
the  gaze  upon  it.  If  we  are  looking  at  a  somewhat  extensive  flat  sur- 
face, it  is  not  the  case  that  we  fix  upon  a  single  point  and  allow  the  atten- 
tion to  wander  over  the  images  of  the  other  points ;  what  we  do  is  to  allow 
the  visual  regard  to  sweep  over  the  entire  surface  of  the  field  of  view  from 
one  point  to  another.  It  is,  therefore,  only  the  horopter  in  the  vicinity  of 
the  fixation-point  that  is  of  especial  importance.  We  have  to  consider  the 
significance  of  this  portion  of  the  horopter ;  and,  further,  the  double  images 
do  not  suddenly  start  up  the  moment  an  object  is  no  longer  exactly  in  the 
horopter.  We  shall  proceed,  therefore,  to  investigate  the  behavior  of 
points  in  the  vicinity  of  the  horopter. 

In  the  first  place,  it  is  easy  to  convince  one's  self  that  the  relief  of  objects 
which  are  in  the  horopter  is  recognized  with  the  greatest  accuracy.  For 
the  horopter  which  is  a  straight  line  Helmholtz  recommends  the  following 
experiment.  A  pin  is  slightly  bent  in  the  middle,  so  that  it  forms  an  angle 
of  about  175°,  and  it  is  then  held  in  the  median  plane  in  such  a  way  that 
both  arms  of  the  angle  fall  as  nearly  as  possible  in  the  horopter  line.  An 
eye  situated  half-way  between  the  two  eyes  would  then  see  the  pin  as  a 
straight  line.  On  fixing  with  both  eyes,  one  recognizes  plainly  the  bend- 
ing ;  but  if  the  pin  is  so  moved  in  the  median  plane  that  it  no  longer  lies 
in  the  horopter  line,  the  fact  that  it  is  bent  can  no  longer  be  determined. 

For  the  horopter  circle  one  can  arrange  the  experiment  in  the  following 
manner.  Three  pins  are  stuck  upright  in  little  wooden  supports,  and  so 
placed  on  a  table  that  the  heads  of  the  pins  are  on  a  level  with  the  eyes. 
A  sheet  of  paper  is  held  in  front  of  the  eyes,  so  that  the  lower  portions  of 
the  pins,  together  with  their  supports,  are  concealed.  If  the  three  heads 
of  the  pins  are  in  a  straight  line  which  is  tangent  to  the  horopter  circle, 
one  can  detect  the  slightest  departure  of  the  middle  pin  from  the  straight 
line.  The  pins  must  be  rather  near  to  each  other,  for  otherwise  an  apparent 
departure  from  a  straight  line  takes  place,  from  a  cause  to  be  mentioned 
farther  on.  Helmholtz  had  the  pins  placed  at  the  distance  of  a  centimetre 
from  each  other  and  situated  about  fifty  centimetres  in  front  of  the  eyes. 
If  the  middle  needle  is  in  the  median  plane,  the  power  of  discrimination 


564  BINOCULAR   VISION. 

for  the  straightness  of  the  line  is  greatest  when  the  line  is  at  right  angles  to 
that  plane;  but  if  the  right-hand  pin  is  in  that  plane,  then  the  line  must  be 
inclined  inward  on  the  left,  because  it  will  then  more  readily  coincide  with 
the  horopter  circle.  If  the  straight  line  in  which  the  pins  lie  cuts  the  circle 
at  an  angle,  the  sensibility  to  its  straightuess  is  considerably  diminished. 

We  have  seen  that,  according  to  Helmholtz,  with  most  persons  (and 
with  himself)  the  ground  is  the  horopter  when  the  visual  regard  is  directed 
horizontally  forward  to  a  remote  point,  and  that  it  is  sufficiently  nearly  so 
when  the  visual  plane  is  slightly  inclined  downward,  as  happens  during 
walking.  Helmholtz  believes  that  he  has  observed  that  relief  in  all  objects 
on  the  ground  is  particularly  well  made  out  when  one  looks  straight  forward. 
It  seemed  to  him  distinctly  less  evident  when  he  looked  underneath  his  arm 
or  between  his  legs,  although  he  placed  himself  so  high  that  the  eyes  were  at 
the  usual  distance  above  the  ground.  That  the  unusual  position  of  the  head 
did  not  contribute  essentially  to  the  effect  was  proved  by  the  fact  that  on 
looking  between  the  legs,  and  at  the  same  time  inverting  the  field  of  view 
by  means  of  a  prism,  the  original  distinctness  of  relief  was  again  obtained. 
Helmholtz  believes  that  he  has  discovered  in  this  circumstance  the  reason 
why  a  convergence  in  the  middle  longitudinal  sections  has  been  developed. 
Other  physiologists — Hering,  for  example — have  either  not  confirmed  this 
observation  or  have  explained  it  differently.  It  must  be  remarked  that 
with  some  observers,  Hering  among  others,  the  convergence  is  not  sufficient 
to  cause  the  longitudinal  sections  to  intersect  at  the  ground.  The  main  sig- 
nification of  the  horopter  lies,  therefore,  in  the  fact  that  in  it  the  perception 
of  relief  is  most  acute. 

If  a  point  is  moved  out  of  the  horopter,  so  that  it  is  seen  with  dis- 
parate portions  of  the  retina,  it  continues  at  first  to  seem  single,  although, 
as  in  the  above-mentioned  experiment  with  the  three  pins,  it  is  seen  not  to 
lie  in  the  horopter  circle.  Only  at  a  considerable  distance  from  the  horop- 
ter does  it  appear  plainly  in  double  images.  It  follows  that  when  the 
disparateness  is  not  great,  double  images  continue  to  suffer  fusion.  On  this 
account,  in  the  determination  of  identical  points  we  have  avoided  using 
lines  which  overlapped  each  other,  choosing  rather  such  as  should  come 
together  in  a  point.  Another  means  for  diminishing  the  fusion  of  double 
images  is  to  hold  differently  colored  glasses  before  the  two  eyes,  or  to  have 
the  two  fields  of  the  haploscope  of  different  colors.  For  the  same  reason 
Fig.  21  is  drawn  white  on  black  on  the  right-hand  side  and  black  on 
white  on  the  left-hand  side. 

The  amount  of  disparateness  at  which  images  cease  to  fuse  is  different 
for  different  observers.  It  also  depends  much  upon  practice :  the  expe- 
rienced observer  can  separate  double  images  when  they  are  very  near  to- 
gether. The  volition  of  the  observer  and  the  direction  of  his  attention  are 
also  of  great  effect  in  the  matter  of  fusion.  If  one  makes  an  effort  to  hold 
in  mind  the  idea  of  position  in  space,  double  images  are  readily  fused ;  but 
if  one  endeavors  to  compare  the  two  monocular  fields  of  view,  the  images 


BINOCULAR  VISION. 


565 


a  b 


are  more  easily  seen  separate.  Another  means  of  separation  is  to  have 
slight  inequalities  in  the  two  figures  to  be  united, — on  one  a  little  mark  on 
the  right  of  a  line,  in  the  other  on  the  left.  In  this  way  the  belief  is  dis- 
turbed that  the  two  images  are  derived  from  a  single  object. 

If,  in  the  haploscopic  drawing,  Fig.  25  (above),  in  which  the  lines  a 
and  b  are  3.2  millimetres  apart  and  c  and 
d  4.6  millimetres,  a  and  c  are  fixed  upon  FIG.  25. 

with  the  right  and  the  left  eye  respectively,  e  a 

b  and  d  will  unite  also,  the  line  bd  appear- 
ing either  behind  or  in  front  of  oc.  If, 
however,  one  introduces  a  point  to  the 
right  of  6  and  to  the  left  of  d  (as  in  Fig. 
25,  below),  and  fixes  as  before,  the  point  is 
seen  single,  and  then  one  easily  succeeds  in 
separating  the  lines  b  and  d, 

It  is  worthy  of  remark  that  vertical 
lines  fuse  much  more  readily  than  hori- 
zontal ones.  Helmholtz  takes  the  reason  a  b 

for   this   to    be   that    in    looking    at   actual    Haploscopic  drawing  illustrating  disparate- 
.  ness  and  fusion. 

objects   the    differences    between   vertical 

distances  upon  the  two  retinas  is  generally  much  less  than  between  hori- 
zontal distances.1 

Volkmann  has  made  experiments  upon  himself  and  others  by  means 
of  a  haploscopic  arrangement  corresponding  to  Fig.  25  (above),  but  with 
one  thread,  d,  movable,  so  that  it  could  be  adjusted  at  different  distances 
from  c,  in  order  to  see  whether,  with  great  disparateness,  fusion  will  still 
occur.  The  absolute  findings  are  not,  of  course,  of  much  consequence,  on 
account  of  individual  differences,  but  they  give  an  indication  of  the  course 
of  the  fusion.  We  give  the  following  table : 


Observer. 

Distance  apart,  in  millimetres. 

Direction  of  Lines. 

ab. 

cd. 

db-cd. 

V  

5.3  - 

3.46 
7.57 
4.88 
6.05 
2.13 
10.00 
4.66 
591 
3/21 
8.48 
4.92 
5.86 

+  1.84) 

—  2.27; 

+  0.42) 
—  0.75  f 
+  3.17) 
—  4.70/ 
+  0.64) 
—  0.61  / 
+  2.09) 
—  3.18  f 
+  0.38  ) 
—  0.56  / 

Vertical. 
Horizontal. 
Vertical. 
Horizontal. 
Vertical. 
Horizontal. 

V  

T.    .                                 ... 

T  

K  

K.               

1  Is  it  not  rather  because  in  real  life,  on  account  of  the  fact  that  we  have  not  an  upper 
and  a  lower  eye  as  well  as  a  right  and  a  left  eye,  actual  horizontal  lines  are  never  seen  in 
double  images  (provided  the  middle  cross -sections  are  in  the  same  plane)  ? 


566  BINOCULAR   VISION. 

The  arrangement  was  equivalent  to  the  observation  of  lines  one  hundred 
and  fifty  millimetres  in  front  of  the  person  looking.  The  individual  dif- 
ferences are  evident,  and  also  the  difference  between  horizontal  and  vertical 
lines. 

The  latter  difference  one  perceives  also  in  looking  haploscopically  at 
two  circles  of  slightly  different  diameters.  Here  the  vertical  portions  unite 
when  the  horizontal  portions  are  distinctly  seen  as  double. 

Volkmann  has  also  made  experiments  concerning  the  greatest  differences 
in  direction  which  can  be  overlooked  with  different  inclinations  of  the  fixed 
line  to  the  vertical.  In  this  case  it  appears  that  nearly  vertical  lines  unite 
much  more  readily  than  those  which  are  nearly  horizontal. 

If  disparate  points  and  lines  can  be  seen  singly,  it  has  seemed  to  some 
persons  to  be  a  logical  consequence  that  points  whose  images  fall  upon  iden- 
tical portions  of  the  retina  can  be  seen  double.  We  have  seen  that  in  Fig. 
25  the  right-hand  lines  unite  when  the  left-hand  lines  are  fixed  upon.  Those 
portions  of  the  diagram  which  correspond  to  the  two  points  of  the  lower 
figure  now  fall  upon  identical  points  in  the  two  retinas.  They,  however, 
lie  the  one  to  the  right  and  the  other  to  the  left  of  the  single  image  cd.  We 
cannot  introduce  these  points  into  the  drawing,  because  the  fusion  of  the 
two  lines  would  be  interfered  with.  Wheatstone,  who  first  gave  expression 
to  the  view  that  we  can  see  double  with  identical  points,  endeavored  to 

prove  it  by  the  diagrams  drawn  in  Fig.  26. 
FIG.  26.  The  heavy  line  on  the  left  corresponds  to  the 

fine  line  on  the  right,  while  the  heavy  line  on 
the  right  is  inclined  to  that  at  an  angle  of 
10°.  On  uniting  them  haploscopically  the 
two  heavy  lines  fuse.  Helmholtz  has  re- 
marked that  for  eyes  accustomed  to  the  sepa- 
ration of  double  images  the  inclination  is  too 
great ;  he  has  changed  the  experiment  sorne- 

Wheatstone's  diagram  to  prove  double         ,  ,    ,      ,  ,  ,    ,      . 

vision  with  identical  points.          what,  and  he  has  also  added  the  experiment 

represented  in  Fig.  27.     In  such  a  figure  the 

fields  A  and  B  are  to  be  colored  red,  C  and  D  green,  and  the  points/ 
and  g  are  to  be  fixed  upon.  The  two  small  crosses  i  and  h,  which  indicate 
two  points  which  are  impinged  upon  identical  portions  of  the  retinas, 
must  not  be  drawn.  Upon  haploscopic  fusion  the  borders  of  the  diagrams 
unite,  and  also  the  lines  ab  and  cd,  and  one  gets  the  single  impression 
of  a  surface  which  is  inclined  to  the  vertical  direction  and  which  is  half 
green  and  half  red.  The  points  indicated  by  the  crosses,  therefore,  are,  in 
accordance  with  Wheatstone's  view,  red  for  one  eye  and  green  for  the  other. 
According  to  Helmholtz,  these  experiments  teach  us  that  "  as  long  as  one 
is  absorbed  in  the  contemplation  of  objects  or  solid  bodies,  even  when  the 
fixation-point  is  kept  constant,  corresponding  impressions  are  utilized  to  fill 
out  different  portions  of  the  total  corporeal  image."  This  view,  however, 
has  found  many  opponents  as  well  as  many  defenders.  Among  the  former 


BINOCULAR   VISION.  567 

is  Hering,  who  interprets  the  experiment  concerning  the  fusion  of  the  lines 
ab  and  cd  to  the  effect  that  the  sensations  corresponding  to  places  indi- 
cated by  the  crosses  do  not  enter  consciousness  at  all,  but  that  the  sensation 
of  color  on  either  side  of  the  bounding  line  always  agrees  with  the  position 
of  that  line. 

In  the  different  experiments  which  we  have  described  concerning  single 
vision,  a  given  point  has  been  required  to  be  constantly  fixed  upon.  We 
cannot,  however,  be  absolutely  certain  that  the  visual  regard  has  not  wan- 
dered from  time  to  time  to  another  portion  of  the  field  of  view,  and  that 
this  wandering,  which  plays  such  an  important  part  in  vision  in  common 
life,  has  not  been  of  some  influence  in  the  perception  of  depth.  Briicke 
has,  in  fact,  proposed  the  hypothesis  that  seeing  single  with  disparate  points 
is  only  apparent,  and  that  the  conception  of  space  is  acquired  through  the 
constant  wandering  of  the  visual  regard,  by  means  of  which  one  point  after 
another  throws  its  image  upon  identical  points  and  is  in  consequence  seen 
single.  This  view  is  contradicted  by  experiments  in  which  the  picture  is 

FIG.  27. 


6  d 

Helmholtz's  diagram  to  prove  double  vision  with  identical  points. 

offered  to  the  eye  for  so  short  a  time  that  there  is  no  possibility  of  a  change 
of  the  point  of  convergence.  In  the  first  place,  Dove  showed  that  in 
instantaneous  illumination  by  the  electric  spark,  perfect  perception  of  depth 
is  obtained  with  the  stereoscope.  In  order  to  keep  the  fixation-point  con- 
stant when  the  illumination  is  not  obtained  by  the  electric  spark,  Aubert 
pierced  fine  needle-holes  in  corresponding  points  of  two  stereographs,  and 
illuminated  them  from  behind.  In  this  experiment  also  there  was  a  perfect 
effect  of  depth  without  the  needle-hole  appearing  double.  Aubert,  indeed, 
has  found  that,  with  instantaneous  illumination,  the  spatial  conception 
prevails  in  cases  where,  with  a  longer  illumination,  one  can  choose  between 
bringing  to  consciousness  double  images  with  no  depth  concept,  or  a  single 
picture  with  extension  in  depth.  For  instance,  if  a  vertical  line  is  presented 
to  one  eye,  and  to  the  other  there  is  given  a  line  inclined  at  an  angle  of 
10°,  one  can  see  at  pleasure  either  two  lines  crossing  each  other  or  a  single 
line  in  a  definite  position  in  space. 

Hering  has  devised  experiments  which  have  had  the  same  result.  He 
looked  through  a  short  tube  at  a  needle  which  was  placed  at  a  convenient 
distance  beyond  the  tube,  while  his  assistant  dropped  little  balls  of  different 


568  BINOCULAR   VISION. 

sizes  at  varying  distances  in  front  of  and  behind  the  thread.  The  observer 
could  always  tell  with  certainty  whether  the  course  of  the  ball  was  behind 
or  in  front  of  the  needle.  Rogers  has  succeeded  in  producing  after-images 
of  stereoscopic  drawings  in  corresponding  eyes,  and  in  uniting  them  after- 
wards into  the  picture  of  an  object  in  space. 

If  points  in  space  are  outside  the  region  within  which  fusion  of  double 
images  occurs  (stereoidentieal  points  in  the  nomenclature  of  Aubert),  double 
images  are  plainly  visible,  and  such  images  might  have  been  supposed  to  be 
of  less  consequence  in  the  perception  of  depth ;  but  in  Hering's  experi- 
ments with  the  falling  balls  it  was  found  that  the  distance  of  a  ball  could 
be  given  no  less  correctly  when  it  appeared  plainly  in  double  images. 

VII.    LOCALIZATION. 

It  was  said  in  the  beginning  that  we  project  outward  into  space  excita- 
tions by  light  which  reach  us  as  sensations,  and  that  we  refer  every  such 
excitation  to  a  more  or  less  definite  point  in  the  visual  space.  We  shall 
proceed  to  determine  the  laws  for  this  projection,  and  to  test  the  accuracy 
and  the  correctness  of  such  localization. 

Very  distant  objects,  even  when  looked  at  with  two  eyes,  we  localize,  as 
we  have  seen,  when  other  means  of  knowledge  are  wanting,  in  one  and  the 
same  distant  surface.  In  this  case  it  is,  therefore,  in  general  a  matter  of 
indifference  whether  we  see  with  one  or  with  two  eyes.  So  also  when  we 
look  at  nearer  objects  which  are  nearly  in  the  vertical  horopter ;  this  case 
occurs  when  the  object  is  a  plane  of  small  extent  perpendicular  to  the  plane 
of  regard  (the  convergence  being  symmetrical).  In  all  these  cases,  and  in 
all  vision  with  one  eye,  only  the  form  and  the  size  of  objects  are  taken 
account  of.  In  general,  we  localize  within  a  given  plane  both  correctly  and 
accurately ;  we  compare  different  objects  by  means  of  movements  of  the 
eyes ;  equal  lines,  for  instance,  we  picture  one  after  another  upon  the  same 
portion  of  the  retina.  We  need  only  consider  here  those  departures  from 
correct  localization  which  appear  when  the  eyes  are  at  rest.  One  error  of 
this  kind  we  have  already  noticed  ;  it  follows  from  the  convergence  of  the 
middle  longitudinal  sections.  In  consequence  of  this,  a  right  angle  with 
one  horizontal  and  one  vertical  side  is  not  seen  as  rectangular.  It  was 
pointed  out  that  the  right  side  of  Fig.  21  represents  a  rectangular  grating 
for  the  right  eye,  the  left  side  for  the  left  eye. 

Other  illusions  are  the  following.  Select  three  stars  in  the  sky,  at  a 
considerable  distance  apart,  which  seem  to  lie  in  a  straight  line  when  the 
eye  wanders  along  this  line ;  if  they  are  looked  at  with  the  periphery  of 
the  retina,  they  will  seem  to  be  in  a  curved  line,  concave  towards  the 
fixation-point.  If  the  drawing  given  in  Fig.  28  be  enlarged  n  times  (five 
or  ten  times,  for  instance),  and  held  at  a  distance  from  the  eye  equal  to  n 
times  the  line  underneath  it,  the  eye  being  just  opposite  the  middle  of  the 
drawing,  and  this  point  being  fixed  upon,  then,  according  to  Helmholtz, 
the  appearance  of  a  rectangular,  chess-board-like  figure  is  obtained.  (For 


BINOCULAR   VISION.  569 

the  explanation,  and  for  other  illusions  connected  with  this,  see  Helmholtz, 
"  Physiologische  Optik"  (1st  ed.),  S.  582,  ff.)  The  curves  in  the  figure 
are  hyperbolas. 

A  given  distance  seems  to  be  longer  if  it  is  cut  up  into  subdivisions. 
Thus  AB,  in  Fig.  29,  looks  longer  than  BC,  although  it  is   in  reality 

FIG.  28. 


Chess-board-like  figure  of  Helmholtz. 

of  exactly  the  same  length.  Acute  angles  appear  too  thick,  obtuse  angles 
too  slender.  In  Fig.  30,  for  example,  ef  appears  to  be  the  continuation  of 
ab  instead  of  cd,  although  cd  is  really  in  the  same  line  with  it.  In  this 
way  is  explained  the  illusion  of  Zollner,  which  is  illustrated  in  Fig.  31. 
The  long  lines,  which  are  inclined  45°  to  the  horizontal  direction,  are  in 
reality  parallel,  but  they  seem  to  be  alternately  convergent  and  divergent. 

FIG.  30. 


FIG.  29. 


Figure  showing  subdivided  line.  Figure  showing  apparent  increase  of  acute  angle. 

If  we  assume  that  the  apparent  increase  of  the  acute  angles  formed  by 
the  short  cross-lines  with  the  long  ones  proceeds  in  such  a  way  that  both 
sides  of  an  angle  are  turned  from  their  true  direction,  the  illusion  may  be 
considered  to  be  dependent  upon  the  size  of  the  angles.  Opinion,  however, 
is  much  divided  as  to  the  proper  explanation  to  be  given  of  this  and  other 
similar  illusions. 


570 


BINOCULAR   VISION. 


As  regards  the  direction  in  which  we  localize,  it  is  clear  that  in  vision 
with  one  eye  we  refer  objects  to  some  point  in  the  line  of  sight :  that  is  a 
consequence  of  the  laws  regarding  the  formation  of  the  image  in  the  eye. 


FIG.  81. 


Zollner's  lines. 


It  is  quite  otherwise,  however,  as  was  first  shown  by  Hering  and  Towne, 
when  we  observe,  as  is  usual,  with  both  eyes.  If  in  Fig.  32  we  fix  upon  an 
object,  a,  the  point  of  a  needle,  say,  at  the  distance  of  distinct  vision,  we  see 


FIG.  32, 


Diagram  illustrating  actual  and  apparent  directions  of  objects. 

other  objects  at  the  same  time ;  for  instance,  an  object  b  somewhere  behind 
a  in  the  line  of  sight  of  the  right  eye,  and  an  object  c,  also  behind  a,  but  in 
the  line  of  sight  of  the  left  eye.  If,  while  fixing  upon  a,  we  direct  the  at- 


BINOCULAR   VISION.  571 

tention  to  6  and  c,  we  shall  notice  that  the  images  a,  6,  and  c  all  reach  the  eye 
in  the  same  direction.  If  the  object  a  is  in  the  median  plane,  this  direction 
of  projection  is  also  in  the  median  plane.  Introduce  now  another  point,  a', 
which  is  in  the  horopter,  and  whose  images,  therefore,  fail  upon  identical 
points.  On  the  sight-line  of  the  right  eye  belonging  to  a'  place  another  ob- 
ject, 6',  and  on  that  of  the  left  eye  another  object,  c'.  The  projection-lines 
of  a',  &',  and  c'  coincide  the  same  as  before.  These  lines  are  called  the  lines 
of  visual  direction,  and  among  them  that  which  contains  the  fixation-point  is 
called  the  principal  line  of  visual  direction.  The  obscurely  seen  objects  6,  c, 
&',  c',  appear,  of  course,  in  double  images,  but  such  images  are  frequently 
so  far  apart  that  we  are  conscious  only  of  the  one  here  taken  account  of. 
Objects  which  lie  really  on  very  different  lines,  and  even  on  different  sides 
of  the  head,  are,  therefore,  localized  on  one  line.  There  is  a  certain  pair  of 
sight-lines  to  which  a  single  direction-line  corresponds  in  such  a  manner  that 
all  objects  on  either  of  these  sight-lines  appear  to  be  on  one  and  the  same 
direction-line.  There  arises,  therefore,  a  sheaf  of  direction-lines,  all  of 
which  proceed  outward  from  a  single  centre.  The  position  of  this  centre  is, 
for  normal  individuals  who  are  in  the  habit  of  using  both  eyes  equally, 
situated  half-way  between  the  two  eyes,  at  the  base  of  the  nose.  If  we  sup- 
pose an  imaginary  eye  to  be  situated  at  this  place,  to  which  the  images  of  the 
two  actual  eyes  are  transferred  in  the  proper  manner,  then  the  sight-lines  of 
this  single  eye  will  correspond  to  the  lines  of  visual  direction  in  binocular 
vision.  This  imaginary  eye  has  been  called  the  Cyclopean  eye.  For  per- 
sons who  are  in  the  habit  of  using  only  one  eye — for  those  who  work  much 
with  the  microscope,  for  instance — it  may  be  customary  to  refer  the  direc- 
tions of  objects  to  that  eye  only ;  but,  as  before,  objects  which  are  pictured 
upon  the  same  points  of  the  retina  seem  to  lie  in  the  same  direction.  The 
illustration  of  this  relation,  which  is  given  in  Fig.  32,  is  due  to  Hering. 
The  drawing  on  the  left  represents  the  directions  of  objects  as  they  actually 
are,  and  that  on  the  right  their  directions  as  they  appear  to  be. 

From  what  has  just  been  said,  it  follows  that  whatever  is  pictured  upon 
the  middle  cross-section  of  the  retina  seems  to  be  situated  in  a  plane  which 
divides  the  visual  space  into  an  upper  and  a  lower  half.  Hering  calls  this 
plane  the  middle  cross-plane  of  the  visual  space.  So  everything  which  is 
pictured  upon  the  middle  longitudinal  section  is  localized  in  a  plane  which 
divides  the  visual  space  into  a  right  and  a  left  half, — the  middle  longitudinal 
plane. 

We  shall  now  investigate  the  effect  upon  localization  of  images  which 
fall  on  disparate  portions  of  the  retina,  when  the  images  are  so  near  to- 
gether as  to  be  nearly  or  completely  fused.  We  have  seen  that  we  estimate 
with  great  accuracy  the  relative  position  of  objects  which  are  nearly  in  the 
horopter.  If  we  look  at  a  row  of  vertical  threads  hung  in  a  plane  at  right 
angles  to  the  median  plane,  and  consequently  nearly  in  the  vertical  horopter, 
and  if  we  move  one  of  the  threads  a  little  out  of  the  plane,  we  can  easily 
detect  the  change.  We  can  also  determine  whether  the  thread  which  has 


572  BINOCULAR  VISION. 

been  moved  is  in  front  of  the  other  threads  or  behind  them.  In  both  cases 
there  is  disparateness  of  images  in  the  transverse  direction  ;  but  when  the 
thread  is  in  front  of  the  plane,  the  left-hand  image  has  been  moved  to  the 
right  and  the  right-hand  image  to  the  left  (heteronymous  images)  •  and  when 
the  thread  is  behind  the  flame,  the  left-hand  image  has  been  moved  to 
the  left  and  the  right-hand  image  to  the  right  (homonymous  images).  The 
eye,  it  is  therefore  evident,  is  able  to  distinguish  between  heteronymous  and 
homonymous  images,  and  to  make  use  of  this  distinction  in  localization. 
This  fact  disproves  the  view  of  Johannes  Miiller,  that  corresponding  por- 
tions of  the  retina  play  a  like  rdle  in  localization.  In  Fig.  33,  let  a  be  the 

FIQ.  33. 


•I 

yu  • 

Diagram  illustrating  heteronymous  and  homonymous  double  images. 

fixation-point  and  let  be  be  the  tangent  to  the  point-horopter.  Let  d  and 
d'  be  identical  retinal  points,  and  also  e  and  e'.  Sight-lines  from  d  and  e' 
will  intersect  in  a  point  within  the  horopter,  as  /,  and  lines  from  d'  and  e 
will  intersect  in  a  point  without  the  horopter,  as/'.  In  the  one  case  the 
double  images  are  heteronymous,  in  the  other  they  are  homonymous ;  and 
the  fact  that  we  can  readily  tell  whether  the  thread  has  been  moved  forward 
or  backward  shows  that  we  can  distinguish  between  these  two  conditions.1 

1  The  subject  of  localization  has  been  put  upon  quite  a  different  footing  by  means  of 
an  experiment  of  Schon's.  (Archiv  fur  Ophthalmologie,  Bd.  xxi.  See  also  Nature,  Feb- 
ruary 13,  1896. )  He  finds  that  the  circumstance  which  is  really  of  moment  in  distinguishing 
between  an  object  at  0  and  one  at  7  (outside  and  inside  the  horopter  circle)  in  Fig.  33a  is 
not  that  in  one  case  the  right  eye  sees  the  right  image  and  the  left  eye  the  left  image, 
while  in  the  other  case  the  images  are  crossed,  as  it  is  impossible  that  the  eye  can  tell 
whether  certain  imaginary  lines  in  space  are  crossed  or  not.  The  determining  difference  is 
due  to  the  fact  that  the  nasal  half  of  the  retina  sees  objects  much  brighter  than  the  tem- 
poral half,  and  that  in  the  one  case  the  bright  image  is  farther  from  the  fovea  than  the 
faint  image,  and  in  the  other  case  it  is  nearer  to  it.  An  object  at  O  is  seen  by  means  of 


BINOCULAR   VISION.  573 

The  thread,  which  in  the  above  experiment  was  moved  out  of  the  plane 
of  the  others,  gave  disparate  images  in  the  transverse  direction  only. 

If  we  rotate  the  threads  by  90°,  keeping  them  still  in  the  same  plane, 
so  that  they  are  horizontal  and  parallel  to  the  plane  of  visual  regard,  and 
then  move  one  thread  a  little  out  of  its  plane,  its  departure  from  ite  original 
position  cannot  be  detected.  In  this  case  the  disparateness  is  longitudinal, 
and  it  follows  that  that  is  of  very  little  influence  (according  to  Helmholtz), 
or  of  none  at  all  (according  to  Hering).1 

In  these  and  in  the  following  experiments  it  is  necessary  to  have  a  plain 
background  against  which  the  objects  to  be  looked  at  are  sharply  defined. 

If,  with  the  primary  position  of  the  head  (upright  and  directed  straight 
forward)  and  with  a  horizontal  position  of  the  plane  of  regard,  one  fixes 
his  vision  upon  a  thread  suspended  in  the  median  plane,  and  if  the  thread 
is  then  rotated  in  the  median  plane  about  the  point  of  fixation,  the  upper 
and  the  lower  halves  of  the  thread  are  seen  in  double  images,  one  pair 
being  heteronymous  and  the  other  homonymous.  In  each  pair,  however, 

the  double  images  n'  and  t,  an  object  at  7  by  means  of  the  double  images  t'  and  n.  In 
both  cases  the  images  fall  upon  points  of  the  retina  which  correspond  to  the  points  A  and  B 
of  the  horopter  circle  ;  but  in  the  one  case  (outside)  B  repre- 
sents the  position  of  the  bright  image  and  A  the  position 
of  its  shadowy  attendant,  and  in  the  other  A  is  the  po- 
sition of  the  bright  (nasal)  image  and  B  that  of  the  (tem- 
poral) shadowy  attendant.  That  this  difference  in  bright- 
ness is  a  sufficient  criterion  for  locating  outside  or  inside  the 
horopter  circle  (there  may  be  others,  as  size  of  image,  color, 
etc.)  is  absolutely  proved  by  this  ingenious  device  of Schon. 
Instead  of  placing  actual  objects  at  the  points  O  and  /,  he 
has  objects,  which  must  be  identical  in  appearance,  some- 
where in  the  sight-lines  through  O  and  /,  but  farther  away, 
as  at  the  points  N,  N',  T,  T' .  The  objects  N'  and  T  form 
images  at  n'  and  t;  the  objects  N  and  T'  form  images  at  n 
and  t'.  If  now  all  the  other  images  of  these  objects  are 
carefully  cut  off  by  little  screens,  then  the  most  probable 
interpretation  of  what  is  seen  is  that  there  is  in  the  one 
case  a  real  object  at  O,  and  in  the  other  a  real  object  at  /, 
the  nasal  and  temporal  distribution  of  images  being  such  as  Diagram  illustrating  Schon's  ex- 
real  objects  at  these  points  would  effect.  By  changing  the  periment. 
actual  relative  illumination  of  the  objects  N'  and  T,  how- 
ever (by  throwing  light  on  T\>\  means  of  a  mirror,  or  by  putting  a  gray  glass  in  front  of 
Nf),  Schon  changes  the  relative  brightness  of  the  images  n'  and  t;  that  is,  he  causes  the 
image  which  is  near  the  fovea  (t)  to  be  brighter  than  that  which  is  farther  away  (n'). 
This  is  the  condition  for  believing  an  object  to  be  at  /,  and  in  fact  he  has  succeeded  in  this 
way,  with  most  observers,  in  changing  the  apparent  position  of  the  illusory  object  from  O 
to  /,  or  buck  again,  at  pleasure.  The  rule  for  localization  should,  therefore,  be  stated  thus  •. 
we  localize  outside  thp  horopter  circle  when  the  principal  image  (the  image  formed  by  the 
near  eye)  has  a  shadowy  attendant  on  the  inside  (that  is,  nearer  to  the  fixation-point)  ; 
and  we  localize  inside  the  horopter  circle  when  the  principal  image  has  a  shadowy  attend- 
ant on  the  outside  (that  is,  farther  from  the  fixation-point).  In  both  cases,  therefore,  there 
may  be  said  to  be  a  sort  of  heteronymy.— TRANS. 

1  In  other  words,  we  are  provided  with  two  eyes  in  a  right-and-left  direction,  but  not 
in  an  up-and-down  one. — TRANS. 


574  BINOCULAR   VISION. 

one  image  is  moved  as  far  to  the  right  as  the  other  is  moved  to  the  left.  In 
such  a  case  as  this  the  double  images  are  said  to  be  symmetrical.  From  the 
fact  that  the  thread  is  constantly  referred  to  the  median  plane,  we  conclude 
that  points  whose  disparate  images  fall  symmetrically  on  either  side  of  the 
middle  longitudinal  section  are  referred  to  the  median  plane.  If,  under 
the  same  conditions,  one  looks  at  beads  which  are  in  a  horizontal  plane  at 
the  level  of  the  eyes,  partly  in  front  of  and  partly  behind  the  horopter,  and 
if  one  bends  the  head  forward  and  backward,  keeping  the  fixation-point 
constant,  the  middle  cross-sections  become  inclined  to  each  other,  and  the 
images  of  the  beads,  in  consequence,  become  symmetrically  disparate  in  a 
longitudinal  direction.  In  this  case,  also,  they  continue  to  be  referred  to 
the  plane  in  which  they  actually  are, — the  plane  of  the  middle  cross-section. 

A  vertical  thread,  looked  at  with  symmetrical  convergence  and  with  the 
plane  of  regard  horizontal  (the  head  being  in  the  primary  position),  appears, 
according  to  Helmholtz,  to  be  vertical,  in  spite  of  the  fact  that,  on  account 
of  the  convergence  of  the  middle  longitudinal  sections,  the  thread  can  be 
plainly  seen  (if  it  is  sufficiently  long)  to  furnish  double  images.  According 
to  Hering,  the  thread  appears  to  be  vertical  when  it  coincides  with  the 
straight  line  of  the  point-horopter. 

If,  while  looking  at  a  vertical  thread,  one  inclines  the  head  backward, 
the  plane  of  regard  being  still  horizontal,  the  lower  end  of  the  thread 
seems  to  be  nearer  to  the  observer :  this  proceeds  from  the  fact  that  with 
this  movement  of  the  eyes  the  middle  longitudinal  sections  converge  more 
above.  According  to  Hering,  the  apparent  vertical  position  of  the  thread 
is  obtained  (upon  moving  its  lower  end  backward)  when  its  images  fall 
upon  the  middle  longitudinal  sections.  Helmholtz  says  that  it  is  obtained 
when  they  fall  upon  the  same  (disparate)  sections  upon  which  the  images 
of  a  vertical  line  fall  when  the  head  is  in  the  usual  position. 

A  row  of  threads  in  a  vertical  plane  perpendicular  to  the  median  plane 
seems  to  lie  in  a  plane  that  is  convex  towards  the  observer  (provided  the 
outer  threads  are  seen  under  a  sufficiently  great  angle).1  In  order  that  they 
may  apparently  lie  in  a  plane,  they  must  actually  be  in  a  surface  slightly 
concave  towards  the  observer.  Hering  holds  that  this  surface,  whose 
intersection  with  the  visual  plane  is  of  less  curvature  than  the  Miiller  circle, 
is  the  real  vertical  horopter,  the  arrangement  of  the  identical  points  not 
corresponding  exactly,  in  his  view,  with  the  representation  which  we  have 
given  of  them.  Helmholtz  asserts  that  the  apparent  curvature  is  to  be 
explained  by  the  fact  that  we  are  in  the  habit  of  underestimating  distances.2 

1  In  the  experiments  hitherto  described  with  threads,  it  was  assumed  that  they  were 
so  near  together  that  the  departure  of  the  horopter  circle  from  a  straight  line  might  be 
neglected  ;  and  so  also  in  Helmholtz's  experiments  with  the  three  needles. 

2  From  the  above  it  appears  that  disparateness  in  a  longitudinal  direction  alone  has  very 
little  effect  in  changing  our  localization  of  objects.    It  follows  that  we  localize  points  whose 
images  are  either  identical  or  differ  longitudinally  only  in  a  surface  which  we  shall  call  the 
fundamental  surface  (Kernflache  of  Hering).    According  to  Hering,  this  surface  seems  to  us 
to  be  a  plane  :  according  to  Helmholtz,  it  appears  to  be  slightly  concave  towards  the  observer. 


BINOCULAR   VISION.  575 

If  we  have  three  threads,  a,  b,  and  c,  in  a  vertical  plane,  6  being  in  the 
median  plane  and  a  and  c  at  equal  distances  to  the  left  and  to  the  right, 
there  is  the  portion  of  the  retina  which  corresponds  to  the  distance  ab. 
This  is  less  in  the  right  eye  than  in  the  left  eye,  and  that  which  corresponds 
to  cd  is  greater  in  the  right  eye  than  in  the  left  eye.  The  difference  in 
these  retinal  distances  is  (for  a  given  distance  of  the  threads  in  front  of  the 
eyes)  a  mark  by  means  of  which  we  estimate  the  surface  to  be  concave,  con- 
vex, or  plane.  If  now  we  refer  the  threads  to  a  distance  that  is  less  than 
their  actual  distance  from  the  observer  (every  other  means  of  estimating 
distance,  except  the  feeling  of  convergence,  being  absent),  we  shall  see  the 
surface  as  convex.  If,  however,  there  are  beads  on  the  threads,  the  illusion 
in  regard  to  the  curvature  of  .the  surface  of  the  threads  will  not  take  place. 

If  one  looks  at  lines,  or  wires,  which  go  through  a  point,  and  which  lie 
in  a  plane  that  is  perpendicular  to  the  median  plane  and  to  the  visual  plane, 
the  visual  regard  being  directed  upward  and  the  intersection  of  the  lines 
being  the  fixation-point,  the  upper  lines  of  a  star  will  seem  to  be  in  a 
concave  surface  of  a  cone,  as  v.  Recklinghausen  has  shown,  while  the  lower 
lines  appear  convex.  This  also  is  explained  by  the  fact  that,  on  account 
of  the  movement  of  the  eyes,  the  middle  cross-section  is  no  longer  in  the 
visual  plane.  The  surface  in  which  the  lines  must  lie,  if  they  are  to  seem  to 
be  in  a  plane  that  is  perpendicular  to  the  median  plane  and  to  the  visual 
plane,  is,  according  to  theory  and  to  v.  Recklinghausen's  measurements,  a 
cone  of  the  second  degree.  For  eyes  with  parallel  longitudinal  sections, 
the  surface  is  the  horopter  of  meridians  for  the  given  position  of  the  eyes : 
v.  Recklinghausen  designates  it  as  the  normal  surface. 

That  the  degree  of  convergence  has  an  effect  upon  the  distance  at  which 
we  localize  an  object  is  beyond  a  doubt.  If  stereoscopic  pictures  are  intro- 
duced into  a  mirror  haploscope  and  a  given  point  is  fixed  upon  (the  axes 
of  the  eyes  being  convergent),  and  if  the  mirrors  with  the  pictures  are 
then  so  rotated  that  the  convergence  is  diminished,  the  objects  looked  at  will 
seem  to  move  farther  away.  Moreover,  since  the  visual  angle  remains  the 
same,  they  will  appear  to  become  larger.  Wundt  has  made  experiments 
to  determine  with  what  degree  of  exactness  changes  of  distance  can  be  esti- 
mated by  change  of  convergence  alone, — this  being  accomplished  by  means 
of  a  thread  suspended  in  the  median  plane  in  front  of  a  white  background 
and  moved  forward  and  backward  by  a  degree  of  motion  that  can  be 
measured.  During  these  experiments  Wundt  found  that  when  the  thread 
was  at  a  distance  of  one  hundred  and  sixty  centimetres  the  change  of  dis- 
tance that  could  be  just  detected  (whether  the  thread  was  placed  nearer  or 
farther  away)  was  three  centimetres,  whilst  for  a  distance  of  fifty  centimetres 
it  was  one  centimetre.  The  estimation  was,  therefore,  very  exact. 

Wundt  has  also  made  experiments  on  the  exactness  of  our  estimation 
of  the  absolute  distance  of  a  thread,  and  has  found  it  to  be  very  slight ; 
the  distance  was  always  underestimated.  His  results  (in  centimetres)  wc-re 
these : 


576  BINOCULAR   VISION. 

Actual  Distance.  Estimated  Distance. 

180 120 

120 58 

80 47 

40 25 

He  says  that  very  distant  objects  are  judged  to  be  too  near  when  other 
means  of  correcting  our  judgment  are  wanting, — when,  for  example,  we  do 
not  know  their  actual  size.  This  is  a  matter  of  daily  experience,  as  in  the 
case  of  distant  lights,  the  moon,  the  sun,  and  the  stars. 

Finally,  we  have  to  consider  localization  in  those  unusual  cases  in  which 
objects  which  are  actually  double  seem  to  us  to  be  single.  This  happens, 
of  course,  when  we  use  the  stereoscope  and  the  haploscope.  With  the  stereo- 
scope, however,  a  number  of  different  considerations  derived  from  experience 
have  a  determining  influence.  We  are  more  interested  in  those  cases  in 
which  such  extraneous  considerations  are  wanting,  and  in  which  also  com- 
parison can  take  place  with  actual  objects  in  space. 

If  we  stand  in  front  of  a  distinct  wall-paper  pattern,  we  can,  by  the 
use  of  strong  convergence,  cause  one  figure  of  the  pattern  to  overlap  the 
adjoining  one.  The  image  which  we  see  then  seems  to  be  swimming  in 
the  air  in  front  of  the  wall.  If  one  looks  at  a  distant  point,  and  if  a 
needle  is  held  in  each  line  of  sight,  two  images  of  two  needles  can  be 
brought  into  fusion  with  each  other.  A  thicker  needle  will  seem  to  be 
seen  at  a  greater  distance.  In  this  case,  therefore,  we  localize  in  accordance 
with  convergence.  If  two  like  coins  are  placed  on  a  table  at  a  distance 
apart  that  is  equal  to  the  distance  between  the  eyes,  and  if  their  images  are 
made  to  coincide,  the  lines  of  sight  being  parallel,  the  coins  are  not  localized 
at  the  distance  that  they  should  be  in  accordance  with  the  convergence.  This 
is  so  on  account  of  the  influence  of  the  near  table  and  other  objects  on  it.  If, 
however,  the  same  coins  are  united  with  the  lines  of  sight  crossed,  a  small  coin 
will  be  seen,  as  in  the  case  of  the  wall-paper  pattern,  floating  in  the  air. 

Upon  summing  up  the  facts  which  we  have  described  in  this  section,  we 
come  to  the  conclusion  that  for  those  portions  of  the  field  of  view  to  which 
the  attention  is  customarily  directed — the  fixation-point  and  its  immediate 
vicinity — the  accuracy  as  well  as  the  correctness  with  which  we  localize 
objects  is  very  great ;  the  exceptions  are,  in  comparison  with  the  extent  of 
the  phenomena  that  are  presented  to  us,  unimportant. 

VIII.    RIVALRY   OF   VISUAL   FIELDS   AND   OF   CONTOURS. 

We  have  now  to  discuss  certain  phenomena  which  escape  our  observa- 
tion in  the  ordinary  use  of  the  eyes  for  practical  purposes,  and  which  are 
discovered  only  upon  making  experiments  with  the  stereoscope  and  the 
haploscope,  but  which  are  nevertheless  of  much  importance  for  the  better 
comprehension  of  the  processes  of  binocular  vision. 

In  the  experiments  with  the  haploscope  for  the  determination  of  the 
identical  points  of  the  retinas  \ve  have  repeatedly  used  fields  of  such  a  char- 
acter that  on  one  side  was  represented  a  black  line  on  a  white  surface, 


BINOCULAR   VISION.  577 

while  on  the  other  side  there  was  no  such  line.  At  the  same  time  certain 
other  lines  appeared  alike  on  both  sides.  In  these  experiments  we  were 
able  to  state  that  in  the  first  case  the  black  lines  presented  themselves  in  the 
binocular  field  exactly  as  in  the  second ;  that  is  to  say,  they  were  not  gray, 
as  one  might  have  expected  on  account  of  the  fact  that  the  corresponding 
portion  of  the  other  retina  was  at  the  same  moment  receiving  the  sensation 
of  white.  If  in  one  haploscopic  field  we  have  two  large  letters,  A  and  J5, 
and  in  the  other  two  similar  letters,  B  and  C,  and  if  we  unite  the  two 
fields  in  such  a  way  that  B  falls  upon  identical  parts  of  the  two  retinas  and 
is  seen  single,  with  A  on  one  side  of  it  and  C  on  the  other,  then  all  three 
letters  look  alike  black  on  a  white  ground.  If,  when  reading  a  book,  we 
hold  a  sheet  of  white  paper  before  the  left  eye,  the  white  ground  of  the 
printed  page  seems  about  equally  bright  whether  the  left  eye  is  kept  open 
or  shut,  though  in  one  case  the  left  field  of  vision  is  white  and  in  the  other 
it  is  nearly  black.  The  white  paper  in  front  of  the  left  eye  produces  some 
effect  only  if  it  is  brilliantly  illuminated,  as  by  the  sun. 


FIG.  34. 


Diagram  for  obtaining  union  of  two  haploscopic  fields. 

We  shall  now  consider  two  haploscopic  fields  containing  diagrams 
which,  upon  being  united,  partially  coincide  with  each  other.  To  make 
the  experiment  as  simple  as  possible,  we  choose  fields  such  as  those  in 
Figs.  34  and  35.  In  Fig.  34  we  have  on  the  left  a  long,  rectangular, 
black  field  perpendicular  to  the  visual  plane,  and  on  the  right  a  similar 
black  field  situated  at  right  angles  to  the  first.  In  the  middle  of  each  field 
is  a  white  cross.  The  fixation-point  is  kept  steady  by  causing  these  white 
crosses  to  coincide  with  each  other.  In  the  binocular  field  we  have  now  an 
appearance  which  cannot  be  very  well  represented  in  a  drawing,  but  which, 
in  succession,  appears  something  like  what  is  given  in  Fig.  36,  a,  b,  and  c. 
(The  fixation-crosses  are  here  left  out.)  At  those  parts  where  the  contours 
of  one  black  band  overlap  those  of  the  other  there  is  a  continually  waver- 
ing appearance,  as  if  there  was  a  constant  rivalry  between  the  two  impres- 
sions which  it  is  possible  for  the  object  to  convey.  This  phenomenon  has, 
VOL.  I.— 37 


578  BINOCULAR   VISION. 

in  fact,  been  called  the  rivalry  of  the  retinas.  The  impression  obtained  is 
not  for  a  moment  that  of  an  evenly  black  or  evenly  gray  cross.  On  the 
parts  where  the  black  bands  overlap  each  other,  one  sees  now  the  contour 
of  one  band,  now  that  of  the  other,  and  again  both  at  once,  as  in  6, — these 
being  so  provided  that  the  attention  is  allowed  to  wander,  and  also  that  (as 
in  our  diagram)  there  is  no  unlikeness  in  the  intensity  of  the  two  fields. 

Fia.  36. 


Diagram  for  obtaining  union  of  two  haploscopic  fields. 

This  circumstance — that  the  borders  of  the  two  objects  make  themselves 
particularly  prominent — is  called  the  rivalry  of  contours.  Close  to  an  edge 
one  always  sees  the  brightness  of  that  object  whose  border  is  for  the  moment 
prevailing.  The  inner  square  of  this  figure  is  always  black.  Just  outside 
of  each  contour  which  prevails  there  is  a  white  spot  which  goes  gradually 
into  black.  If  the  attention,  however,  is  fixed  upon  the  right-hand  band, 

FIG.  36. 


«  b  c 

Diagram  illustrating  the  effect  of  the  union  of  the  two  haploscopic  fields  in  Fig.  34. 

a  is  the  impression  which  prevails ;   if  it  is  directed  upon  the  left-hand 
baud,  c  represents  what  is  seen. 

In  the  second  case  (Fig.  35)  each  field  contains  a  series  of  black  lines 
at  equal  distances  from  each  other,  and  inclined  at  an  angle  of  45°  with 
the  visual  plane,  running  from  right  to  left  on  one  side  and  from  left  to 
right  on  the  other.  Upon  uniting  these  haploscopically,  we  do  not  see 
a  field  of  perfect  squares,  as  in  Fig.  37,  but  simply  notice  a  wavering 
image  whose  separate  parts  correspond  now  to  the  right-hand,  now  to  the 


BINOCULAR   VISION. 


579 


left-hand  figure.  This  is  the  appearance  when  no  particular  direction  is 
given  to  the  attention  ;  but  if  the  attention  is  fixed  upon  either  half  of  the 
dfagram, — if,  e.g.,  we  endeavor  to  count  the  lines  of  one  half,  or  if  we  let 
the  visual  regard  wander  along  the  lines,— then  the  half  that  is  regarded 
distinctly  prevails  for  some  moments  over  the  other. 

Helmholtz  explains  the  rivalry  of  the  visual  fields  as  being  due  to  the 
wandering  of  the  attention,  and  finds  in  the  overpowering  influence  of 
contours  the  effect  of  habit,  which  leads  us  to  examine  especially  the  con- 
tours of  any  object  that  is  presented  to  us  in  order  that  we  may  recognize 
as  quickly  as  possible  what  the  object  is.  The  phenomenon  is  a  proof  that 

FIG.  37. 


Diagram  illustrating  the  effect  of  the  union  of  the  two  hapJoscopic  fields  in  Fig.  35. 

the  contents  of  each  field  of  view  reach  consciousness  distinct  and  separate 
from  those  of  the  other.  It  also  teaches  us  that  such  fusion  as  takes  place 
is  not  conditioned  by  the  organic  structure  of  the  brain.  The  bearing  of 
the  phenomenon  upon  our  powers  of  perception  consists  in  its  showing  that 
when  a  fusion  of  the  sensations  of  the  two  fields  does  not  occur  (in  accord- 
ance with  the  laws  above  explained)  in  the  interests  of  the  perception  of  a 
third  dimension,  each  field  of  vision  preserves  its  independence. 

IX.    BINOCULAR  COLOR-MIXTURE;   LUSTRE. 

The  rivalry  which  we  have  just  described  occurs  also  when  the  two 
fields  are  differently  colored, — when,  for  instance,  one  looks  at  an  object 
with  a  piece  of  red  glass  held  before  one  eye  and  a  piece  of  blue  glass  held 
before  the  other.  The  glasses  must  be  so  chosen  that  the  object  is  of  about 
the  same  brightness  when  looked  at  through  the  two  glasses  separately. 
At  the  first  moment  the  field  of  view  seems  to  be  irregularly  spotted  in 
red  and  blue,  the  two  colors  appearing  alternately  in  rivalry  over  the 
whole  field.  After  some  time  a  condition  of  greater  repose  sets  in,  and  a 
more  or  less  single  impression  is  obtained  (which  is  considered  by  many 


580 


BINOCULAR  VISION. 


physiologists  to  be  a  mixture  of  the  two  colors).  Instead  of  the  colored 
glasses,  two  Nicol  prisms  may  be  held  before  the  eyes,  their  planes  of  polar- 
ization being  placed  at  right  angles  to  each  other ;  or  we  may  look  through 
thin  plates  of  gypsum  or  of  mica  at  a  surface  which  reflects  light  at  the 
angle  of  polarization.  By  this  means  the  two  eyes  see  colors  which  are 
exactly  complementary  to  each  other.  On  rotating  the  Nicol  prisms,  the 
planes  of  polarization  remaining  perpendicular  to  each  other,  different 
pairs  of  colors  are  obtained.  One  can  also  unite  haploscopically  two  fields 
which  are  differently  colored  by  pigments. 

Many  physiologists  affirm  that  in  these  cases  a  mixing  of  the  colors  is 
obtained,  while  others  are  just  as  positive  that  the  correct  color  of  fusion  is 
never  obtained  by  this  means,  and  that  one  is  easily  convinced  of  this  by 
comparing  the  result  directly  with  a  mixture  of  the  two  colors  brought  to- 
gether in  the  ordinary  way.  We  cannot  here  enter  upon  the  numerous 

FIG.  38. 


Figure  for  obtaining  stereoscopic  lustre. 

experiments  which  have  been  devised  and  described  in  order  to  settle  this 
point.  Individual  differences  evidently  play  an  important  rdle  in  the 
phenomenon.  Thus  much,  however,  appears  to  have  been  plainly  made  out : 
the  phenomenon  of  a  fusion  of  colors  occurs  only  under  definite  and  care- 
fully chosen  conditions ;  it  is  very  easily  disturbed  by  the  slightest  differ- 
ences in  the  two  fields  of  view,  which  are  sure  to  bring  out  rivalry ;  exactly 
the  same  effect  as  is  obtained  by  monocular  mixture  of  the  two  colors  (with 
the  color- wheel,  for  instance)  is  seldom  or  never  produced,  but  rather  a 
mixture  which  lies  vaguely  somewhere  between  the  two. 

It  is  also  of  importance  that  the  colors  to  be  mixed  should  be  of  some- 
what the  same  brightness,  for  otherwise  a  peculiar  effect  is  produced.  This 
is  that  of  stereoscopic  lustre,  first  discovered  by  Dove.  A  glance  at  Fig. 
38  with  an  ordinary  stereoscope  will  show  very  well  what  is  meant,  and 
the  condition  assumed. 


NORMAL  COLOR-PERCEPTION.1 

BY  WILLIAM   THOMSON,  M.D., 

Professor  of  Ophthalmology  in  the  Jefferson  Medical  College ;  Attending  Surgeon  to  the 
Wills  Eye  Hospital,  Philadelphia,  Pennsylvania,  U.S.A. 

ASSISTED  BY 

CARL  WEILAND,  M.D., 

Clinical  Assistant,  Eye  Department,  Jefferson  Medical  College  Hospital,  Philadelphia, 

Pennsylvania,  U.S.A. 


THE  rainbow  may  be  regarded  as  one  of  the  best  examples  in  nature  to 
show  us  not  only  the  beauty  of  the  pure  colors  but  also  the  subjective  aspect 
of  them.  Though  the  times  have  long  gone  by  when  Iris  was  thought  to 
bring  down  along  this  path  her  message  from  the  gods,  nevertheless  many 
people  still  regard  the  rainbow  as  an  objective  thing,  and  they  wonder  when 
scientists  tell  them  that  every  person  must  of  necessity  see  his  own  rainbow, 
differing  from  that  of  his  neighbor  not  only  in  apparent  size,  according  to 
the  size  of  the  observer,  but  also  in  its  apparent  position  in  the  sky.  And 
this  may  serve  as  a  good  illustration  of  the  great  difference  between  the  old 
crude  and  the  modern  scientific  view.  Formerly  all  the  qualities  of  the 
surrounding  objects  were  regarded  as  something  inherent  in  them,  some- 
thing that  existed  there  independently  of  ourselves,  whilst  now  we  have 
learned  to  regard  each  quality  in  nature  as  a  product  of  two  factors,  the 
external  object  and  the  perceiving  subject.  But  this  analysis  is  not  always 
easy,  and  it  seems  especially  difficult  to  separate  the  subjective  and  the 
objective  factor  with  regard  to  colors;  so  difficult  that  artistic  minds  like 
Goethe's  find  it  impossible  to  do  so,  and  turn  away  with  disgust  from  an 
analysis  of  our  color-perceptions.  Still,  there  is  no  escape  from  it,  and  it 
will  be  the  object  of  this  paper  to  review  some  of  the  most  important 
facts  with  regard  to  the  subjective  and  the  objective  aspect  of  colors.  Of 
course,  in  the  space  allotted  to  this  subject  it  will  be  impossible  to  go  fully 
into  details  or  to  enter  upon  an  exhaustive  discussion  of  the  different 
theories,  especially  in  face  of  the  fact  that  there  are  a  good  many  questions 
still  unsettled. 

First,  then,  What  is  color  objectively  ?     To  answer  this  we  must  know 

1  The  reader  is  referred  to  the  papers  of  Professors  Piersol,  Cattell,  and  Mays  in  con- 
nection with  this  subject. 

581 


582  NORMAL   COLOR-PERCEPTION. 

what  light  is  objectively.  Light  is  regarded  by  modern  physicists  as  a 
form  of  movement  in  a  hypothetical  medium,  the  ether.  This  is  the 
undulation  theory  of  light,  which  explains  very  completely  all  the  phe- 
nomena so  far  observed.  According  to  this  theory,  all  the  particles  of 
ether  along  a  ray  of  light  are  moving  in  straight  or  curved  lines  near  their 
original  points  of  equilibrium,  like  a  pendulum,  this  movement  occurring 
at  right  angles  to  the  direction  of  the  ray.  As  the  movement  of  each 
succeeding  particle  occurs  slightly  later  than  that  of  the  foregoing,  it  is  clear 
that  aline  touching  all  the  particles  in  their  momentary  positions,  beginning 
at  the  first  particle  when  this  is  just  coming  back  to  its  original  position 
and  extending  to  that  particle  which  just  starts  to  move,  must  have  a 
form  like  this  :  /^~^^^/-  The  distance  between  the  end-points  of  this  wave- 
line  is  called  the  wave-length,  and  is  the  smaller  the  sooner  each  particle 
completes  its  oscillation ;  whilst  the  velocity  with  which  this  oscillatory 
movement  propagates  itself  in  the  air  is  the  same  for  all  wave-lengths, 
amounting  to  about  one  hundred  and  ninety-two  thousand  miles  per  second. 
If  in  a  ray  every  particle  of  ether  moves  always  through  the  same  path 
with  the  same  velocity,  we  call  the  light  thus  resulting  monochromatic  or 
homogeneous  light :  each  particle  takes  then  the  same  amount  of  time  for 
its  excursion.  As  soon,  however,  as,  in  another  ray,  the  time  of  oscillation 
of  each  particle  decreases,  and  as,  therefore,  the  wave-length  gets  smaller, 
then  again  we  get  simple  light,  but  of  a  different  quality  for  our  eye,  or, 
as  we  express  it,  of  a  different  color.  Physically,  therefore,  color  finds  its 
equivalent  in  the  number  of  oscillations  per  second,  or  the  wave-length, 
and  it  has  been  found  by  experiment  that  light  of  the  lowest  number  of 
oscillations  of  the  ether  particles  and  the  greatest  wave-length  gives  us  red, 
while  light  of  the  smallest  wave-length  gives  us  violet.  We  speak  here  of 
greatest  and  smallest  wave-lengths,  but  it  must  be  understood  that  this  refers 
only  to  those  ethereal  waves  that  are  perceived  by  us  as  light.  Indeed,  the 
ether  particles  are  capable  of  all  kinds  of  oscillation,  and  in  the  ether  waves 
of  all  lengths  may  be  excited  and  propagated.  The  long  waves  seem  to  be- 
long to  the  domain  of  electricity  and  magnetism,  according  to  the  recent 
brilliant  researches  of  Hertz.  They  do  not  seem  to  affect  us  directly ;  at 
least  we  have  no  special  organ  for  their  perception.  They  have  been  inves- 
tigated by  purely  electrical  methods,  and  those  electro-magnetic  waves  that 
have  been  measured  have  been  found  to  range  from  a  few  inches  to  many 
yards.  The  shorter  waves  affect  our  nerves  in  the  skin  and  produce  heat  as 
soon  as  they  are  2700 l  w*2  long, — i.e.,  when  there  are  about  one  hundred 
billions  of  oscillations  per  second.  This  effect  of  heat  increases  until  the 
number  of  oscillations  per  second  is  about  four  hundred  billions  and  the 
wave-length  is  only  about  750  w,  when  the  ethereal  waves  begin  to  affect 
our  eye  as  light,  and  continue  to  do  so  till  the  wave-length  comes  down  to 

1  Langley,  Researches  on  Solar  Heat,  1884,  p.  72. 

2  The  unit  //  equals  one-millionth  of  a  millimetre. 


NORMAL   COLOR-PERCEPTION. 


583 


about  380  w*  or  the  number  of  oscillations  increases  to  about  eight  hun- 
dred billions  per  second.  Shorter  waves  are  not  perceived  by  our  eye  as 
light,  but  they  show  their  presence  by  chemical  effect.  These  actinic  rays 
must  be  emitted  by  the  sun  up  to  the  very  smallest  wave-length,  but  they 
do  not  all  reach  us  here  on  the  surface  of  the  earth ;  for  Cornu l  could 
show  that  at  an  elevation  of  2570  metres  the  sunlight  contains  ethereal 
waves  of  only  293  /*//,  and  he  found  also  that  in  the  electric  arc  light  there 
are  waves  as  short  as  211  nn  and  even  156  w,  the  latter  of  which  dis- 
appeared at  one-tenth  metre  from  the  light.  We  may  therefore  speak  of 
electric,  thermic,  photogenic,  and  actinic  rays  in  the  ether. 

Fig.  1  may  serve  to  show  the  different  effects  of  these  different  ether 
waves.     Line  abc  gives  the  thermic  curve  as  observed  by  a  salt  prism, 

FIG.  1. 


UttraRed, 


Uttra-Vwlet. 


curve  hik  is  that  of  the  luminosity  of  the  spectral  colors  as  perceived  by 
our  eye  and  analyzed  by  a  glass  prism,  while  defg  represents  the  chemical 
effect  as  found  by  a  quartz  prism.  The  visible  ether  waves  reach  only 
from  A  to  H ;  at  the  same  time  we  can  observe  how  the  thermic  effect  is 
not  confined  exclusively  to  the  ultra-red  rays,  but  extends  over  the  whole 
visible  spectrum.  It  is  further  apparent  that  the  actinic  effect  is  not  only 
an  exclusive  property  of  the  ultra-violet  wavelets,  but  belongs  in  a  smaller 
degree  also  to  the  luminous  vibrations ;  indeed,  Captain  Abney  has  succeeded 
in  finding  chemical  substances  that  are  even  decomposed  by  the  ultra-red 
rays. 

Here  we  have  to  confine  ourselves  to  those  rays  that  affect  our  sight ; 
and  we  have  mentioned  already  the  rays  which  contain  only  one  kind  of 
wave-length  (from  750  to  380  w),  the  rays  of  monochromatic  light.  But 
the  natural  light  of  luminous  bodies  is  not  usually  of  one  color.  It  con- 
tains in  each  ray  waves  of  very  many  different  lengths,  so  that  we  call  it 
mixed  or  compound  light,  as  each  ray  may  be  regarded  as  consisting  of 
many  rays  of  different  monochromatic  light.  Such  mixed  lights  we  also 
have  in  the  light  of  the  sun.  To  prove  that  this  is  the  case,  we  allow  the 

1  Helmholtz,  Handbuch  der  physiologischen  Optik,  2d  ed.,  p.  288. 


584 


NORMAL   COLOR-PERCEPTION. 


white  sunlight,  after  it  has  passed  through  a  narrow  slit,  to  go  through  a 
prism  which  has  the  property  of  bending  the  different  rays  differently,  ac- 
cording to  the  wave-length  existing  in  each.  We  find  then  the  beautiful 
expansion  of  colors  that  is  called  the  spectrum,  extending  from  red,  the  least 
refracted  light,  through  orange,  yellow,  green,  blue,  to  violet,  which  is  most 
deflected.1  These  colors  are  so  gradually  changing  one  into  the  other  that 
it  is  impossible  not  only  to  give  to  each  color  in  the  spectrum  a  certain  defi- 
nite breadth,  but  also  to  give  names  to  all  the  different  hues,  of  which  more 
than  one  thousand  could  be  distinguished  by  Aubert.  But,  as  it  has  been 
possible  to  measure  the  wave-length  of  each  part  of  the  spectrum,  we  are 
always  able  to  define  a  certain  spectral  color  by  saying  that  it  must  be  of 
such  and  such  a  wave-length.  We  are  also  helped  in  this  determination  by 
certain  dark  lines  in  the  solar  spectrum, — Fraunhofer's  lines, — the  relation 
of  which  to  the  different  colors  with  their  different  wave-lengths  is  given 
in  the  following  table,  combined  from  Helmholtz2  and  Abney,3  and  can 
also  be  seen  in  the  colored  table  I. 


Frauuhofer's  Lines. 

Millions    of    Mil- 
lions   of    Vibra- 
tions per  Second. 

Wave-Length. 

Color. 

A    

395 

760  uu 

Extreme  red. 

B     

437 

686  uu 

Ked. 

c   

458 

656  uu 

Limit  of  red  and  orange. 

D            

510 

589  uu 

Golden  yellow. 

E     

670 

526  uu 

Green. 

F     

618 

486  uu 

Cyanean  blue. 

G    

697 

430  uu 

Limit  of  indigo  and  violet. 

H            

757 

396  uu 

Limit  of  violet. 

According  to  the  instrumental  means  employed,  the  spectrum  can  be 
made  longer  or  shorter.  If  we  regard  such  a  short  spectrum  in  its  entirety, 
so  that  the  whole  affects  our  eye,  it  seems  to  be  composed  of  only  four 
colors, — red,  green,  blue,  and  violet, — because  by  contrast  with  these  main 
colors  their  transitions  into  one  ailother  disappear  almost  entirely ;  at  the 
most,  one  recognizes  that  the  green  becomes  somewhat  yellowish  towards 
the  red.  If  the  spectrum  is  made  longer,  the  transition-colors  are  bettor 
recognized,  but  they  do  not  make  their  full  impression  on  us  if  the  other 
colors  affect  our  eye  at  the  same  time.  To  study  each  color  by  itself,  it 
must  be  isolated  by  shutting  off  all  the  other  colors  except  one,  which  is 
allowed  to  go  through  a  narrow  shutter.  By  shifting  this  shutter  along 
the  spectrum  all  the  different  colors  can  be  separately  studied,  and  it  is 
then  found  that  nowhere  is  there  any  sudden  gap  in  the  color-series,  but 
that  all  color- tones  pass  into  one  another  continuously. 

1  In  this  paper,  as  well  as  in  that  on  detection  of  color-blindness,  the  spectrum  has 
been  assumed  to  be  arranged  in  the  usual  way, — i.e.,  red  to  the  left  and  violet  to  the  right 
of  the  observer. 

*  Physiol.  Optik,  2d  ed.,  p.  287. 

3  Color  Measurement  and  Mixture,  1891,  p.  55. 


NORMAL   COLOR-PERCEPTION.  585 

We  mentioned  before  that  our  eye  is  not  affected  by  the  ultra-red  and 
ultra-violet  rays.  For  the  ultra-red  rays,  it  has  been  proved  by  Briicke  and 
Cima  that  the  eye  transmits  only  about  nine  per  cent,  of  the  heat  falling 
upon  the  eye,  which  circumstance  seems  to  suffice,  according  to  Helmholtz, 
to  explain  their  invisibility.  It  is,  however,  different  with  the  ultra-violet 
rays.  Donders  and  Ilees  have  shown  that  these  rays  passed  largely  through 
glass  vessels  which  were  filled  with  vitreous  fluid  of  the  ox  and  into  which 
cornea  and  lens  had  been  placed.  It  is,  therefore,  not  because  these  rays 
do  not  strike  our  retina  that  they  are  not  perceived,  but  because  the  retina 
is  insensible  to  them.  From  the  fact  that  the  thermic  and  actinic  rays 
do  not  enter  into  our  world  of  vision,  Tyndall  declared  our  eyes  to  be  not 
perfect  yet,  and  he  hoped,  from  the  further  development  of  our  race,  that 
later  we  might  be  enabled  to  see  these  rays  and  so  enjoy  wonderful  sights 
now  hidden  from  us.  Fick,  however,  from  a  teleological  point  of  view, 
justly  objects  that  the  heat  rays,  if  they  should  all  enter  our  eye,  would 
not  allow  our  retina  to  rest,  as  these  thermic  rays  are  emitted  by  all 
bodies  and  even  by  the  neighboring  parts  of  the  eye ;  and  that,  further,  the 
ultra-violet  rays,  because  too  highly  refractive,  wrould  continually  disturb 
the  acuity  of  our  vision  by  their  large  diffusion  circles. 

So  far  we  have  spoken  only  of  the  simple  colors  furnished  by  a  solar 
spectrum.  These  are  simple  because  each  color  cannot  again  be  decom- 
posed by  a  prism,  but  can  only  be  deflected.  Most  colors  in  nature,  how- 
ever, are  not  thus  simple.  If  we  put,  for  example,  an  object  of  a  certain 
color,  such  as  yellow  paper,  on  a  piece  of  black  velvet  in  such  a  manner 
that  the  lustre  is  avoided  as  much  as  possible,  and  look  at  it  with  a  glass 
prism  of  from  thirty  to  fifty  degrees,  we  shall  find  that  not  only  does  the 
color  of  the  object  appear,  but  besides,  on  either  side,  there  appear  colored 
fringes,  indicating  what  simple  colors  make  up  the  color  of  the  object.  It 
was  observed,  for  example,  that  a  piece  of  yellow  paper,  when  looked  at 
in  the  above  manner,  showed  a  red  border  on  one  and  a  green  border  on 
the  other  side,  indicating  that  much  red  and  green  light  was  also  emitted  by 
the  paper.  A  better  way  still  is  to  hold  a  narrow  slit  in  front  of  the 
pigment  color  and  look  at  this  with  the  high  prism.  Then  nothing  will 
be  observed  in  the  slit  but  the  emitted  elementary  colors  alone,  which, 
indeed,  in  the  case  of  the  yellow  paper  were  green,  red,  and  some  yellow. 
In  such  a  manner  the  compound  colors  of  nature  can  be  decomposed.  But 
this  method  can  be  also  employed  for  a  synthesis  of  simple  colors,  as  may 
be  mentioned  here  incidentally,  especially  as  the  books  do  not  give  it. 
This  method  consists  simply  in  looking  at  two  pieces  of  differently  colored 
or  of  white  paper  on  black  velvet  with  a  high  prism,  and  then  bringing 
the  two  pieces  gradually  nearer  until  the  colored  fringes  partially  overlap. 
Beautiful  new  colors  will  then  appear,  to  be  described  later.  One  can 
observe  this  much  better  by  looking  at  two  narrow  strips  of  white  paper 
with  a  Browning  pocket  spectroscope,  after  having  taken  oil'  t IK— lit  and 
the  collimator  lens.  Each  strip  will  then  furnish  a  bright  spectrum 


586  NORMAL   COLOR-PERCEPTION. 

which  may  have  an  apparent  length  of  from  ten  to  twenty  centimetres, 
according  to  the  distance  of  the  prism.  This  arrangement  may  be  even 
used  to  verify  roughly  the  table  of  Helmholtz,  on  page  587.  There  are 
various  other  methods  of  studying  the  effect  of  combining  two  differently 
colored  lights.  One  is  to  allow  two  different  simple  colors  of  two  spectra 
to  fall  upon  the  same  place  on  a  white  screen  or  the  same  spot  of  the 
retina.  Another  is  to  make  a  disk,  upon  which  two  or  more  differently 
colored  sectors  have  been  arranged,  rotate  so  rapidly  that  the  different 
impressions  fuse  into  one  compound  color.  A  third  one  is  to  look  at  the 
limit  of  two  differently  colored  fields  with  a  double- refracting  prism  like 
that  contained  in  Javal's  ophthalmometer.  Then  the  new  compound  color 
appears  in  the  middle  between  the  two  former  color-fields.  Another 
method  is  that  first  employed  by  Lambert,  in  which  one  looks  obliquely 
through  a  plain  glass  plate  at  a  colored  surface,  whilst  that  side  of  the 
plate  which  is  turned  to  the  observer  at  the  same  time  sends  differently 
colored  light  by  reflection  into  his  eyes.  A  very  accurate  instrument  fur- 
ther to  mix  pure  spectral  colors  and  to  analyze  given  colors  like  those  of 
pigments  has  been  described  lately  by  Captain  Abney  in  his  book  on 
"  Color  Measurement  and  Mixture,"  from  which,  later,  a  table  will  be 
given  to  show  the  composition  of  certain  pigment  colors.  These  methods 
all  give  reliable  results,  especially  those  that  mix  spectral  colors ;  but  the 
method  of  mixing  solid  or  fluid  pigments,  though  used  by  Newton,  must 
by  no  means  be  employed. 

To  see  this,  we  must  consider  for  a  little  while  what  pigment  colors 
are.  Let  us  take  a  colored  fluid.  If  white  light  passes  through  it,  some 
colors  in  the  white  light  are  partly  or  wholly  absorbed  and  some  go  through 
without  being  much  affected  by  the  material :  it  is  these  latter  rays  that 
give  the  color  to  the  fluid.  If  we  now  mix  two  differently  colored  fluids 
which  are  chemically  indifferent  to  each  other,  we  can  see  only  those 
colors  which  escape  the  absorptive  power  of  both.  Most  blue  fluids,  for 
instance,  permit  the  blue  rays  to  go  through  unweakened ;  somewhat  less 
well  the  green  and  violet ;  but  badly  the  red  and  yellow  rays.  A  yellow 
fluid,  however,  transmits  almost  all  the  yellow  well,  also  red  and  green,  but 
very  little  blue  and  violet.  Under  these  circumstances  it  is  clear  that 
through  a  mixture  of  these  two  fluids  the  green  rays  alone  will  go  through 
to  a  considerable  degree,  all  the  others  being  absorbed.  The  mixture  will 
appear  green,  while  a  mixture  of  spectral  blue  and  yellow  would  give  white 
or  gray.  It  is  evident  that  in  the  two  fluids  there  occurs  no  summation 
of  the  blue  and  yellow  colors,  but  rather  a  subtraction,1  as  the  yellow 
fluid  takes  away  from  the  rays  that  have  passed  through  the  blue  fluid  all 
those  for  which  it  has  an  absorptive  power.  We  find  a  similar  state  of 
affairs  with  regard  to  powdered  pigments.  We  must  regard  each  little 
particle  of  the  pigment  as  a  small  transparent  body  which  colors  the  light 

1  Helmholtz,  Physiol.  Optik,  2d  ed.,  p.  314. 


NORMAL   COLOR-PERCEPTION. 


587 


by  absorption ;  for  as  the  light  falls  on  it  a  small  part  of  the  white  light 
is  reflected  from  the  outer  part  (about  one-t\venty-fifth  on  perpendicular 
incidence,  otherwise  less),  but  the  rest  of  the  light  enters.  This  becomes 
in  part  absorbed,  is  reflected  from  the  posterior  sides  of  the  particle,  and 
finally  after  another  partial  absorption  enters  our  eye  as  colored  light.  As 
this  occurs  from  each  little  particle,  we  can  understand  how  the  pigment 
must  appear  colored,  and  how,  if  two  such  different  pigments  are  mix.-d, 
the  mixture  can  appear  only  in  a  light  which  has  escaped  being  absorbed 
by  both  substances.  So  it  is  that  blue  and  yellow  pigments  mixed  give  a 
green  color  like  the  blue  and  yellow  fluids.  This  is  an  elementary  fact  to 
painters,1  but  nevertheless  it  is  not  true  for  the  mixture  of  pure  color* ; 
because,  as  already  mentioned  in  the  case  of  blue  and  yellow,  quite  differ- 
ent results  have  been  obtained  by  mixing  the  colors  of  the  spectrum  or  of 
pigments  in  our  eye  in  one  of  the  ways  before  mentioned,  which  are  all 
true  additive  processes.  The  results  of  these  experiments  are  given  by 
Helmholtz  in  the  following  table  : 2 


Violet. 

Indigo- 
Blue. 

Cyanean 
Blue. 

Blue- 
Green. 

Green. 

Green- 
Yellow. 

Yellow. 

Rod    

Purple. 

Dark 

Whitish 

White. 

Whitish 

Golden 

O  mi  lire 

Orange  .... 

Dark 

rose. 
Whitish 

rose. 
White. 

Whitish 

yellow. 
Yellow. 

yellow. 
Yellow. 

rose. 

rose. 

yellow. 

Yellow  .... 

Whitish 

White. 

Whitish 

Whitish 

Green- 

rose. 

green. 

green. 

yellow. 

Green-yellow  . 

White. 

Whitish 

Whitish 

Green. 

green. 

green. 

Green    .... 

Whitish 

Water- 

Blue- 

blue. 

blue. 

green. 

Blue-green   .    . 

Water- 

Water- 

blue. 

blue. 

Cyanean  blue  . 

Indigo- 

blue. 

In  this  table  the  spectral  colors  at  the  top  of  the  vertical  columns  and 
to  the  left  of  the  horizontal  ones  are  the  primary  simple  colors  to  be  mixed, 
and  the  result  of  this  mixture  is  indicated  by  that  color  where  the  corre- 

1  Painters  frequently  ridicule  the  statements  of  scientists  about  color-mixture,  because 
they  do  not  see  the  radical  difference  between  the  term  color-mixture  as  used  by  physicists 
and  the  same  term  as  used  by  themselves.     The  former  mean  by  this  term  the  mixture  of 
two  color-sensations,  while  the  latter  denote  by  it  their  mixture  of  material  pigments ;  the 
first  refers  to  the  addition  of  two  sensations  in  our  color-perceiving  mind,  while  the  second 
refers  to  the  bringing  together  of  two  colored  bodies  which,  after  their  mutual  incorpora- 
tion, do  not  send  the  two  former  single  colors  into  our  eye  at  the  same  time,  but  only  a  re- 
sidual one,  after  the  combined  absorptive  power  of  the  two  substances  has  done  its  work  on 
the  light.     One  may  compare  the  two  colors  entering  into  the  physicist's  color-mixture 
to  two  enemies  that  attack  our  mind  together,  while  the  two  colors  of  the  painter's  mixture 
may  be  regarded  as  two  mutual  enemies  who  first  combat  each  other  violently  before  they 
assault  our  psyche  with  the  remainder  of  their  forces.     That  our  mind  must  be  diffrivntly 
affected  under  such  circumstances  is  clear,  and  we  may  easily  understand  how  painters  and 
scientists  differ  though  each  is  right  in  his  own  field. 

2  Physiol.  Optik,  2d  ed.,  p.  321. 


588 


NORMAL   COLOR-PERCEPTION. 


spending  horizontal  and  vertical  columns  intersect.  We  infer  from  this 
table  that  by  the  simultaneous  effect  of  different  simple  colors  on  the  same 
spot  of  the  retina  only  two  new  colors  have  been  produced,  purple  and 
white.  Purple  is  obtained  by  the  mixture  of  violet  or  blue  with  red,  and 
seems  to  form  for  our  eye  a  bridge  between  the  red  and  the  violet  of  the 
spectrum.  And  still  purple  does  not  exist  as  such  in  nature ;  there  is  no 
part  of  the  spectrum  in  which  the  wave-length  is  such  that  the  correspond- 
ing color  would  be  purple.  It  is  a  color- sensation  only,  produced  in  us  by 
the  simultaneous  impression  of  blue  and  of  red.  Here  we  are  clearly  con- 
fronted with  the  difference  between  color  in  a  physical  sense  as  an  ethereal 
vibration  and  color  in  a  physiological  sense  as  a  sensation,  which  difference 
we  must  always  bear  in  mind.  In  the  same  sense  we  must  call  white  a 
new  color  sensation,  which  can  be  obtained  first,  of  course,  by  combining 
again  all  the  colors  of  the  spectrum.  But  it  has  been  further  observed 
that  even  two  simple  colors,  taken  from  different  parts  of  the  spectrum 
and  mixed  in  a  certain  ratio,  will  give  white.  Two  such  colors  are  called 
complementary,  a  series  of  which  is  here  given  from  Helmholtz  : 


Color. 

Wave-Length. 

Complementary  Color. 

Wave-Length. 

Ratio  of  the 
Wave-Lengths. 

Bed  

656  uu 

Green-blue. 

492  uu 

1.334 

Orange         ...        ... 

607  uu 

Blue 

489  uu 

1.240 

Golden  yellow           .    .    . 

585  uu 

Blue 

485  uu 

1.206 

Golden  yellow   

673  uu 

Blue. 

482  uu 

1.190 

Yellow    

567  uu 

Indigo-blue. 

464  uu 

1.221 

Yellow    

564  uu 

Indigo-blue 

461  uu 

1.222 

Green-yellow      

563  uu 

Violet. 

433  uu 

1.301 

Green  has  no  simple  complementary  color,  but  only  a  compound  one, 
— purple.  Later,  other  observers,  like  Von  Kries,  Von  Frey,  Konig,  and 
Dieterici,  have  each  given  their  table  of  complementary  spectral  colors, 
which  on  the  whole  agree  very  well,  but  show  at  the  same  time  distinctly 
that  there  are  individual  differences  in  the  normal  color-perception  of  men, 
to  which  fact  we  shall  return  later.  For  the  present  we  must  state  again 
that,  as  the  result  of  the  mixture  of  two  or  more  homogeneous  lights,  except 
purple  and  white,  and  their  transitions  into  the  spectral  colors,  no  new  color- 
sensation  can  be  produced.  For  each  resulting  sensation  of  the  combina- 
tion there  exists  in  the  spectrum  a  homogeneous  color  of  a  certain  definite 
wave-length,  which  produces  the  same  or  nearly  the  same  color-sensation, 
and  every  new  sensation  of  purple  and  white  can  be  referred  to  its  spectral 
components.1  That  the  new  color  is  not  always  of  the  same  intensity  will 
be  explained  later. 

We  are  now  in  a  condition  to  remark  that  all  possible  combinations  of 

1  Or,  as  Captain  Abney  expresses  it  in  his  excellent  book,  Color  Measurement  and 
Mixture,  p.  162,  "  The  hue  and  luminosity  of  any  color,  however  compounded,  may  be  regis- 
tered by  a  reference  to  white  light  and  a  single  ray  of  the  spectrum."  This  special  ray  of 
the  spectrum  he  calls  the  dominant  ray,  which  term  will  be  employed  later  in  this  paper. 


NORMAL   COLOR-PERCEPTION. 


589 


different  luminous  wave-systems  of  the  ether  lead  only  to  a  comparatively 
small  number  of  different  states  of  stimulation  of  the  visual  apparatus 
which  appear  in  the  different  color-sensations.  And  in  this  sensation  of 
each  color  we  always  distinguish  three  elements,  or  constants,  as  they  are 
called,  which  are  (1)  hue,  (2)  purity,  (3)  brightness. 

The  first  characteristic  of  any  color  is  its  hue,  which  tells  us  whether 
the  color  is  red,  green,  blue,  etc.  This,  in  the  spectral  colors,  depends  physi- 
cally, as  we  have  seen,  upon  the  wave-length  of  the  ethereal  vibrations  in 
each  ray,  and  thus  we  can  use  the  two  terms  hue  and  wave-length  promis- 
cuously when  we  have  to  do  with  spectral  colors.  But  we  have  rarely  to 
do  with  spectral  colors ;  mostly  the  natural  colors  are  made  up  of  several 
rays  or  groups  of  rays,  and  then  we  can  compare  the  resulting  color-sensa- 
tion with  the  nearest  spectral  color  of  a  definite  wave-length,  or  for  purple 
and  white  with  the  nearest  combination  of  two  spectral  colors,  and  thus 
define  the  hue.  The  accurate  definition  of  a  color  by  reference  to  the 
spectrum  is  absolutely  necessary  for  scientific  experiments,  but  ought  also  to 
be  used  for  an  accurate  description  of  the  colors  of  Holmgren's  wools  and 
the  colored  glasses  used  by  the  railroad  companies.  For  then  alone  can  we 
always  find  the  same  colors  again  after  fading  or  other  deterioration.  Cap- 
tain Abney  has  done  so  for  the  red  and  green  glasses  used  by  the  different 
railroads  in  England,  and  has  given  his  observations  in  the  report  of  the 
Committee  on  Color- Vision  in  the  Proceedings  of  the  Royal  Society,  July, 
1892,  to  which  we  shall  have  occasion  to  refer  frequently.  This  table  is 
given  below,  and  gives  the  wave-lengths  in  the  spectrum  of  the  dominant 
colors  of  the  signals  which  have  been  adopted  by  some  of  the  principal 
railroad  companies  of  England,  these  glasses  being  illuminated  first  by  an 
electric  arc  light  and  secondly  by  gas-light.  The  percentage  of  white  light 
mixed  with  the  spectral  color  is  also  shown,  together  with  the  luminosity 
of  the  light  transmitted. 


Electric  Light. 

Gas-Light. 

t 

"S3 

£tf 

i 

*S5 

-S 

a  *. 

— 

M 

Si 

— 

1 

Glass. 

y 

^8 

p 

sS 

A 

1 

aSg 

Hi 

c  si 

!•-  = 

!!•§ 

p 

£j|§ 

121 

II 

1*3 

^s- 

6 

£ 

s 

Q 

£ 

j 

Reds. 

G~reat  \Vcstem 

625 

7 

10.4 

627 

12 

18.1 

Ruby  Glass  L.  B.  S.  C  

620 

0 

10.4 

620 

0 

13 

Great  Northern    

625 

0 

9 

627 

0 

10 

Greens. 

492 

46 

21.8 

507 

60 

18.1 

L.  B.  S   C     

492 

38 

16.2 

505 

34 

12.5 

510 

61 

19.2 

517 

62 

19.4 

Great  Eastern   

500 

64 

15 

612 

40 

15 

492 
550 

24 
32 

7.6 
9.1 

605 
632 

22 
60 

6.9 
10.6 

District  Railway      

590 


NORMAL   COLOR-PERCEPTION. 


The  definition  of  the  colors  of  Holmgren's  test-wools  will  be  given  in 
the  article  on  "  Detection  of  Color-Blindness." 

The  second  constant  of  color  is  purity,  called  tint  by  Maxwell.  We  call 
a  color  the  purer  the  less  white  is  mixed  with  it.  Purity  must  not  be  con- 
founded with  brightness,  for  many  bright  colors  are  not  pure,  but  contain 
a  large  admixture  of  white ;  nor  must  it  be  thought  that  pure  colors  are 
always  strong,  rich,  and  deep,  for  there  are  parts  in  the  spectrum  which, 
though  of  course  perfectly  pure,  can  be  recognized  only  with  difficulty. 
All  pigments  reflect  also  some  white  light,  and  so  can  never  be  perfectly 
pure,  though  colored  papers  in  certain  positions  and  colored  worsteds  in 
general  may  approach  a  pure  color  very  nearly. 

As  the  third  constant  or  quality  of  color  we  must  mention  luminosity, 
or  brightness,  called  shade  by  Maxwell.  This  factor  depends  objectively 
on  the  vis  viva  of  the  ether  movement,  and  is  therefore  proportional  to 
the  square  of  the  greatest  velocity  of  the  ether  particles ;  but  for  us  it  de- 
pends not  only  upon  the  energy  in  the  ether,  but  also  upon  the  sensitive- 
ness of  our  retina  for  the  different  colors.  This  has  been  shown  by  S.  P. 
Langley,1  whose  table  is  given  here : 


Color. 

Crimson. 

Red. 

Orange. 

Yellow. 

Green. 

Blue. 

Violet. 

"Wave-length   . 
Luminosity  .    . 

750  fj.fi 
1 

650^ 
1,200 

600/i/z 
14,000 

580  ^z 
28,000 

530  fifj. 
100,000 

470^ 
62,OQO 

400  nn 
1,600 

Unity  in  the  third  horizontal  line  is  the  amount  of  energy  required  to 
make  us  see  light  in  the  extreme  red  end  of  the  spectrum  near  Fraun- 
hofer's  line  A,  and  the  higher  numbers  in  this  line  indicate  what  visual 
effect  the  same  amount  of  energy  produces  in  the  respective  colors  :  so 
that,  for  example,  the  green  near  530  ftp  has  about  one  hundred  thousand 
times  more  visual  effect  upon  our  eye  than  has  the  crimson. 

If,  however,  we  determine  the  luminosity  of  the  spectral  colors  directly 
by  comparing  the  intensities  of  the  shadows  thrown  by  a  pencil  in  these 
different  lights,  as  has  been  done  by  Captain  Abney  (loc.  tit.\  we  get  the 
following  results : 


Fraunhofer's  Line. 

Color. 

Luminosity. 

Normal  Eye. 

Red-Blind  Eye. 

A    . 

Dark  red. 
Red    crimson)  . 
Red    crimson). 
Red   scarlet). 
Orange. 
Green. 
Blue-green. 
Blue. 
Violet. 
Faint  lavender. 

i 

8.5 
206 
98.5 
50 
7 
1.9 
0.6 

6 

0.5 
2.1 
53 
49 
7 
1.9 
0.6 

B    

Red  lithium     

C    

D   

E    

F    

Blue  lithium    

G   

H   

1  Energy  and  Vision,  National- Academy  of  Sciences,  vol.  v.,  First  Memoir,  1888. 


NORMAL   COLOR-PERCEPTION.  591 

Here  we  see  that  the  greatest  luminosity  exists  in  the  orange-yellow 
near  line  D.  The  fourth  vertical  column  shows  the  luminosity  of  the 
same  spectral  colors  to  a  red-blind  eye,  and  will  be  referred  to  again  later. 
Of  course  not  only  the  spectral  but  also  the  pigment  colors  differ  in  lumi- 
nosity, as  is  shown  in  the  following  table  from  Captain  Abney,  in  which 
the  luminosity  of  these  pigments  in  electric  light  is  determined  when  that 
of  white  paper  under  the  same  conditions  is  equal  to  100  : 


Pigment  Color. 

Luminosity  in 
Electric  Light. 

White      

100 

Vermilion'  

36 

30 

Ultramarine  . 

44 

Orange    

39.1 

Black  ... 

3.4 

Black  (different  surface)      ... 

5.1 

Having  thus  obtained  a  clear  conception  of  the  three  color-constants, 
we  are  now  in  a  position  to  define  exactly  any  color  by  its  hue  (comparing 
it  with  a  spectral  color  or  a  combination  of  them),  its  purity  or  tint  (absence 
of  white  light  entering  into  it),  and  its  brightness  or  shade  (the  amount  of 
energy  in  it  for  our  eye  or  the  amount  of  black  being  mixed  with  it).  But, 
instead  of  doing  it  by  these  three  variable  quantities,  a  color  may  be  also 
defined  as  a  mixture  of  variable  quantities  of  three  colors,  the  so-called 
primary  colors,  as  which  formerly  red,  yellow,  and  blue  were  regarded.  If 
one  should  take  this  doctrine  objectively,  as  if  there  were  three  objective 
simple  colors  in  the  spectrum,  by  the  combination  of  which  there  might  be 
produced  the  same  impression  upon  the  eye  as  by  any  other  simple  or  com- 
pound light,  he  would  be  quite  wrong.  There  are  no  three  simple  colors  in 
the  spectrum,  as  Brewster  thought,  by  the  combination  of  which  the  inter- 
mediate colors  of  the  spectrum  could  be  constructed.  These  intermediate 
colors  always  appear  much  more  saturated  in  the  spectrum,  even  if  one 
takes  a  much  better  selection  for  the  three  primary  colors, — namely, 
violet,  green,  and  red.  Then,  indeed,  one  may  from  red  and  green  obtain 
yellow ;  but  this  is  by  no  means  like  the  brilliant  color  of  the  spectrum. 
The  reason  for  this,  as  will  be  given  more  in  detail  later,  is  that  even  the 
spectral  colors,  though  the  purest  in  nature,  excite  in  us  not  pure  color- 
sensations  alone,  but  some  white  also. 

At  present  we  must  conclude  from  the  foregoing  facts  that  it  is  not 
necessary  to  assume  as  many  primary  color-sensations  in  us  as  there  are 
innumerable  colors  in  the  spectrum  and  combinations  of  them.  We  may 
reduce  them  very  much.  Two  color-sensations  will  not  suffice,  but  by  the 
help  of  three  we  can  produce  all  the  color-sensations  that  the  spectrum  can 
give.  This  fact  has  given  origin  to  different  theories  of  color-perception,  a 
few  of  which  will  be  described  later.  Before  doing  that  we  must  refresh 


592  NORMAL   COLOR-PERCEPTION. 

the  reader's  memory  about  some  important  facts  of  the  physiology  of  the 
nerves,  as  this  will  assist  him  very  much  to  judge  about  these  theories. 

When  a  stimulus  is  applied  to  a  nerve  a  peculiar  change  occurs  in  it, 
which  manifests  itself  in  the  motor  nerve  by  the  contraction  of  a  muscle 
and  by  a  sensation  in  the  sensory  nerve.  The  most  important  peculiarity 
about  the  two  kinds  of  nerves  is  that,  in  the  words  of  Helmholtz,  "  so  far 
no  difference  in  the  structure  and  function  of  the  sensory  and  motor  fibres  is 
known  which  could  not  be  derived  from  their  different  connections  with 
other  organic  systems.  The  fibres  themselves  seem  to  play  only  the  part 
of  indifferent  conducting  fibres,  which  work  as  motor  or  sensory  nerves, 
according  to  whether  they  are  connected  with  a  muscle  or  a  sensitive  part 
of  the  brain."1  Another  important  fact  is  "that  by  the  stimulation  of 
every  sensory  nerve-fibre  only  such  sensations  arise  as  belong  to  the  region 
of  the  special  sense,  and  that  every  stimulus  which  is  able  to  excite  the 
nerve-fibre,  be  it  mechanical,  chemical,  electrical,  etc.,  produces  only  sen- 
sations of  this  special  character."  2  This  is  well  shown  by  a  sudden  blow 
on  the  eye,  which  produces  a  suddenly  appearing  and  disappearing,  often 
very  bright,  light-sensation  over  the  whole  visual  field.  If,  in  a  man,  the 
eyeball  is  extirpated  and  the  optic  nerve  not  yet  disorganized,  great  masses 
of  light  appear  at  the  moment  the  nerve  is  being  cut.  That,  according  to 
Foster,  this  has  not  been  observed  always  may  easily  be  explained  either  by 
the  nerve  being  in  a  pathological  non-excitable  condition  or  by  the  excite- 
ment and  pain  which  arise  from  the  cutting  of  the  numerous  small  nervi 
nervorum,  and  which  may  easily  overpower  the  light-sensation  in  the  con- 
sciousness of  many  a  patient.  That  the  rough  blow  or  the  cutting  of  the 
nerve  can  compare  with  the  subtle  ethereal  wavelets  nobody  will  assert, 
and  yet  they  produce  the  same  effect.  Again,  if  light  strikes  the  retina  we 
know  that  we  see  it  only  after  it  has  reached  the  layer  of  rods  and  cones. 
These  latter  are  struck  by  the  ethereal  impulse  of  from  four  hundred  to 
eight  hundred  billion  times  per  second.  It  is  impossible  that  the  cones 
and  rods  should  vibrate  in  unison  with  the  ether  particles,3  for  then  they 
either  would  simply  burn  up  or  would  show  the  phenomena  of  fluores- 
cence or  phosphorescence,  which  have  never  been  observed,  at  least  in 
the  macula.  Such  fluorescence  would  further  mean  that  the  actinic  rays 
which  produce  this  most  strongly  ought  to  be  most  powerful  on  our  retina, 
whilst  we  know  that  the  retina  is  not  sensitive  to  them.  It  is,  therefore, 
necessary  to  assume  that  it  is  a  different  change  that  is  going  on  in  the 
nerve  and  in  the  ultimate  elements  of  the  retina.  What  occurs,  very  likely, 
may  be  compared,  with  Clifford,4  to  what  happens  in  a  train  of  gunpowder. 
All  the  molecules  here  are  in  a  state  of  unstable  equilibrium,  and  a  spark 

1  Helmholtz,  loc.  cit.,  p.  232.  2  Ibid.,  p.  233. 

3  As  the  Committee  on  Color-Vision  remarks,  p.  303,  "  It  is  difficult  to  conceive  that 
matter  which  is  so  comparatively  gross  as  the  rods  and  cones  which  are  situated  on  the 
retina  can  be  affected  by  the  merely  mechanical  action  of  the  vibrations  of  light." 

4  Seeing  and  Thinking. 


NORMAL   COLOR-PERCEPTION.  593 

to  one  end  will  upset  this  equilibrium  there ;  the  molecules  changing  their 
positions  and  entering  into  new  combinations  will  cause  the  next  particles 
to  do  the  same,  until  finally  all  the  molecules  are  again  in  equilibrium  and 
the  powder  is  burnt. 

This  is  a  chemical  process,  and  in  a  similar  way  we  must  imagine  that 
the  ether  molecules  set  energy  free  in  the  rods  and  cones,  producing  a 
chemical  decomposition  there  as  they  do  in  the  sensitive  plate  of  the 
photographer,  which  molecular  change  travels  up  the  nerve  to  the  brain. 
From  what  has  been  already  said  about  the  identity  of  all  nerve-fibres,  it 
is  most  probable  that  the  nerves  are  capable  of  only  one  kind  of  chemical 
change,  so  that  the  most  unlike  stimuli  to  the  optic  nerve  produce  the 
same  effect  of  a  chemical  decomposition  in  it;  just  as  a  drop  of  nitro- 
glycerin  becomes  decomposed  in  the  same  manner  whether  the  hammer 
or  the  thermic  ether- waves  strike  it.  And  if  that  is  so,  if  there  is  no 
vibratory  but  only  a  chemical  change  of  the  same  kind  in  all  nerve- 
fibres,  whatever  stimulus  is  applied,  whether  a  blow  or  colored  light,  it 
is  evident  that  we  have  to  drop  all  theories  which  suppose  that  each  fibre 
can  conduct  different  vibratory  changes  when  it  is  stimulated  by  different 
colors ;  we  have  to  give  up  the  idea,  advanced  by  Prince  Kropotkin,  "  of 
different  undulations  travelling  along  the  nerves  and  being  the  source  of 
different  sensations."  l  Such  a  theory  has  also  been  broached  in  a  very 
learned  essay  by  Dr.  Swan  M.  Burnett,3  which  ought  to  be  fully  discussed 
if  space  allowed  it.  His  theory,  too,  makes  the  same  nerve  the  con- 
ducting apparatus  for  different  colors,  so  that  the  brain  is  made  the  dif- 
ferentiating organ  and  the  nerve-fibre  is  supposed  to  undergo  different 
changes,  according  to  the  different  colors  striking  the  terminal.  But  there 
is  another  fact,  in  addition  to  those  from  the  physiology  of  nerves,  which 
makes  it  very  probable  that  the  retina  is  the  differentiating  or  sifting 
organ.  There  are  well-authenticated  cases  of  uniocular  color-blindness 
which  can  be  explained  only  by  an  ante-commissural  defect,  which,  from 
the  fact  that  the  visual  acuity  is  not  reduced,  must  most  probably  be 
placed  in  the  retina.  As  Professor  Rutherford3  says,  "By  some  it  is 
believed  that  congenital  color-defect  is  due  to  the  brain.  If  there  had 
been  defective  color-sense  on  one  side  of  the  brain,  it  would  not  have 
implicated  the  whole  of  one  eye,  but  the  half  of  each  eye.  Its  limita- 
tion to  one  eye,  therefore,  seems  to  me  to  suggest  that  the  fault  was  in 
the  eye  rather  than  in  the  brain."  And  if  that  is  so,  then  we  would  seem 
to  be  justified  in  assuming  from  this  fact  alone  that  the  retina  is  also  the 
differentiating  organ  in  normal  color-perception,  because  when  it  is  at  fault 
the  remaining  parts  of  our  visual  apparatus  are  not  able  to  distinguish  the 
colors  normally.  Oliver,  in  the  American  Journal  of  the  Medical  Sciences 

1  Kecent  Science,  in  August  number  (1893)  of  Nineteenth  Century. 
*  American  Journal  of  the  Medical  Sciences,  July,  1884. 

3  Our  Sense  of  Color.     Opening  presidential  address,  Section  D,  Biology,  British 
Association,  1892. 
VOL.  I.— 38 


594  NORMAL   COLOR-PERCEPTION. 

for  1885,  gives,  in  a  very  elaborate  article,  "a  correlation  theory  of  color- 
perception,"  in  which  he  states  that  "  each  and  every  healthy  optic-nerve 
filament  transmits  to  the  color-centre  for  recognition  nerve-energies  equal 
to  as  many  special  sensations  as  its  peripheral  tip  is  capable  of  receiving," 
and  that  "  color-perception  takes  place  through  each  and  every  optic-nerve 
filament."  But  against  such  a  theory  the  words  of  Donders1  may  be 
cited  :  "  Moreover,  more  than  one  process  in  the  same  form  element  [of  the 
retina]  would  have  supposed  more  than  one  process  of  conduction  in  the 
corresponding  fibre,  against  which  physiology  wishes  to  put  in  its  veto."  2 
Oliver  justly  lays  great  stress  on  a  comparison  with  the  sense  of  touch,  and 
says,  "  It  would  be  foolish  to  assert  that  there  may  be  special  divisions  of 
peripheral  tactile  nerves  especially  adapted  for  the  three  empirical  sensory 
impressions, — cold,  warm  and  hot,"  etc.  But  has  it  not  been  proved  by 
the  researches  of  Goldscheider  that  there  are  special  nerve-endings  for 
heat,  cold,  pain,  and  touch?  It  seems  that  this  reference  to  the  skin  ought 
to  strengthen  the  theory  which  is  here  proposed  and  which  agrees  well  with 
the  "  law  of  specific  energy." 

These  facts  narrow  down  considerably  the  region  for  possible  color- 
theories,  which  will  be  further  reduced  by  another  consideration,  given 
by  Helmholtz 3  as  follows :  "  For  the  motor  nerve  we  know  only  the 
antagonism  between  the  state  of  rest  and  that  of  activity.  In  the  first 
state  the  nerve  can  be  left  unchanged  for  a  long  time  without  consider- 
able metabolism  or  development  of  heat;  hereby  the  muscle  connected 
with  this  nerve  remains  limp.  If  the  nerve  is  stimulated,  there  is  de- 
veloped heat  in  it,  material  change,  electric  oscillations  can  be  demonstrated, 
the  muscle  contracts.  In  the  dissected  muscle  preparation  the  conductivity 
is  soon  lost  on  account  of  the  consumption  of  those  chemical  constituents 
that  are  necessary  for  activity.  Under  the  effect  of  atmospheric  oxygen,  or, 
better  still,  of  the  arterial  blood  containing  oxygen,  the  irritability  is  slowly 
reproduced,  wholly  or  partly,  without  these  assimilative  processes  producing 
any  contraction  in  the  muscle,  or  without  giving  rise  to  changes  in  the 
electric  behavior  of  muscle  and  nerve  accompanying  their  activity.  Fur- 
ther, we  know  no  outer  means  which  could  excite  the  assimilative  process 
so  rapidly  or  intensely  and  could  initiate  or  stop  it  so  suddenly  as  would 

1  Arch,  fur  Ophthalmol.,  xxvii.  1,  p.  173. 

2  This  must  also  he  urged  against  a  new  theory  lately  proposed  by  Chr.  L.  Franklin. 
Her  articles  upon  this  subject  in  Mind,  New  Series,  vol.  ii.,  1893,  Zeitschr.  f.  Psych,  u. 
Physiol.  der  Sinnesorgane,  Bd.  iv.,  etc.,  are,  nevertheless,  of  profound  interest  and  ought 
to  be  carefully  perused  by  the  student  of  this  field  of  physiological  optics.     Quite  in  agree- 
ment, however,  with  this  physiological  law,  a  new  theory  has  been  lately  propounded  by 
Dr.  J.  Wallace,  who  tries  to  establish  a  definite  relation  between  the  length  of  a  cone  and 
the  color  acting  on  it.     Mauthner,  in  his  Farbenlehre,  1894,  refers  to  it  as  an  important 
contribution  to  the  subject.     But  according  to  this  theory  each  cone  ought  to  have  always 
the  same  length,  which  does  not  seem  to  be  the  fact,  according  to  Stort  (Akad.  zu  Amster- 
dam, June,  1884). 

8Loc.  cit.,  p.  349. 


NORMAL   COLOII-PEIICEPTION.  595 

be  necessary  if  this  process  should  serve  as  a  physiological  foundation  of 
vivid  and  regularly  starting  sensations."  Now,  as  we  have  every  reason 
to  believe  that  the  sensory  nerves  possess  the  same  properties  as  the  motor 
nerves,  it  must  be  clear  that  all  theories  which  make  this  process  of  assimi- 
lation as  active  as  that  of  dissimilation  for  our  sensations  are  in  direct 
antagonism  with  the  best-established  facts  of  our  nerve  physiology.  This  is 
done  by  Bering  in  his  color-theory,  of  which  we  shall  speak  more  hereafter. 
We  shall  now  give  a  theory  of  color-perception  which  dates  back 
to  the  beginning  of  this  century,  and  was  in  its  rudiments  then  given  by 
Thomas  Young.  This  was  later  taken  up  by  Helmholtz  and  made  the 
basis  for  explanations  in  the  first  edition  of  his  celebrated  "  Physiologische 
Optik."  We  shall  not  enter  into  the  historical  development  of  this  theory, 
but  shall  simply  state  it  in  its  latest  modified  and  improved  form  as  given 
in  the  second  edition  of  Helmholtz's  book,  which  is  now  being  published. 
Helmholtz  has  made  some  important  additions,  with  regard  to  which  he 
modestly  remarks  on  page  349,  "  That  objections  to  these  additions  do  not 
refute  Young's  hypothesis  (of  three  primary  color-sensations)  I  need  not, 
of  course,  explain  any  further." 

1.  In  the  eye  are  three  kinds  of  nerve-fibres,  all  identical  in  their  struc- 
ture and  conducting  processes,  but  supplied  with  different  end-organs  (rods 
and  cones).     One  kind  of  end-organs  we  will  call  the  red-perceiving  or 
red-sensitive  end-organs,  because  they  are  supplied  with  a  photo-chemical 
substance  which  is  mostly  affected  by  the  red  rays  of  the  spectrum,  though 
the  other  rays  affect  it  also  to  some  extent.     The  second  kind  of  end- 
organs,  the  green-sensitive,  are  endowed  with  a  photo-chemical  substance 
mostly  sensitive  to  green  light ;  and  the  third  kind  of  fibres,  the  blue-sensi- 
tive,1 with  a  third  substance  mostly  sensitive  to  blue. 

2.  By  the  decomposition  of  each  of  the  sensitive  photo-chemical  sub- 
stances the  fibre  connected  with  it  is  stimulated.     There  is  only  one  kind 
of  sensation-producing  activity  in  each  nerve-fibre,  which  is  accompanied 
by  decomposition  of  the  organic  substance  and  development  of  heat  in  the 
same  way  as  we  know  it  to  be  in  the  muscle-nerves.     These  processes 
in  the  three  fibre-systems  are,'  therefore,  probably  of  entirely  the  same 
character,  and  they  affect  us  differently  only  because  they  are  connected 
with  functionally  different  parts  of  the  brain.     The  nerve-fibres  need  play 
only  the  part  of  electric  wires,  which  are  traversed  by  the  same  electric 
current  and  yet  may  produce  the  most  different  results,  according  to  the 
instruments  with  which  they  are  connected.      These  stimulations  of  the 
three  fibre-systems  play  the  rdle  of  three  elementary  stimulations,  provided 
that  the  strength  of  the  stimulation,  for  which  as  yet  we  have  no  general 
measure,  is  put  down  as  proportional  to  the  strength  of  the  light.     This 
proportionality  does  not  prevent  us  from  assuming  that  the  intensity  of  the 

1  Helmholtz  speaks  usually  of  violet  sensitive  fibres,  but,  as  he  himself  later  inclines 
to  blue,  this  has  been  selected  here. 


596 


NORMAL   COLOR-PERCEPTION. 


elementary  stimulation  may  be  any  complex  function  of  the  metabolism  or 
the  negative  variation  in  the  nerve. 

3.  In  the  brain  these  three  fibre-systems  are  connected  with  three 
functionally  different  systems  of  ganglion-cells,  which  perhaps  are  locally 
so  placed  to  each  other  that  those  are  near  together  which  correspond  to 
the  same  retinal  area. 

A  little  diagram,  perhaps,  will  serve  to  make  this  somewhat  plainer. 

FIG.  2. 


In  Fig.  2,  abc  are  the  identical  nerve-fibres.  R  is  the  red-sensitive  end- 
organ  of  a,  G  the  green-sensitive  of  6,  and  B  the  blue-sensitive  of  c.  R', 
G',  B'  are  the  three  brain-cells,  probably  in  the  cuneus,  whose  specific 
energy  is  for  us  identical  with  a  sensation  of  red  or  green  or  blue,  no 
matter  in  what  way  that  energy  has  been  aroused,  whether  by  the  usual 
way  of  the  nerve  or  by  central  irritation  in  the  brain,  as  in  sleep.  We  see 
that  this  theory  makes  the  retina  only  the  differentiating  or  selecting  organ, 
while  the  brain  alone  is  the  perceiving  one.  With  regard  to  the  three  dif- 
ferent photo-chemical  substances,  it  must  be  admitted  that  at  present  they 
are  only  hypothetical ;  but  as  we  have  already  found  one  substance  in  the 
retina  that  is  very  sensitive  to  light, — the  visual  purple,  which  was  discovered 
only  a  few  years  ago,  in  1876, — we  may  well  suppose  that  there  are  other 
chemical  substances  yet  to  be  discovered  by  actual  experiment.1  Perhaps 
we  may  then,  indeed,  find  that  the  visual  purple  itself  plays  a  very  im- 
portant part  in  color-vision,  as  has  been  suggested  lately  by  Ebbinghaus, 
but  that  can  be  decided  only  by  further  investigations.  Later  we  shall  get 
some  idea  about  the  relation  of  the  three  photo-chemical  substances  to  each 
other. 

At  present  we  must  see  in  what  way  these  three  chemical  substances  are 
affected  by  the  different  spectral  colors.  The  diagram  on  the  opposite  page 
may  serve  to  show  this  in  a  rough  way. 


1  Kiihne,  in  Hermann's  Handbuch  der  Physiologie,  p.  341,  is  of  the  same  opinion 
•when  in  his  article  "  Chemische  Vorgange  in  der  Netzhaut"  he  remarks  that  "every- 
thing urges  us  to  assume  also  colorless  visual  substances  in  the  visual  cells." 


NORMAL   COLOR-PERCEPTION. 


597 


Fid.  3. 


Here  the  spectral  colors  must  be  imagined  to  be  placed  at  the  letters 
R  O  Y  G  B  V.  Then 
the  three  curves  may  rep- 
resent the  amount  of  stim- 
ulation that  is  given  to 
each  substance  by  the  dif- 
ferent spectral  colors,  No. 
1  being  for  the  red-sensi- 
tive, No.  2  for  the  green- 
sensitive,  and  No.  3  for  the 
blue-sensitive  substance. 

The  spectral  color  red 
stimulates  strongly  the  red- 
sensitive  substance,  less  the  green,  and  least  the  blue  one.     Sensation  equals 
red. 

The  simple  yellow  excites  moderately  the  red-  and  the  green-sensitive 
material,  little  the  blue.     Sensation  equals  yellow. 

The  simple  green  excites  strongly  the  green-sensitive,  but  less  the  other 
two  substances.     Sensation  equals  green. 

The  simple  blue  excites  strongly  the  blue-sensitive,  little  the  red-  and 
the  green- sensitive  material.  Sensation  equals  blue. 

The  simple  violet  excites  strongly  the  blue-,  less  the  green-  and  the  red- 
sensitive  substance.  Sensation  equals  violet. 

Stimulation  of  all  the  three  substances  to  about  the  same  degree  gives 
the  sensation  of  white  or  of  very  whitish  colors.  We  see  here  that  every 
spectral  color  stimulates  all  the  substances  more  or  less,  so  that  really  the 
spectral  colors  do  not  represent  the  pure  sensations.  To  obtain  these  we 
are  obliged  to  have  recourse  to  an  artifice,  which  will  be  mentioned  later. 
Helmholtz  has  lately  calculated,  from  very  careful  experiments  performed 
by  Kdnig  and  Dieterici  as  well  as  by  Brodhun,  what  must  be  the  primary 
color-sensations  if  Weber-Fechner's  law1  is  assumed  to  have  not  been 
violated  in  those  observations.  He  found  that  carmine  red,  the  green  of 
vegetation,  and  ultramarine  blue  are  nearest  to  our  primary  sensations.  He 
also  found 2  "  that  all  simple  colors  stimulate  all  the  light-sensitive  nervous 
elements  of  the  trichromic  eye  simultaneously  and  with  only  moderate  dif- 
ferences of  intensity.  If  we,  therefore,  hypothetically  reduce  the  stimula- 
tion to  the  presence  of  three  photo-chemical ly  changeable  substances  in 
the  retina,  we  must  conclude  that  all  these  three  have  nearly  equal  limits 
of  light- sensitiveness,  and  that  they  show  only  secondary  deviations  of 
moderate  amount  in  the  curve  of  the  photo-chemical  effect  for  the  different 

1  This  law  states  that  within  certain  limits  the  increase  dE  in  our  sensation  E  is 
directly  proportional  to  the  increase  of  the  stimulus  dH,  and  inversely  proportional  to  the 

stimulus  H  ;  or  mathematically  dE  equals  A1—  ,  where  A  is  a  constant. 

II 

2  Loc.  cit.,  p.  457. 


598  NORMAL   COLOR-PERCEPTION. 

wave-lengths.  Similar  changes  by  admixture  of  other  substances,  substi- 
tution of  analogous  groups  of  atoms,  etc.,  occur  in  other  photo-ehemically 
changeable  substances  as  they  are  used  in  photography, — for  example,  in 
the  different  haloid  salts  of  silver." 

Objections  to  this  Theory. — Helmholtz's  theory  supposes  that  each  ter- 
minal element  has  its  separate  nerve-fibre ;  but  this  cannot  be,  as  Salzer 
found  that  there  are  about  three  millions  of  cones  alone  in  the  human  retina, 
while  the  optic  nerve  cannot  have  more  than  one  million  fibres.  This  objec- 
tion, however,  is  not  valid  ;  for  though  we  must  assume  one  separate  fibre 
for  each  cone  in  the  fovea  centralis,  there  is  no  necessity  of  doing  the  same 
for  the  peripheral  parts  of  the  retina.  On  the  contrary,  Helmholtz  himself 
has  given  a  theory  which  shows  how  the  facts  of  peripheral  vision  can  be 
explained  much  better  by  the  assumption  that  many  nervous  end-elements 
are  connected  with  one  fibre  .alone.1 

Another  objection — raised  by  Fick  years  ago — has  been  lately  brought 
forward  by  Professor  Rutherford  in  his  opening  presidential  address  to 
Section  D,  Biology,  in  the  British  Association.  He  says  that  according 
to  Helmholtz's  theory  a  colorless  small  pencil  of  light — as  from  a  star — 
ought  not  to  be  seen  without  a  color  unless  it  fall  on  three  cones  at  the 
same  time,  which,  from  astronomical  observations,  is  not  the  case.  And, 
indeed,  this  seems  to  be  a  fact  fatal  to  the  theory ;  because  we  see  the  white 
star  white,  and  know  that  its  image,  on  account  of  its  small  size,  can  cover 
only  one  cone.  But  it  only  seems  so ;  for  we  must  not  forget  that  our 
eye  is  continually  going  through  very  small  movements,  which,  leaving 
out  of  consideration  other  factors,  have  their  principal  cause,  perhaps,  in 
the  rhythmical  innervation  of  the  muscles  that  hold  the  eye  fixed.  The 
distance  between  two  cones  in  the  fovea  is  at  the  most  0.008  millimetre, 
corresponding  to  an  angle  of  about  2'  measured  from  the  second  nodal 
point  of  the  eye.  Now,  if  the  eye  in  these  small  oscillations  moved  through 
an  angle  of  only  2',  this  movement  in  all  directions  would  bring  the 
point  of  light  on  the  retina  in  relation  with  at  least  seven  different  cones. 
The  cornea  would  move  through  a  circle  of  only  0.0075  millimetre  radius, 
which  cannot  be  observed  with  the  naked  eye.  We  see,  therefore,  how 
small  a  movement  of  the  eye  suffices  to  expose  very  many  little  cones, 
one  after  another,  to  the  same  point  of  light,  so  that  the  brain  can  easily 
perceive,  by  a  quick  comparison  of  the  impressions  obtained  successively 
by  the  three  color-cells  Rr,  Gf,  B'  (Fig.  2),  whether  the  light  from  the 
distant  star  is  white  or  colored.  For  if  all  the  cells  are  excited  to  the 
same  degree,  rapidly,  one  after  the  other,  we  judge  the  star  to  be  white ;  but 
if,  for  example,  the  R'  cell  gets  the  strongest  impression  comparatively, 
we  call  the  star  or  point  of  light  red.2  This  explanation  will  also  give  an 


1  Loc.  cit.,  p.  264. 

2  This  explanation  of  the  difficult  question  is  not  given  by  Helmholtz,  but  is  brought 
forward  here  for  the  first  time,  so  far  as  known  to  the  writers. 


NORMAL   COLOR-PERCEPTION.  599 

answer  to  the  following  passage  from  Rutherford's  lecture:  "Whichever 
view  [whether  light  stimulates  the  optic  terminals  by  inducing  vibrations  or 
by  setting  up  chemical  changes]  we  adopt,  it  seems  to  me  necessary  to  suppose 
that  all  the  processes  for  the  production  of  nerve-impulses  can  take  place  in 
one  and  the  same  visual  cell,  and  are  transmitted  to  the  brain  through  the 
same  nerve-fibre,  because  the  image  of  a  colored  star  small  enough  to  fall 
only  upon  one  cone  is  seen  of  a  fixed  and  definite  color,  which  does  not  alter 
when  the  position  of  the  eye  is  changed.  It  seems  to  me  that  if  there  are 
special  cones  for  red,  green,  and  blue,  the  color  of  the  star  should  change 
when  its  image  falls  on  different  terminals ;  but  I  am  assured  by  Lockyer 
that  such  is  not  the  case."  It  seems  that  in  this  regard  the  retina  may  be 
compared  to  the  skin.  Here  also  we  find  different  end-elements,  one  sensi- 
tive to  pressure,  another  to  heat,  and  still  another  to  cold  ;  so  that  only  by 
exposing  different  parts  of  the  skin  to  very  small  objects  do  we  find  out 
these  properties. 

Another  objection  raised  by  Fick  is  the  following.  Every  ray  of  light, 
while  exciting  a  color-sensation  if  it  fall  on  a  sufficient  area  of  the  posterior 
polar  part  of  the  eye,  provided  it  acts  on  an  extremely  limited  part  of  the 
retina,  even  if  it  be  colored  light,  produces  a  whitish  impression.  This  is 
exactly  the  opposite  of  what  we  should  expect, — viz.,  the  smaller  the  area 
of  the  retina  acted  on,  the  more  easily  should  the  peculiar  nerve-ending  be 
excited  and  a  pure  color-sensation  result.  Here  it  is  to  be  replied  that 
this  is  true  only  if  the  amount  of  light  falling  to  the  "  extremely  limited 
part"  is  small,  because  we  really  do  see  distant  stars  colored.  And  the 
reason  why  we  see  the  small  amount  of  colored  light  whitish  depends  upon 
the  fact  that  with  weak  light  the  curves  for  the  sensitiveness  of  the  three 
photo-chemical  substances  to  the  different  colors  almost  coincide,  so  that  in 
our  brain  the  three  different  color-perceiving  cells  are  stimulated  to  about 
the  same  degree,  giving  the  whitish  sensation.  If,  however,  the  retinal 
area  of  the  same  dim  colored  light  be  greater,  then  more  color-perceiving 
cells  are  stimulated,  and  the  small  difference  in  their  stimulation  may  then 
become  large  enough  to  enter  the  threshold  of  consciousness :  we  see  again 
color. 

A.  Charpentier l  objects  to  the  Young-Helmholtz  theory  because  it  does 
not  explain  the  fact,  proved  by  him,  that  even  the  central  parts  of  the  retina 
are  less  sensitive  to  color  than  to  white  light.  But  it  seems  that  this  theory 
can  account  for  it  very  easily,  because  it  assumes  that  white  light  stimulates 
all  the  three  cones  equally,  so  that  now  three  brain-cells  are  excited,  while 
by  monochromatic  light  only  one  cell  is  considerably  aroused. 

The  great  majority  of  human  eyes  belong  with  regard  to  color-per- 
ception to  one  and  the  same  class.  As  three  colors,  or,  more  correctly 
expressed,  three  primary  color-sensations,  are  required  to  explain  their 
color-perception,  these  eyes  are  called  normal  trichromic  eyes.  But  the 

1  Cited  by  Prince  Kropotkin  in  his  article  before  mentioned. 


600  NORMAL   COLOR-PERCEPTION. 

three  primary  sensations  are  not  always  exactly  alike  in  different  men, 
or  even  in  the  two  eyes  of  the  same  observer.  This  has  been  already 
mentioned  in  the  case  of  different  persons,  and  is  also  shown  for  the 
same  individual  in  the  observations  with  the  leukoscope1  of  Helmholtz,  in 
which  instrument  one  endeavors  to  make  two  complementary  colors  pro- 
duced by  quartz  plates  in  polarized  light  as  equal  as  possible  to  each  other ; 
and  as  a  coloring  of  the  media  of  the  eye  sufficient  to  explain  this  phe- 
nomenon has  never  been  observed,  we  are  driven  to  the  assumption  that 
even  in  normal  trichromic  eyes  the  intensity  of  the  stimulation  of  each 
primary  color-sensation  is  a  function  of  the  wave-length  of  the  light, 
slightly  different  in  different  individuals.  But  all  those  persons  who  can 
distinguish  all  the  colors  of  the  spectrum  must  be  called  normal  with 
regard  to  color-perception.  There  are,  however,  persons — the  so-called 
color-blind — who  do  not  see  all  the  colors  in  the  spectrum.  Some,  indeed, 
— the  so-called  achromatics  or  monochromatics, — see  no  different  colors  at 
all  there,  but  only  different  shades.  Such  cases  are  very  rare.2  More  fre- 
quently we  find  people  who  can  see  only  two  colors  in  the  whole  spectrum. 
They  are  called  dichromatics,  and  the  study  of  their  defect  is  of  great  prac- 
tical importance  to  the  employees  of  railroad  and  navigation  companies. 

In  general  there  are  two  kinds  of  dichromatics.  If  the  spectrum  is 
shown  to  the  first  kind,  they  will  say  that  red,  orange,  and  yellow  are  all 
yellow,  red  being  described  as  dark  yellow,  orange  as  less  dark,  and  yellow 
as  bright  yellow,  whilst  the  green  part  of  the  spectrum  bordering  on  the 
yellow  will  be  described  as  yellow  diluted  with  white.  In  the  normal 
blue-green  they  point  out  a  white  or  gray  band,  the  so-called  neutral  band  ; 
the  blue  near  it  they  describe  as  a  whitish  blue,  whilst  the  rest  of  the  blue 
appears  light  blue  and  the  violet  dark  blue.  (See  Plate,  vol.  ii.,  article 
"  Color-Blindness."  If  they  are  given  certain  light  green  and  dark  red 
worsteds,  they  will  often  not  see  the  difference  in  hue.  They  cannot  always 
distinguish  a  dark-green  pattern  on  a  black  background,  but  they  can  see 
well  a  red  one  on  the  same  ground.  The  spectrum  is  not  shortened  for 
them  at  the  red  end,  and  the  maximum  of  intensity  is  for  them  in  the 
yellow.  As  the  neutral  line  is  in  the  green,  we  must  assume  that  these 
persons  do  not  perceive  the  green  as  we  do,  and  they  are  for  that  reason 
called  green-blind.  Purple,  which,  as  we  know,  excites  a  blue  and  a  red 
sensation,  the  green-blind  can  distinguish  from  blue,  as  both  colors  excite  in 
him  the  corresponding  sensations ;  but  he  confounds  it  with  green  and  gray. 

This  is  not  so  in  the  second  class.  Here  the  individuals  cannot  distin- 
guish purple  from  blue.  The  spectrum  appears  shortened  to  them,  as  is 
well  shown  by  Captain  Abney's  table  on  page  608  ;  it  begins  usually  in  the 
orange,  and,  as  they  do  not  see  the  extreme  red,  they  are  called  red-blind. 

1  Diro  Kitato,  Zur  Farbenlehre,  Dissertation,  Gottingen,  1878;   A.  Konig,  Wiede- 
mann's  Annalen,  Bd.  xvii.,  1882;  Helmholtz,  Physiol.  Opt.,  p.  372. 

2  To  obtain  some  idea  of  the  way  in  which  the  different  colors  appear  to  them,  vide 
photograph  of  Thomson's  color-stick,  in  article  on  Color-Blindness. 


NORMAL   COLOR-PERCEPTION.  601 

To  them  the  spectrum  also  consists  of  only  two  colors,  yellow  and  blue,  but 
the  neutral  white  or  gray  line  lies  in  the  green-blue,  more  towards  the  blue 
than  in  the  first  class  :  they  have  their  maximum  of  brightness  in  the 
spectrum  also  more  towards  the  blue,  in  the  yellow-green.  They  frequently 
confound  light  red  with  dark  green.1 

That  both  these  classes  of  dichromatics  can  mix  all  the  colors  by  yellow 
and  blue  was  shown  by  Bonders,2  who  made  two  different  color-blind  persons 
copy  a  color-circle  containing  one  hundred  different  colors  with  these  two 
colors  alone.  Each  regarded  his  copy  as  perfect,  but  ridiculed  that  of  the 
other.  Now,  how  does  the  Young-Helmholtz  theory  explain  the  color- 
perception  of  the  color-blind  ?  Formerly,  Young  and  Helmholtz  explained 
these  defects  by  assuming  an  entire  absence  of  the  red-  or  green-sensitive 
fibres ;  but  lately,  in  the  second  edition  of  the  "  Physiologische  Optik," 
Helmholtz  has  given  an  explanation  that  seems  to  be  more  probable. 
We  have  seen  that  he  assumes  three  photo-chemical  substances  which  are 
comparatively  only  slightly  different,  and  which  for  brevity's  sake  may 
be  called  the  red-,  green-,  and  blue-sensitive  substances.  He  further 
assumes  that  in  normal  eyes  there  is  such  a  genetic  relation  between  the 
red-  and  green-sensitive  substances  that  the  green  one  in  some  cones  is 
further  developed  into  the  red  one.3  Now,  in  those  persons  who  are  called 
green-blind  this  normal  metabolism  is  perverted.  All  those  cones  which 
in  the  normal  eye  carry  the  green-sensitive  substance  have  this  latter  also 
developed  at  once  into  the  red-sensitive  substance,  so  that  there  exist  then 
only  cones  with  red-sensitive  and  blue-sensitive  material.  In  other  words, 
looking  back  to  Fig.  2,  G  has  become  equal  to  R,  while  all  the  rest — the 
fibres  a,  b,  c,  and  the  brain-cells  R',  G',  B' — remain  the  same.  If  now  red 
light  strikes  the  cones,  all  these  R  cones  are  affected,  thus,  however,  stimu- 
lating both  -R'  and  GJ  cells  of  the  brain.  But  if  in  the  normal  eye  R'  and 
G'  are  equally  aroused,  as  is  done  by  the  yellow  of  the  spectrum,  it  has  the 
sensation  of  yellow.  This  same  spectral  yellow  affects  now  in  our  green- 
blind  person  all  the  R  cones,  and  therefore  the  G'  and  R'  cells,  equally,  so 
that  yellow  must  make  the  same  impression  upon  his  brain  as  in  the  normal 
eye,  if  we  suppose  with  Helmholtz  that  the  brain-cells  do  not  differ  much 
in  either  case.  Further,  as  the  spectral  red  also  arouses  the  cerebral  R' 
and  G',  it  is  clear  that  the  patient  cannot  distinguish  between  yellow  and 
red,  both  affecting  his  brain  in  almost  the  same  manner.  He  will  call  the 
red  yellow,  and  it  is  evident  that  all  his  color-sensations  will  be  com- 

1  It  must  be  confessed  that  there  is  a  great  deal  of  difference  in  different  authors  in 
the  description  of  the  manner  in  which  the  color-blind  see.     The  present  description,  how- 
ever, is  given  by  the  best  observers,  and  borne  out,  at  least  for  the  green-blind,  by  persons 
with  uniocular  green-blindness  and  by  the  careful  self-observations  of  Dr.  W.  Pole. 

2  Arch,  fiir  Ophthalm.,  xxvii.  155  et  seq.,  1881. 

3  It  would  seem  that,  judging  by  the  fact  that  the  green  is  usually  first  lost  in  patho- 
logical cases,  the  green-sensitive  material  might  be  developed  from  the  red  one.    But,  how- 
ever this  may  be,  that  would  not  change  the  following  explanation. 


602  NORMAL   COLOR-PERCEPTION. 

posed  of  a  yellow  and  a  blue  sensation.  That  our  green-blind  individual 
does  not  see  the  spectrum  shortened  can,  of  course,  be  easily  explained  by 
the  fact  that  his  cones  are  red-sensitive.  To  explain  the  neutral  band,  we 
must  remember  that  in  the  green,  where  they  see  this  band,  the  red  and 
blue  cones  are  stimulated  about  equally  (as  Fig.  3  shows),  and  that  there- 
fore in  our  case  all  the  three  brain-cells  R',  G',  Bf  are  aroused  to  about  the 
same  degree.  Now,  whenever  in  normal  or  color-blind  eyes  all  three  brain- 
cells  are  equally  stimulated,  the  sensation  of  white  or,  on  equal  feeble  stimu- 
lation, of  gray  must  arise.  Thus  we  see  that  there  must  be  a  neutral  band 
of  white  or  gray  in  the  spectrum  of  the  green-blind,  that  purple,  green,  and 
gray  cannot  be  distinguished  by  their  hues ;  and  we  observe  further  that 
he  must  see  white  very  much  as  we  see  it,  because  white  light  (the  red  and 
blue  parts  of  it)  excites  his  R',  G'y  Bf  cells  equally.  Thus  the  objection 
is 'answered  that  is  often  made  to  this  theory, — namely,  that  it  does  not 
explain  how  the  red-  or  green-blind  can  see  white.  This  explanation  also 
does  away  with  another  difficulty  which  is  well  stated  by  Berry,1  thus :  "  In 
the  first  place,  there  can  be  no  doubt  that  an  individual  who  is  blind  for 
one  particular  hue  is  at  the  same  time  blind  for  its  complement.  That  this 
is  the  case  is  shown  by  the  following  facts.  It  is  possible  by  the  rapid  rota- 
tion of  a  disk  to  obtain  from  three  or  more  suitably  selected  colored  sectors 
an  impression  which  is  identical  with  that  of  a  mixture  of  black  and  white 
produced  in  the  same  way ;  the  colors  taken  in  certain  proportions  can  be 
got  to  neutralize  each  other,  so  that  the  resulting  impression  is  colorless. 
The  slightest  removal  of  any  portion  of  one  of  the  colors  entering  into  the 
combination  can  at  once  be  detected,  and  some  color-sensation  is  the  result. 
If,  on  the  other  hand,  the  disk  should  contain  two  sectors  of  exactly  com- 
plementary colors,  their  simultaneous  removal  does  not  destroy  the  colorless 
effect ;  the  remaining  colors  continue  to  neutralize  each  other,  so  that  the  im- 
pressions they  give  rise  to,  following  each  other  in  rapid  succession,  resolve 
themselves  into  gray.  Now,  it  is  found  that  the  same  mixture  which  to  a 
normal  individual  appears  similar  to  a  mixture  of  black  and  white  also 
appears  so  to  the  color-blind  individual,  whence  it  follows,  as  they  are 
known  to  be  blind  for  one  color,  that  they  must  either  be  insensitive  to  two 
complementary  hues  in  both  disks  or  to  only  one  in  each.  If  they  only 
fail  to  perceive  one,  both  disks  must  appear  to  them  colored ;  but  this  is 
extremely  unlikely,  because  then  all  objects  which  appear  to  the  normal  eye 
colorless  must  appear  to  them  colored." 

The  facts  are  here  stated  well,  but  it  must  be  remarked  that  the  expres- 
sion "  blind  for  one  color"  is  misleading,  as,  for  instance,  the  green-blind  is 
somewhat  affected  in  his  eyes  by  the  green  rays  of  the  spectrum,  because  the 
red-  and  blue-sensitive  substances  are  still,  though  very  moderately,  excited  ; 
but  the  effect  is  (1)  much  weaker  than  in  the  normal  eye  and  (2)  differently 
distributed  tb  the  brain-cells,  so  that  a  different  sensation  must  result. 

1  Diseases  of  the  Eye,  p.  336. 


NORMAL   COLOR- PERCEPTION.  603 

Suppose,  for  example,  that  the  revolving  disk  contained,  besides  other  com- 
plementary hues,  green  and  purple  sectors.  This  to  our  green-blind  must 
appear  gray,  just  as  to  the  normal  eye;  for  though  his  eye  has  no  green- 
sensitive  material,  still  the  green  does  affect  moderately  both  his  other 
photo-chemical  substances,  and  so  gives  gray,  while  the  purple  affects  also  all 
his  cones — the  red-  and  blue-sensitive  ones — and,  therefore,  all  the  R',  G',  B' 
cells,  thereby  giving  likewise  the  sensation  of  gray  or  white.  From  this 
it  follows  that  neither  the  removal  of  the  green  sector  alone  nor  that  of  the 
purple  one  can  change  the  former  sensation  of  gray,  though  he  is  by  no 
means  blind  to  the  red  and  blue  color  of  which  the  purple  is  composed. 

With  regard  to  the  other  class  of  color-blind  people,  the  so-called  red- 
blind,  the  theory  supposes  that  the  change  of  the  green- sensitive  into  the 
red-sensitive  substance,  which  normally  always  takes  place  in  a  certain  num- 
ber of  cones,  does  not  occur  at  all  here ;  so  that,  besides,  the  blue-,  only  the 
green-sensitive  substance  exists  in  such  eyes.  In  other  words,  the  R  cones 
have  become  equal  to  the  G  cones,  whilst  the  rest  of  the  visual  apparatus 
remains  the  same.  Here,  also,  yellow  must  be  seen  as  yellow,  for  this 
color  stimulates  all  the  G  cones,  and,  therefore,  all  the  G'  and  R'  cells,  as 
yellow  does  in  the  normal  eye.  For  the  same  reason  green  alone  must  be 
seen  as  yellow,  or  at  least  make  the  same  cerebral  impression  as  yellow.  In 
short,  we  have  almost  the  same  state  of  affairs  as  in  the  green-blind,  with 
the  following  exception.  The  spectrum  must  appear  shortened  to  the  red- 
blind,  especially  in  dim  light,  as  the  red  rays  stimulate  the  green-sensitive 
substance  only  very  slightly.  The  neutral  band  and  the  greatest  brightness 
of  the  spectrum  are  displaced  farther  towards  the  blue,  because  the  eye  is 
sensitive  to  green.  Purple  stimulates  much  their  blue-  but  hardly  their 
green-sensitive  substance,  so  that  purple  to  the  red-blind  must  appear  blue 
or  violet,  whilst  the  green-blind,  as  we  have  seen,  confuses  gray  or  green 
with  purple. 

These  two  classes  of  color-blind,  the  red-blind  and  the  green-blind,  are 
often  comprehended  in  one  group, — namely,  that  of  red-green  blindness, — 
especially  by  the  adherents  of  Hering's  theory.  And  indeed  it  must  be 
admitted  that  the  persons  who  cannot  see  green  as  we  do  cannot  see  red 
either  in  the  same  way  that  we  see  this  color.  As  Dr.  W.  Pole,  himself  a 
green-blind,  in  his  latest  contribution 1  to  the  subject,  remarks,  "  The  true 
solution  is  that  I  am  blind  to  both  colors"  (red  and  green).  This  fact  has 
had  special  stress  laid  upon  it  in  this  paper  in  order  that  the  reader  might 
not  get  the  impression  that  the  term  green-blindness  signified  a  condition  in 
which  the  individual  could  not  see  green,  but  in  which  he  could  see  red,  as 
it  is  perceived  in  the  normal  eye.  The  terms  red-blindness  and  green-blind- 
ness are  simply  retained  here  to  give  precision  to  the  clinical  difference  in 
the  two  classes  of  red-green  blind,  and  to  give  at  the  same  time  a  clue  to 

1  On  the  Present  State  of  Knowledge  and  Opinion  in  Regard  to  Color-Blindness.  Trans- 
actions of  the  Royal  Society  of  Edinburgh,  vol.  xxxvii.,  Part  II.,  September,  1893,  p.  459. 


604  NORMAL   COLOR-PERCEPTION. 

the  etiology  according  to  Helmholtz's  theory.  The  old  view  of  the  entire 
absence  of  the  red  or  green  fibres  has,  of  course,  been  abandoned. 

There  still  exists  a  third  class  of  color-blind,  the  so-called  blue-blind. 
Here  the  blue-sensitive  substance  has  changed  its  character,  it  having 
become  either  equal  to  the  red-  or  equal  to  the  green-sensitive  material.  If 
the  B  cones  become  equal  to  the  G  cones,  the  spectrum  must  appear  more 
or  less  shortened  ;  the  red  is  well  recognized,  but  the  blue  and  green  are  not 
distinguished,  as  seems  to  have  been  the  case  with  the  five  blue-blind 
patients  of  Magnus.  In  Cohn's  five  cases  the  spectral  yellow  was  con- 
founded with  gray,  because  in  these  patients  yellow  excites  all  their  cones 
(the  R  and  G  ones),  which  excites  all  the  R',  G',  B'  cells,  thus  producing  the 
same  cerebral  effect  that  gray  does.  If,  however,  the  B  cones  become  equal 
to  the  R  cones,  then  the  patient  will  confound  strong  blue  and  weak  red, 
which  appears  to  have  been  the  case  in  the  patient  reported  by  Stilling 
(Centralblatt  fur  prakt.  Augenheilkunde,  ii.  99,  May,  1878).  Blue-blind- 
ness, on  the  whole,  is  a  very  rare  condition,  and  has,  therefore,  not  been 
studied  very  thoroughly,  especially  as,  further,  it  is  not  of  much  practical 
importance. 

A  priori  we  might  expect  many  more  classes  of  color-blindness.  In- 
deed, twenty-seven  combinations  of  the  three  photo-chemical  substances 
into  sets  of  three  would  be  possible,  which,  as  only  one  set  could  be  the 
right  one,  would  give  us  twenty-six  classes  of  color-blindness.  But  it  lies 
in  the  nature  of  things  that  all  these  twenty-six  combinations  are  not  equally 
likely  to  occur,  because  these  substances  bear  a  certain  genetic  relation  to 
each  other,  so  that  certain  combinations  will  be  very  improbable  and  even 
impossible.  Indeed,  except  one  class,  still  to  be  mentioned,  the  three 
classes  just  given  seem  to  cover  the  ground  pretty  well,  though  the  "sur- 
prising multiplicity  of  the  individual  deviations"  (Geissler)  from  the  gen- 
eral types  might  sometimes  have  to  be  referred  to  a  different  combination. 

The  fourth  class  of  color-blind  people  are  the  totally  color-blind,  to 
whom  the  spectrum  appears  perfectly  achromatic.  Yellow,  or  mostly  the 
green,  appears  as  the  brightest  gray,  from  which  towards  both  ends  the 
spectrum  becomes  gradually  darker  till  it  finally  becomes  almost  black. 
Foster  believes  that  such  a  case,  well  authenticated,  would  furnish  a  com- 
plete refutation  of  the  Young-Helmholtz  theory  ;  but  the  vision  of  achro- 
matics  may  be  easily  explained  by  assuming  that  the  three  photo-chemical 
substances  have  all  become  alike  the  green-sensitive  substance.  Green 
must  be  taken  because  the  greatest  luminosity  in  their  spectrum  is  usually 
not  in  the  line  D  (yellow),  as  in  the  normal  eye,  but  in  the  green  of  the 
line  E.  It  is  here  of  great  interest  to  remark  that  in  a  very  dim  light  and 
after  prolonged  stay  in  darkness  the  normal  eye  sees  the  spectrum  in  the 
same  way  as  is  done  by  the  achromatic,  and  with  the  greatest  brightness  near 
the  line  E.  That  the  normal  eye  sees  the  spectrum  gray  under  these  condi- 
tions is  explained  by  Helmholtz  as  follows.  Every  spectral  color  stimulates 
ail  the  three  different  cones,  only  in  slightly  different  strengths ;  but  in 


NORMAL   COLOR-PERCEPTION.  605 

order  that  these  objective  differences  in  the  amount  of  stimulation  may 
enter  our  consciousness  and  thus  produce  color-vision,  it  is  necessary  that 
they  overstep  certain  amounts,  the  so-called  threshold  values.  If  this  is  not 
so,  as  in  a  dim  illumination,  we  perceive  only  the  stimulation  of  all  the 
corresponding  cerebral  cells ;  we  see  gray.  The  displacement  of  the  site  of 
the  greatest  luminosity  towards  the  blue  must  be  explained  by  reference 
to  Purkinje's  phenomenon,  according  to  which  our  eye  in  a  very  dim  light 
is  much  more  sensitive  to  blue  than  in  a  strong  illumination.  Although 
th\s  explanation  seems  so  natural  if  we  accept  this  new  theory  of  Helm- 
holtz,  still  this  phenomenon  has  been  made  the  starting-point  of  a  con- 
troversy against  the  Young-Helmholtz  theory  by  Professor  Ebbinghaus.1 
He  admits,  however,  that  if  we  make  the  above  assumption  of  all  the 
photo-chemical  substances  having  become  equal  and  more  or  less  like  the 
green-sensitive  substance,  at  least  in  so  far  that  the  new  substance  has  its 
maximum  of  intensity  in  that  color,  the  observed  phenomenon  can  be 
explained.  But  he  then  urges  the  following  difficulty.  If  a  spectrum  of 
mean  intensity  is  gradually  darkened  while  the  eye  views  the  whole  extent 
of  it,  then  the  yellow  and  cyanean  blue  disappear  gradually,  until  finally 
only  red,  green,  and  violet  blue  remain.  If  the  dimness  increases,  then 
green  first  loses  its  color  and  changes  into  gray ;  the  same  occurs  later 
at  the  spaces  where  formerly  the  yellow  and  blue  were  seen ;  only  the  red 
retains  its  color  very  long.  But  here  we  must  bear  in  mind  that  under 
these  conditions  we  have  to  do  with  a  very  complicated  process, — namely, 
with  the  positive  after-effect  of  colors  on  and  fatigue  of  the  retina,  which 
phenomena  follow  different  laws  for  different  colors,  as  we  shall  see  later. 
It  must,  however,  be  observed  in  this  connection  that  W.  von  Bezold  and 
later  E.  Briicke  concluded,  from  the  fact  that  in  a  very  short  and  not  too 
bright  spectrum  only  red,  green,  and  violet  blue  are  distinguished,  that 
these  three  colors  must  be  the  physiological  primary  colors,  inasmuch  as 
they  regard  those  elements  of  a  mixed  sensation  which  do  not  trespass  the 
threshold  of  stimulation  (Reizschwelle)  as  ineffectual  also  in  the  mixed 
sensation.  This  phenomenon,  then,  offers  no  argument  against  the  Young- 
Helmholtz  theory. 

But  here  must  be  mentioned  another  fact  that  has  been  used  against  this 
theory.  It  is  this :  all  simple  colors,  if  of  very  high  intensity,  produce  a 
whitish  sensation.  This  fact  may  be  explained  in  two  ways;  but  from 
the  other  experience,  that  in  a  spectrum  of  great  brightness  the  colors  are 
first  reduced  to  yellow  and  blue,  we  may  assume  that  in  an  intense  light 
the  cones  of  a  special  color  become  exhausted  of  their  differentiating 
material,  so  much  so  that  their  further  stimulation  is  only  slightly  different 
from  that  of  the  other  two  kinds,  so  that  all  the  three  cones  and  therefore 
all  the  three  brain-cells  for  one  retinal  area  are  equally  stimulated,  thus 
producing  the  impression  of  white.  Thus,  there  is  no  difficulty  for  the 

1  Zeitschrift  fiir  Psychologic  und  Physiologic  der  Sinnesorgane,  Bd.  v.  Heft  8,  p.  149. 


606  NORMAL   COLOR-PERCEPTION. 

Young-Helmholtz  theory,  if  we  only  remember  that  these  three  photo- 
chemical substances  must  not  be  regarded  as  fixed  constant  elements  like 
three  chemical  atoms,  but  as  three  genetically  related  substances  which  may 
change  their  mutual  relations  under  certain  circumstances. 

Color-  Vision  in  the  Peripheral  Parts  of  our  Retina. — Deviations  from 
normal  color-vision  similar  to  those  observed  in  the  color-blind  occur 
also  in  the  peripheral  part  of  the  normal  retina.  We  do  not  recognize  the 
color  of  an  object  at  the  very  extreme  part  of  our  field  of  vision  ;  every- 
thing makes  the  impression  of  gray.  Farther  towards  the  middle  of  the 
visual  field  the  difference  between  blue  and  yellow  is  first  recognized,  though 
blue  is  usually  seen  a  little  more  peripherally.  In  this  yellow-blue  zone 
deep  red  appears  almost  dark  or  dark  yellow,  blue  and  leaf-green  of  a 
yellowish  white.  Still  nearer  to  the  middle,  red  and  green  are  also  differ- 
entiated and  recognized  as  such.  We  may  say,  therefore,  that  the  middle 
zone  of  our  retina  is  green-blind,  and  explain  that  according  to  the  Young- 
Helmholtz  theory  by  the  assumption  that  in  this  zone  the  red-  and  green- 
sensitive  substances  have  become  alike  and  equal  to  the  former.  Here  re$ 
and  green  appear  more  or  less  yellowish,  as  both  colors  excite  the  R'  and  Gf 
cells  equally  in  the  same  way  as  dark  or  light  yellow  would.  The  outmost 
zone  is  totally  color-blind,  because  here  all  the  terminals  contain  the  same 
photo-chemical  substance. 

We  may  refer  here  to  the  speculation  of  Donders,  who  considered  that 
he  could  trace  in  the  retina  vestiges  of  several  evolutionary  steps,  some- 
what as  follows : 

1.  Sensations  of  light  and  shade  only. 

2.  Dichromic  imperfect  vision  like  red-blinduess,  with  short  spectrum 
and  low  sensitiveness  to  the  long-waved  rays. 

3.  Dichromic  perfect  vision  like  green-blindness,  with  full  length  of 
spectrum. 

4.  Imperfect  normal  vision,  with  low  sensitiveness  to  certain  colors. 

5.  Perfect  normal  vision.1 

Apparently  against  this  statement  are  the  observations  of  Landolt,2  who 
found  that  colored  papers  (about  one  square  centimetre  in  size),  if  illumi- 
nated by  direct  sunlight,  and,  further,  the  spectral  colors,  if  intense  enough, 
are  recognized  in  their  color  up  to  the  extreme  limits  of  our  visual  field ; 
but  Dobrowolsky  and  other  observers  after  him  objected  that  the  color- 
perception  in  Landolt's  experiment  is  due  to  diffracted  light,  which,  falling 
on  more  central  parts  of  the  retina,  is  here  recognized.  It  seems,  therefore, 
fully  justifiable  to  assume  such  an  extreme  achromatic  zone  with  Helrn- 
holtz,  Donders,  Ole  Bull,  Preyer,  Woinow,  and  other  careful  observers. 
At  the  same  time  it  must  be  admitted  that  by  practice  our  peripheral 
color-vision  can  be  somewhat  improved  by  a  better  education  of  the  cor- 

1  Dr.  Pole,  loc.  cit.,  p.  468. 

1  Annales  d'Oculistique,  Jan.,  Fevr.,  1874. 


NORMAL   COLOR-PERCEPTION. 


607 


responding  brain  parts.     The  extent  of  the  peripheral  field  for  white  and 
colors  is  given  in  the  following  table : 


White. 

Blue. 

Red. 

Green. 

Externally     

90° 

80° 

65° 

<iO0 

Out  and  up   

60° 

55° 

4.V 

4/V> 

Upward                 

45° 

40° 

W 

orjo 

Up  and  in     

50° 

46° 

30° 

25° 

Internally  

60° 

55° 

50° 

40° 

In  and  down     .    . 

60° 

50° 

35° 

30° 

Downward     

70° 

60° 

45° 

:;.v 

Down  and  out      .    .                       

85° 

75° 

55° 

45° 

We  must  not  expect  to  find  the  same  numbers  for  all  normal  eyes, 
especially  as  the  pigments  used  in  these  examinations  are  by  no  means 
always  the  same.  Here  must  be  remembered  what  Landolt1  says,  that 
"  no  experience  about  the  perception  of  colors  is  complete  that  does  not  take 
into  consideration  (1)  the  degree  of  the  general  illumination,  (2)  the  bright- 
ness, and  (3)  the  area  of  the  color  employed."  That  our  peripheral  color- 
vision  depends  also  on  the  area  of  the  colored  object  was  first  quantitatively 
shown  by  Aubert,2  who  at  the  same  time  demonstrated  the  influence  of  the 
background  upon  which  the  colored  object  is  shown.  According  to  him, 
the  color  of  colored  squares  at  twenty  centimetres  from  the  eye  disappears 
at  the  following  angles  of  deviation  from  the  visual  line : 


Side  of  the  Square  in  Milli- 
metres. 

Red. 

Blue. 

Yellow. 

Green. 

1 

2 

4 

8 

1 

8 

4 

8 

1 

2 

50° 

45° 
470 

White  background  .... 
Black  background  .... 
Average     ...        .... 

16° 
30° 
23° 

19° 
32° 
26° 

26° 
42° 
34° 

37° 
53° 
45° 

15° 

36° 
26° 

22° 
48° 
35° 

36° 

54° 
45° 

49° 
72° 
61° 

21° 
30° 
26° 

31° 

32° 
32° 

440 

49° 
42° 

47° 

20° 
24° 
22° 

36° 

27° 
32° 

44° 

35° 
40° 

Aubert  also  found  that  a  blue  square  of  one  millimetre  side  on  a  white 
background  appeared  black  at  ten  feet  distance,  and  so  did  a  red  one  at 
twenty  feet  distance.  A  yellow  and  a  green  one  fused  completely  with  the 
white  ground  at  twelve  feet  distance.  On  a  black  background,  however, 
the  green  or  the  yellow  square  millimetre  appeared  as  a  gray  point  at  six- 
teen feet,  the  red  at  twelve  feet.  Blue  appeared  blue  when  it  was  seen  at 
all.  Oliver3  states  lately  that  in  order  to  be  recognized  on  a  black  back- 
ground and  under  diffused  daylight  at  a  distance  of  five  metres  it  requires 
squares  of  the  following  sides  for  the  different  colors :  two  and  two-thirds 
millimetres  for  red,  a  little  more  for  yellow,  eight  and  three-fourths  milli- 
metres for  blue,  ten  and  three-fourths  millimetres  for  green,  and  twenty- 
two  and  three-fourths  millimetres  for  violet. 


1  Loc.  cit.,  p.  3. 

3  Text-Book  of  Ophthalmology,  1893. 


2  Graefe's  Arch.  f.  Ophth.,  Bd.  iii.  p.  2. 


608 


NORMAL   COLOR-PERCEPTIOX. 


Here,  again,  attention  must  be  called  to  the  fact  that  the  pigment  colors 
are  by  no  means  always  the  same,  so  that  for  scientific  experiments  these 
pigments  must  be  analyzed  in  the  same  manner  as  has  been  done  by  Captain 
Abney  in  the  following  table : 


Colored  Papers. 

Wave-Length  of 
Dominant  Ray. 

Percentage  of 
White  Light. 

Percentage  of  Lumi- 
nosity if  White  Pa- 
per equals  100. 

Vermilion  

610  uu 

2.5 

14.8 

522  uu 

69 

22.7 

472  uu 

61 

4.4 

Brown  paper  ...           

594  uu 

50 

25 

Brown  paper             

587  uu 

67 

19.5 

591.5  uu 

4 

62.2 

583.5  uu 

26 

77  7 

500.5  uu 

42.5 

14.8 

Eosine  dye  

640  uu 

72 

44.7 

Cobalt          

482  uu 

55.5 

14.5 

This  would  mean  for  vermilion  that  this  pigment  being  taken  as  100,  it 
can  be  exactly  reproduced  by  mixing  97.5  parts  of  a  spectral  color  of  the 
wave-length  610  fj.fi.  with  2.5  parts  of  white  light.  Cattell1  has  shown  that 
when  the  light  reflected  from  colored  pigments  impinges  upon  the  retina  for 
too  short  a  time,  gray  alone  is  seen,  and  that  the  time  required  for  recog- 
nizing the  color  of  a  surface  one  by  three  centimetres  on  a  white  background 
with  daylight  from  a  clear  sky  varies  between  0.0006  and  0.0027  of  a 
second.  The  retina  in  these  experiments  was  most  sensitive  to  orange 
and  yellow,  requiring  about  0.0009  second  for  their  recognition,  while  it 
required  0.00121  second  for  blue,  0.00128  second  for  red,  0.00142  second 
for  green,  and  0.00232  second  for  violet.  As  pigments  reflect  light  not 
only  of  one  color,  but  usually  of  two  or  more  colors,  these  results  cannot 
be  used  to  draw  exact  conclusions  from  for  the  relative  inertia  of  the  three 
photo-chemical  substances,  but  they  certainly  show  us  that  time  is  required 
to  perceive  the  colors,  and  different  times  for  different  colors. 

Konig,  a  normal  trichromatic,  and  Brodhun,  a  dichromatic,  have  both 
made  very  careful  experiments  as  to  the  minimum  intensity  of  light  that 
must  exist  to  enable  us  to  recognize  the  existence  of  an  objective  illumi- 
nation in  comparison  with  the  perfectly  dark  background  of  the  objective 
field.  They  found  that  this  minimum  amount  of  light — this  "  untere  Reiz- 
schwelle,"  as  Helmholtz  calls  it — is  quite  different  for  differently  colored 
lights,  and  that  it  is  least  for  blue.  One  might  expect  that  the  eye  would 
observe  the  smallest  amount  of  light ;  but,  apart  from  the  fact  already  men- 
tioned,— the  inertia  of  the  retina, — there  is  another  factor  here  that  must  be 
considered  now  as  having  a  bearing  on  many  phenomena  mentioned  in 
our  article.  The  visual  field  of  even  a  healthy  man  is  at  no  time  entirely 
free  from  a  dim  light,  in  which  often  changing  spots  of,  it  may  be,  fan- 

1  The  Inertia  of  the  Eye  and  Brain.     Brain,  vol.  viii.  p.  301. 


NORMAL   COLOR-PERCEPTION.  609 

tastic  appearance  arise.  This  subjective  light,  that  changes  with  every 
movement  of  the  eye  and  lids  and  every  act  of  accommodation,  has  been 
called  by  Helmholtz  the  intrinsic  light  of  the  retina.  We  recognize  its 
presence  only  by  the. slight  fluctuations  of  its  intensity,  the  so-called  light 
dust  or  light  chaos,  whilst  the  intensity  itself,  as  calculated  by  Helmholtz 
according  to  the  modified  Fechner's  law,  is  by  no  means  so  slight.  This 
helps  us  to  understand  why  the  incoming  light  can  be  observed  only  after 
it  has  reached  a  certain  intensity, — namely,  that  of  the  intrinsic  light  of 
the  retina. 

Lately,  Konig  and  Brodhun l  have  made  very  careful  and  extended 
experiments  about  the  smallest  observable  difference  in  differently  colored 
lights.  They  have  found  that  if  one  selects  a  suitable  unity  of  illumina- 
tion for  the  different  spectral  colors,  the  curve  which  represents  the  depend- 
ence of  the  smallest  observable  differences  from  the  absolute  light  intensities 
shows  for  greater  intensities  only  small,  uncertain  differences.  In  a  weaker 
illumination,  however,  there  is  a  considerable  difference  between  the  more 
refrangible  and  the  less  refrangible  colors.  In  the  first  named,  the  blue 
colors,  one  can  then  observe  much  smaller  differences  of  the  objective  light 
intensity  than  in  the  red-yellow  colors.  These  observations  are  the  more 
important  as  they  form  the  basis  for  Helmholtz's  new  inquiry  into  the 
three  primary  color-sensations  referred  to  above.2 

So  far  we  have  spoken  of  color  only  as  a  momentary  sensation,  without 
regard  to  the  subsequent  changes  that  may  be  produced  by  its  after-effect ; 
for  all  light,  of  course,  affects  our  retina  not  only  at  one  moment,  but  for  a 
considerable  time  after  its  objective  disappearance,  the  simplest  proof  of 
which  lies  in  the  fact  of  our  seeing  a  light,  if  quickly  moved  in  a  circle,  not 
as  a  multitude  of  different  lights,  but  as  a  circle  of  light,  because  the  retinal 
impression  of  the  light  on  one  point  of  its  circular  path  still  remains,  while 
at  the  same  time  in  another  and  then  in  another  it  makes  new  impressions  on 
our  retina.  It  belongs  to  another  paper  in  this  System  of  Ophthalmology 
to  describe  these  phenomena  in  detail.  We  only  remind  the  reader  here 
that  these  after-effects  of  light  give  rise  to  after-images,  which  are  called 
positive  when  at  the  closure  of  the  eye  the  light  parts  of  the  after-image  cor- 
respond to  the  light  parts  of  the  object,  and  negative  when  each  light  part 
of  the  object  appears  dark  in  the  after-image.  If  the  light  has  affected  the 
eye  only  a  short  time,  and  the  visual  field  is  kept  free  from  all  traces  of 
external  light,  we  see  usually  a  positive  after-image,  which  disappears  gradu- 
ally without  becoming  negative.  This  latter  appears  at  once  if,  while  the 
positive  after-image  still  exists,  one  turns  the  eye  to  an  equally  illuminated 
surface.  The  color  of  the  object  and  of  the  positive  after-image  is  called 
the  primary  one,  whilst  that  of  the  field  later  affecting  the  eye  and  producing 
the  negative  after-image  is  called  the  reacting  light,  because  it  shows  the 

1  Sitzungsber.  der  Akad.  der  Wissensch.  zu  Berlin,  July  26,  1888. 

2  Helmholtz,  loc.  cit.,  pp.  405  and  448. 
Vol..  1—39 


610  NORMAL   COLOR-PERCEPTION. 

reaction  of  the  retina.  By  the  study  of  these  after-images  it  has  been 
shown  (1)  that  after  disappearance  of  the  primary  light  the  state  of  stim- 
ulation in  the  retina  and  visual  apparatus  lasts  for  some  time,  whence 
positive  after-images  result ;  and  (2)  that  the  same  nervous  substance 
affected  before  now  perceives  new  light  much  less  than  do  the  parts  of  the 
retina  not  stimulated  before,  so  that  if  new  light  falls  into  the  eye  the 
formerly  stimulated  area  appears  relatively  dark  as  compared  with  the  other 
visual  field,  thus  producing  negative  after-images.  If  now  we  have  looked 
at  a  colored  object,  and  regard  the  after-image  on  a  quite  dark  ground  or  a 
bright  ground  of  different  brightness,  then,  according  to  circumstances,  a 
positive  or  a  negative  after-image  will  appear.  The  positive  after-image  is 
first,  in  the  phases  of  its  greatest  luminosity,  of  the  same  color  as  the  object, 
whilst  the  negative  image,  at  least  as  soon  as  it  is  fully  developed,  shows  a 
color  complementary  to  that  of  the  object.  Thus,  the  negative  after-image 
of  red  is  blue-green ;  of  yellow,  blue ;  of  green,  rose  red ;  and  vice  versa. 
This  may  serve  as  a  good  argument  in  favor  of  the  Young-Helmholtz 
theory ;  for,  as  the  colored  light  does  not  stimulate  the  three  kinds  of  cones 
or  photo-chemical  substances  equally,  there  must  also  follow  an  unequal 
degree  of  fatigue.  If  the  eye  has  looked  at  red,  the  red-sensitive  cones  are 
strongly  stimulated  and  fatigued,  but  the  green  and  blue  ones,  to  only  a 
very  moderate  degree.  If,  now,  white  light  falls  into  the  eye,  the  green- 
and  blue-sensitive  cones  of  the  formerly  stimulated  area  will  be  more 
strongly  affected  than  the  red  ones.  There  must,  therefore,  appear  a  color- 
sensation  in  which  red  is  weak,  but  green  and  blue  are  dominant.  In  short, 
the  after-image  must  appear  blue-green.  Corresponding  results  are  obtained 
if  one  regards  negative  after-images  of  colored  objects  or  colored  grounds. 
From  the  color  of  the  ground  principally  those  constituents  disappear  which 
are  most  prominent  in  the  primary  color.  Thus,  a  green  after-image  will 
appear  red-yellow  on  a  yellow  ground,  because  the  yellow  of  the  ground 
(arousing  as  it  does  the  red  and  the  green  sensations)  excites  the  tired 
green  fibres  much  less. 

Of  special  interest  are  those  cases  in  which  the  primary  color  of  the 
object  is  complementary  to  the  reacting  one  of  the  ground.  If,  for  example, 
a  blue-green  piece  of  paper  is  placed  on  a  red  ground,  and,  after  having 
been  fixed  steadily  for  a  while,  is  taken  away,  there  appears  a  beautiful 
saturated  red  after-image  of  the  blue-green  object,  more  saturated  than  if  a 
black  object  on  the  same  red  ground  had  been  looked  at.  The  same  obser- 
vations have  been  made  and  verified  by  Helmholtz  with  pure  spectral  colors, 
whence  he  has  drawn  the  important  conclusion  that  the  most  saturated  objec- 
tive colors  existing — the  pure  spectral  colors — do  not  produce  in  the  untired 
eye  the  most  saturated  color-sensations  which  are  possible  for  us,  but  that 
we  obtain  these  only  when,  before  looking,  we  have  tired  our  eye  to  the 
complementary  colors.  This  seems  to  prove  that  even  the  spectral  colors 
are  by  no  means  quite  saturated.  They  contain,  as  the  Young-Helmholtz 
theory  especially  calls  attention  to,  a  great  deal  of  white  from  the  simulta- 


NORMAL   COLOR-PERCEPTION.  611 

neous  stimulation  of  all  the  photo-chemical  substances.  Thus,  it  is  evident 
that  if  the  blue  and  green  elements  have  been  much  fatigued,  as  in  the 
previous  experiment,  they  take  almost  no  part  in  the  production  of  the 
following  sensation,  so  that  the  following  red  stimulates  only  the  red- sensi- 
tive fibres,  and  appears,  therefore,  in  a  beautiful  saturation. 

Not  only  colored  but  also  white  objects  give  rise  to  colored  after-images ; 
but  the  complete  description  of  this  phenomenon  cannot  be  given  here,  for 
want  of  space.  We  can  only  mention  that  thereby  the  differences  in  the 
remaining  after-stimulation  and  the  following  fatigue  for  the  different  colors 
have  been  carefully  studied  by  Helmholtz  and  others,  and  that  the  results 
are  valuable  for  the  explanation  of  the  appearance  of  the  whole  spectrum 
in  gradually  reduced  light,  as  before  stated.  The  reader  must  be  referred 
to  larger  books,  such  as  Helmholtz's  "  Physiologische  Optik,"  for  a  study 
of  this  subject. 

Having  described  how  succeeding  colors  influence  each  other  mutually, 
we  must  now  see  how  colors  affect  each  other  if  at  the  same  time  in  the 
visual  field.  These  phenomena  of  subjective  colors  are  usually  described 
under  the  name  of  contrast,  and  especially  simultaneous  contrast,  whilst 
those  before  mentioned  fall  under  the  head  of  successive  contrast  This  suc- 
cessive contrast  plays,  however,  an  important  part  also  in  many  instances 
of  apparently  simultaneous  contrast,  when  one  compares  colors  which  are 
beside  each  other  in  the  visual  field ;  for  it  is  a  fact  that  in  the  common 
use  of  our  eyes  we  let  the  point  of  fixation  unconsciously  wander  in  the 
field,  so  that  it  glides  successively  over  the  different  parts  of  the  object 
looked  at.  When,  therefore,  the  eye  slowly  wanders  over  an  object,  an 
after-image  is  produced,  which,  however,  as  being  only  an  indistinct  spot, 
is  usually  not  recognized,  though  an  attentive  observer  will  see  it.  If  now 
the  eye  look  at  a  neighboring  field  of  another  color,  of  course  this  color 
must  be  changed  by  the  influence  of  the  after-image  of  the  first  one.  If 
we  place,  for  instance,  upon  a  sheet  of  colored  paper  a  small  circle  of 
white  or  gray  paper,  this  will  appear  in  the  complementary  color.  The 
same  principle  helps  us  to  understand  how  the  effect  of  contrast  is  the 
greater  the  nearer  the  two  colors  are  to  each  other,  because  the  eye  under 
those  circumstances  has  not  time  to  recover  as  quickly  as  it  could  if  the 
two  colors  were  far  apart.  We  cannot  give  more  examples  here,  but  the 
results  of  all  the  experiments,  as  Church  *  remarks,  confirm  in  every  par- 
ticular the  Young-Helmholtz  theory. 

With  regard  to  the  phenomena  of  pure  simultaneous  contrast,  however, 
we  cannot  make  use  of  the  wandering  of  the  eye,  because  they  appear 
even  if  the  eye  is  held  as  steady  as  possible.  Helmholtz  regards  them  as 
due  to  deceptions  of  judgment,  because  we  are  inclined  to  look  upon  those 
differences  which  are  distinctly  to  be  observed  in  the  perception  as  greater 
than  those  that  are  seen  only  indistinctly  in  the  perception  or  that  have  to 

1  Color,  by  Professor  A.  H.  Church,  1887,  p.  102. 


612  NORMAL   COLOR-PERCEPTION. 

be  judged  by  the  help  of  memory.  A  colored  field,  if  looked  at  steadily, 
will  soon  begin  to  appear  gray,  and  a  small  square  of  gray  on  a  surface  of 
green,  when  covered  with  a  transparent  sheet  of  tissue-paper,  appears  in  a 
distinct  rose  red.  Helmholtz  says,  "When  in  the  visual  field  a  special 
color  preponderates,  this  gradually  appears  in  a  whitish  shade,  while  real 
white  then  appears  in  a  complementary  color.  Our  mind  changes  the 
standard  of  what  we  call  white."  The  best  example  of  simultaneous  con- 
trast is  seen  in  the  colored  shadows,  especially  if  the  colors  are  not  too 
saturated.  If,  for  example,  a  sheet  of  white  paper  is  illuminated  at  the 
same  time  by  daylight  and  by  candle-light,  and  a  pencil  is  placed  on  the 
paper,  two  shadows  will  appear.  One  is  thrown  by  the  daylight,  but,  being 
illuminated  by  the  red-yellow  light  of  the  candle,  appears  red-yellow.  The 
other  is  thrown  by  the  candle  and  illuminated  by  the  white  daylight.  It 
appears  blue,  complementary  to  the  color  of  the  other  shadow.  That  this 
blue  is  produced  by  an  error  of  judgment  the  following  experiment  seems  to 
show.  If  one  looks  through  a  blackened  tube  on  a  spot  that  belongs  partly 
to  the  white  ground,  partly  to  the  shadow  produced  by  the  candle-light,  the 
second  part  appears  blue.  If  the  tube  is  now  moved  so  that  nothing  is 
seen  but  the  shadow  of  the  candle-light,  the  whole  visual  field  appears 
-blue,  and  remains  so  even  if  the  candle  is  removed  entirely.  The  blue  dis- 
appears only  if  now  the  tube  is  taken  from  the  eye.  This  explanation 
of  these  phenomena  of  pure  simultaneous  contrast  by  errors  of  judgment 
has  often  been  objected  to,  and  indeed  it  seems  to  be  insufficient,  though  the 
psychical  factor  cannot  be  denied.  It  is  this  class  of  subjective  colors 
that  has  given  such  prominence  to  a  new  theory  of  color-perception, 
that  of  Hering.  A  long  time  before  Hering,  Plateau  had  attempted  to 
explain  these  colors  on  the  assumption  that  the  light  on  one  part  of 
the  retina  produced  an  indirect  effect  on  the  adjacent  parts,  a  kind  of 
antagonistic  activity  of  the  retina  that  gave  rise  to  the  complementary 
colors.  But  though  on  such  a  theory  it  is  difficult  to  explain  how  the 
gray  circle  on  the  green  paper  appears  in  a  more  distinct  rose  red  when 
the  .green  color  is  made  more  indistinct  by  the  tissue-paper,  still  nothing 
hinders  us  from  assuming  such  a  spreading  activity  also  among  the 
Young-Helmholtz  color-sensitive  terminals  in  the  retina,  if  decisive  ex- 
periments oblige  us  to  take  such  a  stand.  That  would  in  no  way  affect 
the  Young-Helmholtz  theory,  but  would  only  oblige  us  to  accept  Plateau's 
principle. 

Lately,  Alfred  M.  Mayer1  has  studied  these  phenomena  of  pure  simulta- 
neous contrast  anew  very  carefully,  and  has  estimated  the  interval  between 
an  electric  flash  that  probably  lasted  less  than  one-millionth  of  a  second 
and  the  perception  of  the  complementary  contrast-colors  as  less  than  one- 
fifteenth  of  a  second.  Mayer  considers  this  time  as  too  short  for  errors  of 
judgment ;  but  this  is  certainly  not  correct,  as  there  are  many  instances 

1  American  Journal  of  Science,  vol.  xlvi.,  July,  1893. 


NORMAL  COLOR-PERCEPTION. 


613 


which  show  that  we  make  such  false  judgments  instantaneously,  as,  for 
instance,  Zollner's  line1  will  demonstrate  easily. 

And  now  we  have  to  describe  in  a  few  words  Bering's  theory  of  color- 
perception,  which  in  the  last  twenty  years  has  sprung  up  as  a  rival  theory 
to  that  of  Helmholtz.  This  theory,  as  Helmholtz  remarks,2  is  a  modifica- 
tion of  Young's.  It  assumes  the  existence  in  the  retina  of  three  visual 
substances, — a  whitish-black,  a  red-green,  and  a  yellow-blue  substance.  In 
each  pair  the  one  color  is  complementary  to  the  other,  so  that  the  red  must 
be  more  of  a  purple.  The  theory  further  assumes  that  the  different  colors 
affect  the  different  substances  differently.  Red  light,  for  example,  induces 
a  katabolic  change  (Dissimilirung)  in  the  red-green  substance,  giving  rise 
to  a  sensation  of  red ;  while  green  light  gives  rise  to  constructive  or  ana- 
bolic changes  (Assimilirung)  in  the  same  substance,  thereby  producing  the 
sensation  of  green.  In  the  same  way  katabolic  changes  in  the  yellow-blue 
substance  induced  by  yellow  rays  of  light  give  rise  to  the  sensation  of 
yellow,  while  anabolic  changes  in  the  same  substance  are  induced  by  blue, 
giving  the  sensation  of  blue.  The  white-black  substance,  however,  is 
affected  in  a  katabolic  way  by  all  the  colors  of  the  spectrum,  though  in  a 
different  degree,  producing  in  us  the  sensation  of  white,  whilst  its  anabolism 
gives  the  sensation  of  darkness.  If  now  red  and  green  light  together 
strike  the  retina,  the  anabolic  and  katabolic  changes  are  in  equilibrium  in 
the  red-green  substance,  and  no  color-sensation  is  produced,  except  that 
which  results  from  the  simultaneous  action  of  both  colors  on  the  white- 


Fio.  4. 


r  a 


black  substance,  by  which  we  see  white.  The  accompanying  diagram,  from 
Foster's  "  Text-Book  of  Physiology"  (sixth  edition,  part  iv.  p.  95),  may 
serve  to  illustrate  this  view  better. 

The  lines  R,  0,  Y,  G,  B,  V  indicate  the  position  on  the  spectrum  of 
red,  orange,  yellow,  green,  blue,  and  violet.     The  line  r.g,  which  includes  a 

1  Text-Book  of  Human  Physiology,  by  Landois  and  Stirling,  third  American  edition, 
p.-  843. 

2  Loc.  cit.  p.  376. 


614  NORMAL   COLOR-PERCEPTION. 

space  shaded  vertically,  is  intended  to  represent  the  effect  of  rays  of  different 
wave-lengths  on  the  red-green  visual  substance.  In  the  red,  orange,  and 
yellow,  up  to  the  line  F,  the  effect  is  katabolic,  one  of  dissimilation  (red 
sensation).  Y  marks  the  position  of  equilibrium ;  beyond  this  the  effect 
is  anabolic,  one  of  assimilation  (green  sensation).  The  line  y.b  similarly 
represents  the  behavior  of  the  yellow-blue  substance  shaded  horizontally, 
katabolic  (yellow)  up  to  G  and  anabolic  (blue)  beyond.  The  line  w  indi- 
cates the  white-black  substance,  unshaded,  katabolic  (sensation  of  white) 
along  the  whole  length  of  the  spectrum. 

The  main  feature  of  this  theory  consists  in  establishing  an  antagonism 
between  red  and  green  and  between  yellow  and  blue,  by  connecting  each 
sensation  with  opposite  states  in  the  metabolism.  On  page  595  attention 
has  been  called  to  this  point,  and  there  the  antagonism  of  this  theory  to 
the  best-established  facts  of  our  nerve-physiology  has  been  pointed  out. 
This  last  objection  is  also  admitted  by  Professor  Ebbinghaus  in  his  latest 
contribution  to  this  subject,  mentioned  before.  He  still  clings  to  the  three 
visual  substances,  but  he  allows  only  katabolic  changes  in  each  to  pro- 
duce the  corresponding  sensations  of  white,  red,  green,  yellow,  and  blue, 
although  by  this  means  he  loses  the  plausible  explanation  of  an  antago- 
nistic relation  between  the  two  colors  in  each  pair,  and  it  is  difficult,  if 
not  impossible,  to  understand  him  when  he  says  (page  236,  op.  ciY.),  "  Blue 
and  yellow  are  antagonistic  colors.  If  the  stimulation  of  each  stands 
in  a  certain  quantitative  ratio,  the  chromatic  character  of  the  sensation 
is  abolished."  For  Ebbinghaus  the  yellow-blue  substance  is  identical 
with  the  visual  purple,  which  he  supposes  to  exist  not  only  in  the  rods 
but  even  in  the  cones,  where  it  has  never  been  found,  an  absence  which 
he  explains  as  only  apparent  and  due  to  its  mixture  with  the  red-green 
substance  that  is  present  only  in  the  cones  and,  being  complementary  in 
color  to  the  other  color-substance,  makes  the  cones  appear  colorless.  It 
seems,  therefore,  as  if  Ebbinghaus,  providing  as  he  does  each  cone  with 
the  red-green  and  the  yellow-blue  substance,  makes  the  nerve  transmit 
different  impulses  according  as  the  cone  is  affected  by  red,  green,  yellow,  or 
blue  light.  But,  as  observed  before,  this  appears  very  questionable  in  the 
face  of  our  present  nerve-physiology,  so  that  Ebbinghaus's  theory  does  not 
seem  to  be  able  to  displace  the  Young-Helmholtz  theory.  It,  however, 
weakens  Hering's  theory  the  more,  as  it  again  calls  attention  to  the  vital 
physiological  difficulty  of  that  view.1 

Color-blindness,  by  Hering's  theory  as  well  as  by  Ebbinghaus's  modi- 
fication of  it,  is  produced  by  the  absence  of  the  red-green  visual  substance, 
giving  red-green  blindness,  or  by  the  rare  absence  of  the  yellow-blue  sub- 
stance, giving  rise  to  yellow-blue  blindness,  or  by  the  combined  absence  of 

1  The  reader  will  find  a  very  able  discussion  of  Ebbinghaus's  theory  of  color-vision  in 
Mind,  New  Series,  No.  9,  January,  1894,  by  Chr.  L.  Franklin,  who  shows  that  the  visual 
purple  cannot  be  employed  for  the  explanation  of  color-perception  in  the  manner  assumed 
by  Ebbinghaus. 


NORMAL   COLOR-PERCEPTION.  615 

the  red-green  and  yellow-blue  substances,  producing  total  color-blindness. 
The  difference  between  red-blindness  and  green-blindness  Hering  attributes 
to  a  different  coloration  of  the  media  of  the  eye,  partly  of  the  macula  lutea 
and  partly  of  the  crystalline  lens.  The  latter,  however,  occurs  only  in 
diseased  or  very  old  eyes,  whilst  the  coloring-matter  of  the  yellow  spot 
shows  its  undeniable  effect  mostly  on  greenish-blue  rays,  and  hardly  any 
on  those  rays  that  would  establish  the  difference  above  referred  to.1 

Of  course  it  does  not  lie  within  the  scope  of  this  paper  to  give  anything 
like  an  exhaustive  critical  consideration  of  Bering's  theory.  However  in 
the  future  the  intellectual  fight  may  be  decided,  whether  in  favor  of  Helm- 
holtz  or  of  Hering  or  of  some  new  theory,  at  present  the  modified  Young- 
Helmholtz  theory  seems  to  explain  best  all  the  phenomena,  and  it  has  there- 
fore been  accepted  as  the  clue  through  the  labyrinth  of  the  complex  facts 
of  color-perception. 

A  few  words  may  be  added  about  the  development  of  our  own  color- 
sense.  Although  the  efforts  to  prove  that  on  account  of  their  limited 
vocabulary  for  colors  the  ancients  had  an  imperfect  appreciation  of  colors 
have  been  shown  to  rest  on  invalid  grounds,  still  there  can  hardly  be  any 
doubt  that  our  color-sense  was  gradually  developed,  and  the  question  may 
be  asked,  How  did  these  three  primary  sensations  of  color  arise  ?  The  first 
perception  was  very  likely  that  of  simple  light  and  darkness,  which  is  all 
we  possess  even  now  at  the  most  peripheral  parts  of  the  retina.  Later  the 
retina  became  better  differentiated  to  distinguish  between  those  lights  that 
were  of  most  interest  to  the  life  of  our  prehistoric  ancestors  and  those  that 
were  most  powerful  to  affect  the  eye.  It  seems  that  blue  must  be  regarded 
as  the  most  primitive  sensation  of  color,  for  "  the  first  sensation  of  light  is 
what  answers  to  the  blue  sensation  when  it  is  strong  enough  to  give  the 
sensation  of  color."  As  Captain  Abney  says,2  "  It  appears  probable  that 
even  in  insect  life  this  violet  (blue)  sensation  is  predominant,  but  at  all 
events  existent.  Insects  whose  food  is  to  be  found  on  flowers  seek  it  in  the 
gloaming,  when  they  are  comparatively  safe  from  attack.  Professor  Huxley 
states  that  the  greatest  number  of  wild  flowers  are  certainly  not  red,  but 
more  or  less  of  a  blue  color.  This  means  that  the  insect  eye  has  to  distin- 
guish these  flowers  at  dusk  from  the  surrounding  leaves,  which  are  then  of 
a  dismal  gray ;  a  blue  flower  would  be  visible  to  us,  while  a  red  flower 
would  be  as  black  as  night.  That  the  insects  single  out  these  flowers  seems 
to  show  that  they  participate  in  the  same  order  of  visual  sensations."  Blue 
is  the  color  of  the  heavens,  where  at  a  much  later  period  man  placed  his 
gods.  As  such,  long  before  that  time,  this  blue  entered  his  eye  continually 
in  his  out-of-doors  life  when  the  sky  was  not  clouded,  and  even  now  it  is 
recognized  farthest  toward  the  periphery  of  our  retina. 

1  Helmholtz,  loc.  cit.  p.  383. 

3  The  Sensitiveness  of  the  Eye  to  Light  and  Color.  Lecture  delivered  at  the  Royal 
Institution  of  Great  Britain. 


616  NORMAL   COLOR-PERCEPTION. 

Next  to  blue,  probably  yellow  became  first  developed  in  man's  visual  sen- 
sation, so  that  all  lights  of  long  wave-lengths  (red,  yellow,  green)  appeared 
yellow,  while  those  of  short  waves  appeared  blue,  as  is  still  the  case  in 
certain  peripheral  parts  of  the  retina.  Later  the  yellow  sensation  became 
differentiated  into  a  green  and  a  red  one,  when  the  green  of  the  vegetation 
made  its  deep  impression  upon  his  eye  and  mind,  which  color,  according  to 
the  latest  investigations  of  Helmholtz,  seems  to  correspond  best  to  our  green 
sensation.  At  the  same  time  red  followed,  impressing  him  most  deeply 
from  the  beauty  of  the  rising  and  of  the  setting  sun,  and,  above  all,  "  from 
the  embers  of  the  hearth-fire,"  which  must  have  played  so  important  a  part 
in  prehistoric  times.  To  this,  as  remarked  by  Gould,1 — who  advocates,  how- 
ever, four  primary  sensations, — we  must  add  "  the  rdle  that  war  and  blood- 
shed, blood- sacrament  and  blood-rites,  have  acted  in  the  history  of  the  race 
from  man's  egress  out  of  animalism  and  progress  to  nineteenth-century 
militarism."  Perhaps  thus  we  may  explain  how  from  the  mere  light-per- 
ception our  three  primary  color-sensations  have  arisen,  by  means  of  which 
we  are  now  enabled  to  enjoy  the  marvellousness  of  our  whole  color- 
universe. 

1  The  Meaning  and  Method  of  Life,  by  G.  M.  Gould,  M.D.,  1893. 


PHOTO-CHEMISTRY  OF  THE  RETINA. 

BY  CARL  MAYS,  M.D., 
Assistant  in  the  Physiological  Laboratory  of  the  University  of  Heidelberg,  Germany. 

TBANSLATED  BY 

JAMES  A.  SPALDING,  A.M.,  M.D., 

Ophthalmic  Surgeon  to  the  Maine  Eye  and  Ear  Infirmary,  and  to  the  Maine  General 
Hospital,  Portland,  Maine,  U.S.A. 


THE  "  adequate"  irritations  which  impinge  upon  the  peripheral  terminal 
apparatus  of  our  organs  of  special  sense  vary  so  much  in  quality  and 
quantity  from  those  which  the  conducting  paths  of  this  apparatus  (the 
nerve  fibres)  are  able  to  excite,  that  they  must  first  be  transformed  into 
nervous  irritations  in  the  terminal  organs  in  order  to  attain  the  central 
organ  along  these  paths.  Nervous  irritations  are  of  a  mechanical,  chemical, 
thermic,  or  electrical  nature,  and  into  such  irritations  must  external  ones 
be  transformed  when  they  vary  in  quantity,  or  when  they  are  of  a  different 
quality,  such,  e.g.,  as  waves  of  sound  or  of  light.  For  example,  the 
chemical  processes  which  are  capable  of  exciting  the  olfactory  mucous 
membrane  must  be  altered,  at  least  in  quantity. 

The  first  observation  of  such  a  transformation,  objectively  visible,  was 
made  by  Hensen,1  who  recognized  that  the  aerial  vibrations  of  sound  cause 
vibration  of  the  fine  hairs  in  the  ears  of  the  mysis  in  the  same  way  as  a 
mechanical  irritation  could  produce  it.  In  the  other  higher  organ  of  sense, 
the  eye,  Holmgren2  first  demonstrated  the  transformation  of  light  into 
electrical  processes,  whilst  later  it  has  been  shown  in  the  same  organ  that  not 
only  electrical  but  other  phenomena  take  part  in  this  alteration.  Subse- 
quently Boll  made  the  far-reaching  discovery3  that  the  retina  of  most 
animals  appears  of  a  purplish  red,  and  that  during  life  this  color  disap- 
pears under  the  influence  of  light.  It  was  reserved  for  Kuehne,4  however, 
first  to  prove  that  this  process  is  really  chemical. 

Finally,  even  mechanical  alterations  due  to  the  action  of  light  have  been 
recognized  in  the  eye  ;  for  example,  those  first  seen  by  Boll 8  in  the  retinal 

1  Zeitsch.  f.  vergleich.  Zoologie,  xiii.  S.  319. 

2  Upsala  Fort  und  Lingar,  Band  i.,  January,  1866.  Ss.  184-198. 

3  Monatsber.  d.  Berliner  Akndemie,  November  12,  1876. 

4  Verhand.  d.  Naturhist.  Med.  Vereins  zu  Heidelberg,  January  5,  1877. 

5  Monatsber.  d.  Berliner  Akademie,  February  15,  1877. 

617 


618  PHOTO-CHEMISTRY   OF   THE    RETINA. 

epithelium,  and  later  correctly  explained  by  Kuehne.1  Moreover,  the 
alterations  in  the  form  of  the  cones  in  the  illuminated  and  non-illuminated 
retina  have  been  observed  and  explained  conjointly  by  Eugelmann  and 
Van  Genderen  Stort.  Whether  all  these  processes  have  their  direct  primal 
cause  in  light  itself,  or  are  of  a  secondary  nature,  is  not  known,  but  it  is 
positive  that  they  are  so  intimately  connected  with  one  another  that  those 
which  are  not  chemical  must  be  considered  in  this  chapter.  Moreover, 
our  knowledge  of  these  chemical  processes  demands  that  we  should  make  a 
thorough  study  of  the  chemical  structure  of  the  retina,  and  this  we  shall 
now  lay  out  in  detail,  so  far  as  it  is  known  to  date,  though  acknowledging 
that  the  results  of  such  a  study  are  still  far  from  perfection. 

THE   RETINA   AS  A  WHOLE. 

The  extreme  instability  of  the  retina  is  shown  by  its  rapid  disintegra- 
tion after  death.  It  yields  with  extraordinary  lack  of  resistance  to  decay  ; 
but  even  before  it  falls  a  victim  to  bacteria  it  exhibits  many  striking  alter- 
ations. In  life  it  is  smooth  and  transparent ;  in  death  it  becomes  wrinkled 
and  opaque.  The  opacification  begins  in  the  anterior  layers ;  dissolution 
of  the  external  layer,  of  the  rods  and  cones,  as  described  by  Max  Schultze, 
into  columns  of  delicate  rods  next  appears ;  later  the  internal  and  exter- 
nal membranes  separate  from  one  another,  and  the  latter  curves  over  into 
the  shape  of  a  shepherd's  crook,  or  even  curls  into  complete  circular  coils. 
The  satiny  sheen  described  by  Boll,2  after  the  disappearance  of  the  purple 
tint,  as  the  second  stage  of  the  alteration,  is  not,  -in  Kuehne's  opinion,  due 
to  death,  because  this  condition  can  be  produced  by  pressure,  and  can  occa- 
sionally be  made  to  disappear.  This  phenomenon  must  be  regarded  as 
simply  due  to  a  slanting  and  a  dislocation  of  the  rods.  Inasmuch  as  the 
destruction  of  tissues  in  general  is  closely  connected  with  the  cessation  of 
the  circulation  of  the  blood,  it  is  remarkable  to  find  so  few  blood-vessels  in 
the  retina.  In  the  inner  layer  there  is  nothing  but  a  capillary  net-work, 
which  does  not  pass  farther  forward  than  the  external  granular  layer.  The 
external  half,  therefore,  of  the  epithelial  layer  of  the  retina  is,  like  many 
peripheral  glands,  nourished  by  lymphatics,  the  material  for  which  is 
chiefly  furnished  by  the  vessels  of  the  choroid. 

On  account  of  the  strongly  alkaline  reaction  of  the  vitreous,  it  is  not 
easy  to  establish  the  reaction  of  the  retina.  At  Rollet's  suggestion,  Chodin 
made  a  series  of  careful  studies  to  decide  this  question,3  and  found  such  a 
reaction  to  exist  in  fresh  retinae,  as  well  as  in  transverse  sections  of  the 
optic  nerve,  it  being  acid  especially  when,  with  the  neatest  possible  re- 
moval of  the  vitreous,  the  retina,  at  first  reacting  alkaline,  was  crushed  and 

1  In  various  articles  which  will  be  specified  later  in  this  paper. 

2  Zur  Anatomie  u.  Physiol.  d.  Netzhaut,  Monatsbericht  d.  Berliner  Akademie,  No- 
vember 12,  1877. 

3  Ueber  d.  chem.  Keaction  d.  Netzhaut  und  d.  Sehnerven,  Sitzungsber.  d.  Wiener 
Akad.,  January  19, 1877. 


PHOTO-CHEMISTRY   OF   THE   RETINA.  619 

exposed  to  light.  The  reaction  in  the  retina  of  the  frog  is  less  constant, 
but  is  generally  inclined  to  be  acid.  The  retinre  of  animals  kept  in  the 
dark  for  a  long  time  previous  to  death  seem  to  react  with  less  acidity. 
Colin  asserts l  that  fresh  retinae  exhibit  an  acid  reaction  on  the  side  of  the 
rods  and  cones.  He  also  says  that  if  the  retina  is  kept  for  a  long  time,  aud 
especially  in  the  dark,  the  reaction  rapidly  becomes  alkaline.  Kuehne  has 
always  found2  the  retinae  of  frogs  remaining  in  the  dark  to  be  alkaline. 
He  states  that  this  is  so  even  after  careful  removal  of  the  vitreous  and 
crushing  under  light.  Therefore,  since  Gscheidlen3  has  always  found  the 
gray  substance  of  the  brain  and  spinal  cord  to  be  acid,  and  Chodin  the 
white  substance  in  dogs'  brains  the  same,  though  less  distinctly  marked, 
Kuehne  believes  that  the  acid  reaction  of  the  retina  must  be  referred  to  the 
conducting  portion  of  the  apparatus. 

C.  Schmidt  has  found  albumin  in  the  retina.4  Kuehne  attributes  the 
opacification  of  the  retina  after  death,  which  appears  in  frogs  that  are  placed 
at  a  temperature  of  45°  C.  (112°  F.),  to  a  substance  resembling  rnyosin. 
Colin  insists  on  the  presence  of  muscular  and  serous  albumin.  C.  Schmidt 
has  discovered  a  peculiar  substance  which  does  not  coincide  with  glutin, 
mucin,  or  so-called  chondrin.  Cohn  has  found  genuine  mucin,  but  has 
not  determined  the  presence  of  any  glutin.  C.  Schmidt  has  discovered  a 
body  which  crystallizes  with  platinum  chloride  and  smells  strongly  of 
trimethylamin,  the  latter  originating  from  cholin  as  a  product  of  the  de- 
composition of  lecithin. 

In  ultra-violet  light  the  gray  substance  of  the  retina  shows  a  pale 
whitish-blue  and  slightly  changeable  fluorescence.5 

THE   RETINAL   EPITHELIUM. 

The  cells  composing  the  retinal  epithelium  consist  of  a  summit,  a  base, 
and  a  process. 

The  base  is  to  be  regarded  as  a  disk,  which  is  perforated  with  cylin- 
drical channels  for  the  reception  of  the  external  members  of  the  retina,  the 
rods.  The  processes  vary  in  form,  and  may  extend  as  far  as  the  external 
limiting  membrane.  Schwalbe  has  described6  a  system. of  cementing  lines 
or  beams  which  exist  between  the  cells,  but  says  that  they  are  actually  opti- 
cal sections  of  a  firmer  portion  of  the  cells  that  are  composed  of  neuro- 
keratin,  which,  owing  to  the  manner  in  which  it  rests  upon  the  cells,  has 
been  called  the  hat.  Kuhnt 7  has  found  it  extending  almost  to  the  basis  of 

1  Zeitsch.  f.  Physiol.  Chemie,  Band  v.  S.  213. 

2  Untersuchungen    aus  dcm   Physiologischen    Institut   der  Universitat   Heidelberg 
(for  the  sake  of  brevity,  hereafter  H.  U.),  i.  S.  22. 

3  Arch.  f.  d.  Ges.  Physiologic,  viii.  S.  171. 

4  Communicated  by  Blessig.     De  Retinae,  Inaug.  Diss.,  Dorpat,  1856. 

5  Ewald  and  Kuehne,  H.  IT.,  i.  S.  317. 

6  Handbuch,  i.  S.  224. 

7  Monatsbl.  f.  Augenheilk.,  xv.  Jahrgang,  S.  72. 


620  PHOTO-CHEMISTRY   OF   THE    RETINA. 

the  cells,  where,  when  near  the  margin,  according  to  Angelucci,1  it  coalesces 
with  the  hats  of  the  adjoining  cells.  The  summits  of  the  cells  have  a 
slightly  striated  protoplasm,  while  a  majority  of  them  have  a  bright,  ellip- 
soidal, transverse  nucleus.  The  soft  protoplasm,  which  swells  a  little  in 
acids,  gradually  breaks  down  in  a  ten  per  cent,  solution  of  salt,  and  dis- 
solves with  the  greatest  ease  in  a  solution  of  biliary  acid  and  in  alkalies 
that  vary  from  one  to  five  per  cent.  The  brown  pigment 
consists  mostly  of  globules  and  granules,  and  extends  slightly 
upward  at  the  margin  of  the  summit.  A  short  time  after 
death  it  retracts  somewhat  at  the  margin,  so  that  the  borders 
of  the  cells  appear  broader. 

In  many  animals  the  summit  contains  a  remarkable 
amount  of  fat.  In  the  frog,  in  a  few  owls,  and  in  rabbits, 
especially  those  that  are  albinotic,  fat  is  always  present.  It 
has  not  yet  been  discovered  in  man,  cattle,  or  swine.  It 
Ceil  of  the  ret-  generally  appears  in  the  form  of  a  single  drop  which  is  as 
inai  epithelium  ]arge  as  the  nucleus  of  the  cell.  If  there  are  several  drops, 
myeioidin  gran-  a  number  of  small  ones  group  themselves  around  the  larger 
uies;  b,  nucleus;  ones>  Occasionally  segmentation  of  some  of  the  smaller 
globule  of  fat ;  d,  drops,  or  a  change  in  its  form,  or  an  arrangement  in  layers 
ucuehne  f™  ^  on  ^  sur^ace>  may  be  seen.  The  fat  is  almost  always  found 
in  the  anterior  portion  of  the  summit,  near  the  nucleus.  In 
frogs  it  varies  from  a  deep  golden  yellow  to  a  pale  lemon.  It  is  very 
brilliant  where  segmentation  has  occurred,  this  probably  not  resulting 
from  the  minuteness  of  the  drops  alone,  but  from  their  lack  of  pigment. 
A  few  species  of  owls  are  the  only  birds  in  which  fat  has  been  thus  far 
discovered  ;  in  some  it  is  colorless,  in  others  it  extends  from  yellowish  to 
orange.  The  finely  granular,  zigzag,  deeply  tinted  specimens  are  probably 
excreted  pigment  with  a  trace  of  fat.  The  gigantic  drops  of  the  rabbit's 
retina  are  but  slightly  pigmented.  As  this  epithelial  fat  remains  fluid  at 
low  temperatures,  it  probably  consists  chiefly  of  olein.  It  can  be  easily 
extracted  by  alcoholic  ether,  benzol,  or  sulphuretted  carbon.  Alkalies  and 
bile  do  not  affect  it.  Osmic  acid  quickly  tints  the  globules  a  deep  brown. 

The  yellow  pigment  of  the  fat  globules  in  frogs — the  lipochrin — is  ex- 
hibited thus.2  The  retinae  of  a  large  number  of  frogs  are  placed  in  alcohol, 
which  they  tint  very  slightly.  Ether  is  used  to  extract  the  pigment  fat. 
This  form  of  fat  cannot  be  distinguished  from  that  which  is  found  in  the 
folds  of  the  abdominal  cavity  of  frogs.  It  gives  the  same  spectrum  in 
ether  and  in  sulphuretted  carbon.  When  removed  into  a  hot  alcoholic  solu- 
tion with  caustic  soda,  it  yields  a  yellow  soap  from  which  ether,  except  for 
a  slight  contamination  with  soap,  extracts  the  pigment  almost  pure.  After 
evaporating  the  ether,  the  pigment  is  found  to  be  amorphous.  The  spec- 


1  Arch.  f.  Anat.  u.  Physiol.,  Phys.  Abth.,  1878,  S.  353. 

2  Kuehne  and  Ayres,  H.  U.,  i.  S.  341. 


PHOTO-CHEMISTRY   OF  THE   RETINA.  621 

truru  of  the  ethereal  solution  is  identical  with  that  of  frog's  fat,  whilst  that 
of  the  deep  orange  sulphuretted  solution  is  slightly  different.  If  the  latter 
is  evaporated  and  again  absorbed  by  ether,  the  genuine  ether  spectrum  can 
once  more  be  produced. 

In  solutions  of  iodine  and  iodide  of  potassium  and  a  trace  of  alcohol, 
lipochrin  becomes  greenish  to  bluish  green ;  in  nitric  acid  it  becomes  yellow- 
green  ;  in  concentrated  sulphuric .  acid  it  becomes  dark  violet  to  brown. 
If  it  be  kept  in  solutions  and  exposed  to  intense  sunlight  it  bleaches  in 
two  or  three  hours.  The  color  is  not  restored  in  darkness.  In  the  dark  it 
is  bleached  by  ozone. 

Although  lipochrin  exhibits  some  similar  reactions,  such  as  the  blue 
color  in  death  and  the  reactions  with  nitric  and  sulphuric  acids,  it  is  not 
the  same  as  lutein.1  It  differs  from  lutein  in  its  destructibility  by  light ;  it 
is  never  crystalline,  and  it  gives  a  different  spectrum.2  The  skin  of  frogs 
gives  off  a  pigment  to  ether,  especially  after  previous  dilution,  which  bears 
some  resemblance  to  lipochrin,  this  being  occasionally  mixed  with  another 
greenish  or  bluish  pigment. 

The  summits  of  the  epithelial  cells  contain,  in  addition  to  the  fat,  some 
colorless  masses,  shining  like  wax.  These  are  oval,  oblong,  zigzag,  semi- 
lunar,  or  cylindrical  in  shape.  Boll  probably  saw  them,3  but  erroneously 
regarded  them  as  colorless  fat.  Ewald  and  Kuehne  called  them  myeloidin 
granules,  and  differentiated  them  from  fat.  They  are  generally  situated 
directly  behind  the  nucleus,  and  are  often  so  numerous  that  they  fill  the 
entire  summit.  They  yield  somewhat  to  ether,  chloroform,  and  benzol, 
without  swelling  or  entirely  dissolving.  Under  osmic  acid  they  slowly 
become  a  pale  or  dirty  bluish  green.  They  swell  greatly  in  alcohol,  like 
the  myeloidin  of  the  rods,  and  gradually  decay.  They  are  readily  soluble 
in  from  one  to  five  per  cent,  of  bile.  They  have  been  discovered  in  frogs 
(not  invariably,  but  when  present  always  in  some  epithelial  cells),  in  a  few 
owls,  and  in  the  buzzard.  Light  has  no  influence  upon  their  number. 

The  base  and  the  processes  of  the  retinal  epithelium  are  filled  with  epi- 
thelial pigment.  The  pigment  is  absent  in  albinos.  The  cells  in  front  of 
the  tapetum  are  more  or  less  scantily  provided  with  pigment.  In  many 
animals  (beasts  of  prey  and  fishes)  the  pigment  is  replaced  by  larger  or 
smaller  iridescent  crystals.  The  pigment  is  always  brown,  even  in  thick 
sections.  For  that  reason  Kuehne  calls  it  fuscin.  It  looks  most  like 
choroidal  pigment.  It  is  darkest  in  frogs,  lighter  in  men  and  birds,  from 
a  chocolate  to  a  cinnamon  brown  ;  in  fishes  traces  of  reddish  or  purple  are 
seen ; 4  in  invertebrates  a  black  pigment  is  found ;  and  in  the  astacus 5  a 
fine,  yellow,  crystalline  pigment  can  be  recognized. 

1  Capraniva,  Arch.  f.  Anat.  u.  Physiol.,  Phys.  Abth.,  1877,  S.  285. 

2  Kuehne  and  Ayres,  loc.  eit. 

3  Archiv  f.  Anat.  u.  Physiol.,  Phys.  Abth.,  1877,  S.  1. 
*  K.  Wagner,  Lehrbuch  d.  Zoologie,  1843,  S.  250. 

5  W.  Szczawinska,  Arch,  di  Biol.  de  Gend. 


622  PHOTO-CHEMISTRY   OF   THE   RETINA. 

Whilst  choroidal  pigment  is  granular,  the  fuscin  of  the  retinal  epithelial 
cells  is  mostly  rod-shaped,  suggesting  a  form  of  crystallization.  Rosow  l 
has  attempted  to  fix  this  pigment  by  macerating  retinae  in  water  up  to 
putrefaction,  but  in  this  attempt  the  bacteria  gave  rise  to  a  most  dis- 
agreeable pollution. 

Kuehne2  exhibited  small  masses  of  fuscin  by  dissolving  the  retinal  epi- 
thelium in  five  per  cent,  bile  and  washing  it  with  water,  alcohol,  and  ether. 
At  Kuehne's  suggestion,  the  writer  has,  by  the  aid  of  trypsin  digestion,3 
been  able  to  show  larger  masses  of  choroidal  and  retinal  pigment  combined. 
These  were  obtained  from  the  eyes  of  hens.  The  eyes  were  placed  in  alco- 
hol, extracted  with  ether,  boiled  with  water,  and  then  subjected  to  a  whole 
day  of  trypsin  digestion.  The  masses  were  filtered  through  gauze,  washed 
with  alkali  to  dissolve  the  nuclein,  and  the  alkali  in  turn  was  washed  out 
with  water. 

Fuscin  is  not  easily  affected  by  chemicals.  Concentrated  alkalies  and 
acids  absorb  but  little  of  it,  even  after  prolonged  boiling.  Rosow  asserts 
that  it  dissolves  easily,  with  a  violet  tint,  after  treatment  with  nitric  acid 
in  alkalies,  from  which  by  neutralization  it  is  again  precipitated.  The 
writer  has  verified  this  ready  solubility  after  the  nitric  acid  treatment,  but 
the  solutions  were  yellow,  and  from  them  he  has  failed  to  get  a  pigment 
precipitate  by  acids.  Moreover,  he  has  found  that  the  solubility  in  alka- 
lies depends  on  the  amount  of  light  and  warmth,  or  even  of  light  alone. 
Solutions  made  in  this  manner  yielded  on  neutralization  a  precipitate  of 
pigment  in  brown  masses.  As  nitric  acid  produces  oxidation,  he  tried 
other  oxidizing  materials,  but  found  that  ozone,  e.g.,  had  no  influence  upon 
the  solubility.  If  the  solution  is  not  acid,  nothing  is  dissolved,  even  with 
warmth.  It  is  remarkable  that  this  substance,  which  acids  can  hardly 
touch,  should  be  so  sensitive  to  light,  as  discovered  by  Kuehne  and  later 
carefully  investigated  by  himself.  He  has  found  that  even  desiccated 
fuscin  shows  more  or  less  sensibility  to  light :  it  is  slight  in  frogs  and  hens, 
moderate  in  owls,  and  most  noticeable  in  fish  (Abramis  brama)*  Moist 
fuscin  grows  paler  in  the  presence  of  acids  than  when  dry,  this  being  par- 
ticularly so  in  alkaline  fluids.  Fuscin  discovered  in  these  fluids  is  gener- 
ally heavier  than  the  pigment  granules  that  are  suspended  in  the  same. 
The  latter  at  last  become  colorless,  but  maintain  their  form,  so  that  it  be- 
comes necessary  to  imagine  a  substratum  that  is  first  impregnated  with  the 
pigment.  If  oxygen  is  present,  even  the  moist  preparations  fail  to  bleach. 
On  the  contrary,  solutions  grow  paler  in  the  dark  when  treated  with  ozone, 
whilst  in  the  light  the  same  agent  rapidly  bleaches  them.  Pure  fuscin 
contains  nitrogen  and  iron.5 

1  Archiv  f.  Oph.,  ix.  3,  S.  63. 

J  H.  IL,  ii.  1,  S.  112. 

»H.  IT.,  iii.  S.  324. 

*  Kuehne  and  Sewall,  H.  U.,  iii.  3  and  4,  S.  237. 

5  Mays,  Archiv  f.  Ophth.,  xxxix.  3,  S.  89. 


PHOTO-CHEMISTRY   OF   THE   RETINA.  623 

THE   VISUAL   CELLS   (RODS   AND   CONES). 

Whenever  the  rods  and  cones  appear  together,  the  rods  alone  extend  to 
the  base  of  the  epithelial  cells.  When  the  cones  predominate,  they,  too, 
extend  partly  into  the  summit  free  of  fuscin.  In  the  retina  of  snakes, 
which  have  no  rods,  all  the  conal  processes  extend  beyond  the  posterior 
fuscin  zone.  The  same  may  occur  in  the  fovea  centralis  of  man  and  the 
ape. 

The  internal  members  have  a  transparent  protoplasm.  There  are 
nuclear  bodies  which  produce  marked  and  excessive  refraction  of  light. 
Particularly  is  this  so  with  those  ovoidal,  moon-shaped,  and  parabolic  bodies 
which  occur  in  the  lower  vertebrates  in  the  rods  and  in  the  higher  verte- 
brates in  the  cones.  Finally,  the  fibrous  baskets  that  were  discovered  by 
Max  Schultze  are  most  developed  in  the  cones  of  man.  All  these  struc- 
tures become  slightly  tinted  by  osmic  acid,  but  grow  dark  much  earlier 
and  to  a  greater  degree  than  the  protoplasm.  They  seem  partly  related  to 
fat  and  partly  to  myelin.  Many  reptiles  and  birds  have  actual  pigmented 
fat  globules.  Moreover,  the  inner  members  of  the  cones  in  many  birds  and 
reptiles  are  interspersed  with  minute  granules  of  rose-colored  or  yellowish 
pigment,  which  has  probably  been  contaminated  with  fat. 

The  transition  from  the  inner  to  the  outer  members  is  apparently  a  sud- 
den one,  yet,  according  to  Dreser,1  a  process  of  the  inner  member  seems  to 
extend  like  a  mesh-work  into  the  interior  of  the  outer  member  of  the  rods. 
The  latter  (cylinders)  have  a  cylindrical  or  ellipsoidal  section,  are  often 
hollow,  and  have  irregular  ends,  looking  as  if  they  had  been  gnawed  off. 
The  cones  have  ninepin-shaped  external  members,  with  gently  rounded 
tips.  These  external  members  are  soft,  pliant,  and  perishable.  They  are 
of  weak  refraction,  and  are  easily  separated  from  the  inner  members.  The 
transverse  striation  of  the  tips  is  somewhat  broader  after  death.  Osmic 
acid  tints  the  cylinders  darker  than  the  tips. 

The  cylinders  swell  in  a  half  per  cent,  solution  of  salt 2  in  the  same 
way  as  Rurnpf  discovered  that  the  axis-cylinders  of  the  nerve-fibres  do. 
They  have  a  separable  envelope,  which  reacts  like  neurokeratin5  and  has 
a  resistance  like  that  of  the  brain.  Owing  to  this  attribute,  they  may  be 
regarded  as  cellulose.  Neurokeratin,  however,  contains  nitrogen  and  sul- 
phur, and  these  remnants  of  rods  do  not  show  a  cellulose  reaction,  but  one 
of  xanthoproteinic  acid.  Plates  and  interstitial  tissue  cannot  be  differen- 
tiated chemically. 

The  pigmentation  by  osmic  acid,  first  observed  in  the  rods  by  Max 
Schultze  and  Rudenew,4  affects  the  myeloidi  n  of  the  rods.  Owing  to  its  rapid 
appearance,  it  reminds  one  of  that  of  the  nerve  medulla,  but  it  is  of  another 
shade.  With  osmic  acid,  fat  becomes  yellowish  brown  or  reddish  brown,  and 

1  Zeitsch.  f.  Biol  ,  xxii.  S.  23.  2  Dreser,  loc.  cit,  Ss.  33,  84. 

3<Kuehne,  loc.  cit.  *  Arch.  f.  Mic.  Anat.,  5.  S.  300. 


624  PHOTO-CHEMISTRY   OF   THE   EETINA. 

finally  black ;  nerve  medulla,  bluish  gray  or  bluish  black  ;  rod  cylinders, 
greenish  brown  or  greenish  black,  like  that  of  the  myeloidin  of  the  epithelial 
cells,  except  that  the  latter  are  much  brighter,  as  are  the  ends  of  many  of 
the  rods. 

Dreser l  believes  that  the  myeloidin  ought  to  be  identified  with  vitellin. 
It  is  just  as  difficult  to  extract  any  solutions  which  will  react  the  same  to 
osmic  acid  after  evaporation  from  myeloidin  as  from  nerve  medulla.  Al- 
though alcohol-ether  extracts  everything  from  them  which  osmic  acid  stains, 
yet,  like  fat,  they  react  brownish.  The  cylinders  of  frogs  are  rich  in  mye- 
loidin. The  tips,  many  rods  in  mammals,  and  the  granules  of  the  epithe- 
lium have  but  little  myeloidin. 

The  ether  in  which  retinae  have  been  treated  in  warmth  deposits  in 
small  amounts,  on  cooling,  a  substance  which  is  presumably  lecithin.  From 
warm  alcohol  a  substance  acting  like  cerebrin  is  obtained.  In  diluted  alco- 
hol the  cylinders  swell  enormously  and  roll  into  clumps.  Rapid  freezing 
followed  by  thawing  produces  the  same  results.  Most  surprising  of  all  is 
their  behavior  in  one  to  five  per  cent,  solutions  of  crystalline  bile.  Here 
the  cylinders  and  tips  of  all  the  visual  cells  of  all  animals  easily  dissolve, 
leaving  empty  delicately  walled  sheaths.  With  fresh  rods  the  solution  is 
so  sudden  that  they  look  like  bursting  rockets  filled  with  coins.  Since  gall 
dissolves  on  the  one  hand  albumin,  and  on  the  other  lecithin,  cerebrin,  etc., 
it  unites  the  properties  of  the  aqueous  and  the  alcoholic  solutions. 

PIGMENTS   OF   THE   VISUAL   CELLS. 

Hannover 2  was  the  first  to  describe  pigments  in  the  oil  globules  of  rep- 
tiles and  birds.  He  found  all  the  colors  of  the  spectrum,  and  purple,  in 
birds.  Many  reptiles  have  yellow  oil  globules.  Max  Schultze3  has  de- 
scribed red  ones  in  the  turtle.  Carmine-red  pigment  had  long  been  recog- 
nized in  the  eyes  of  many  invertebrates,  but  it  was  first  noted  by  Krohn,4 
in  1839,  as  the  beautiful  rose  color  of  the  rods  of  cephalopods.  Leydig 
found  rose  and  violet  pigments  in  insects ;  Max  Schultze  found  the  same  in 
crabs.  H.  Mueller5  first  saw  a  few  red  rods  in  the  frog.  Leydig  again6 
investigated  this  point  in  amphibia,  and  saw  with  the  naked  eye  a  brilliant 
sheen  in  the  fresh  retina  of  the  frog.  He  described  the  color  as  rose  red,  as 
Schultze  had  done.7  He  also  saw  it  in  the  rods  of  the  rat  and  the  duck. 
These  observations  remained  unnoticed  till  Boll 8  turned  universal  attention 
to  the  extensive  and  constant  occurrence  of  pigmented  rods.  He  also  dis- 

1  Loc.  cit. 

2  Arch.  f.  Anat.  u.  Phys.,  1842,  S.  320. 

3  Article  "  Retina"  in  Strieker's  Handbuch  d.  Mic.  Anat. 

4  Verhand.  d.  Leopold.  Carolin.  Akad.,  xix.  2,  S.  45. 
5Zeitsch.  f.  Wiss.  Zool.,  iii.  S.  234,  and  viii.  S.  71. 

6  Lehrb.  d.  Histologie,  1857,  S.  238. 

7  Archiv  f.  Mic.  Anat.,  ii.  199  and  208. 

8  Monatsb.  d.  Berliner  Akad.,  November  12,  1877,  and  Accad.  di  Lincei,  December  3, 
1877. 


PHOTO-CHEMISTRY  OF  THE   RETINA.  625 

covered  green  rods  in  the  retina  of  the  frog.  He  further  found  that  frogs 
kept  in  the  light  had  paler  retinae,  and  that  if  they  were  held  in  the  sun- 
light their  retinae  appeared  bleached.  He  asserted  that  the  color  in  the 
enucleated  retina  of  a  frog  persisted  scarcely  for  a  minute,  and  in  the  eye 
of  mammals  but  a  few  seconds,  after  death.  He  found  that  frogs  exposed 
to  direct  sunlight  had  the  retinal  color  restored  almost  instantaneously  after 
the  animal  was  removed  to  a  darkened  room.  Boll  suggested  that  the  «ye 
should  be  halved  quickly,  the  retina  removed  without  pressure  (which 
destroys  the  color),  and  examination  made  with  the  naked  eye  or  the  micro- 
scope. He  believed  that  in  mammals  the  conditions  should  be  examined 
with  the  ophthalmoscope.  He  ascribed  the  "  proper  color,"  the  "  purple 
color,"  to  the  stratified  substance  of  the  rods  and  cones  of  all  animals  (ex- 
cept the  cones  of  reptiles),  but  he  failed  to  express  any  opinion  as  to  whether 
this  tint  depended  upon  a  pigment  or  upon  the  phenomena  of  interference. 
Whilst  Boll  regarded  the  disappearance  of  the  pigment  after  death  as  a 
post-mortem  condition,  Kuehne  found  that  the  retina  in  a  weak  light  (from 
a  small  hole  in  a  shutter  covered  with  a  yellow  cloth)  preserved  its  color  for 
hours.  He  also  determined  that  in  daylight  the  tint  was  lost  at  once. 
This  test  succeeded  with  disintegrated,  crushed,  and  minced  retinae,  as  well 
as  with  those  that  had  been  treated  with  ammonia  or  had  been  hardened  in 
alum.  In  this  way  it  was  decided  that  the  bleaching  is  not  due  to  death, 
but  to  the  action  of  light,  and  that  the  color  is  not  due  to  structural  rela- 
tions, but  to  a  pigment  (the  visual  purple)  which  is  decomposed  by  light; 
thus  proving  that  the  whole  resulted  in  a  rapid  photo-chemical  disintegra- 
tion of  the  retina. 

THE   VISUAL   PURPLE  (RHODOPSIN). 

The  visual  purple l  can  be  seen  by  the  naked  eye  or  the  microscope, 
under  the  light  of  a  sodium  flame,  in  the  fresh  retinae  of  animals  which  have 
been  kept  for  an  hour  or  two  in  the  dark  or  in  a  moderate  light.  (Fig.  2.) 
In  frogs,  the  optic  nerve  can  be  snipped  with  scissors  and  the  eye  halved 
equatorial ly.  In  mammals,  the  papilla  in  the  halved  eye  is  struck  out  on 
a  lead  plate  with  a  punch,  in  a  half  per  cent,  solution  of  salt.  In  birds,  the 
fundus  is  prepared  by  two  cuts  with  scissors  running  nearly  parallel  to  the 
pecteu  and  meeting  at  its  central  terminus.  Human  eyes  kept  on  ice  for  a 
day  can  still  be  used.  If  the  retina  sticks  to  the  epithelium,  as  in  apes,  the 
eye  should  be  left  for  a  day  in  a  four  per  cent,  solution  of  potassic  alum. 

Visual  purple  occurs  in  almost  all  vertebrates,  but  only  in  the  rods. 
The  exceptions  are  one  bat  (Rhmolophus  hipposideros),  the  hen,  and  the 
pigeon,  although  Van  Genderen  Stort 2  asserts  that  he  has  discovered  it  in 
one  pigeon  (Columba  Lima),  whilst  Boll  has  seen  traces  of  it  in  all  pigeons. 
It  has  been  recognized  in  man  by  Fuchs  and  Walpouer,3  and  by  Schenk 

1  Compare  Fi£.  K. 

2  Archives  NY-derlandoises,  xxi.  p.  40.     (Reprint.) 

3  Wiener  Med.  Wochensch.,  1877,  S.  221. 
VOL.  I.— 40 


626  PHOTO-CHEMISTRY   OF   THE   RETINA. 

and  Zuckerkandl.1  In  late  years  Kuehne  and  Nettleship2  have  often  seen 
it.  Michel  and  Rosenthal,  on  the  contrary,  never  could  find  any  traces 
of  it.3  Animals  living  in  the  dark  possess  it,  and  it  has  been  discovered 
in  embryos.4  The  anterior  portion  of  the  retina  over  a  zone  of  three  or 
four  millimetres  wide  in  man,  apes,  and  other  animals  is  free  from  visual 
purple.  The  rods  of  the  macula  lutea  in  man  do  not  seem  to  possess  it. 
Rabbits,  oxen,  sheep,  and  full-grown  dogs  and  cats  exhibit  it  in  horizontal 
line  as  a  more  darkly  tinted  stripe.'  In  rabbits  it  is  distinctly  elevated, — 
the  visual  i^dge.  In  other  animals  it  is  flatter, — the  visual  girdle.  Owls, 

FIG.  2. 


Microscopic  view  of  the  frog's  retina  from  behind.— a,  purple  rods;  b,  green  rods.    (Kuehne.) 

most  fishes,  man,  and  sheep  have  a  purple  shading  to  violet,  which  may 
be  regarded  as  a  mixture  of  visual  red  with  visual  violet  or  blue.  This, 
however,  is  not  so  :  it  is  a  different  substance  chemically,  as  Kuehne 
and  Sewall  have  shown  in  the  fish.5 

In  order  to  exhibit  the  visual  purple,  solutions  of  from  one  to  five  per 
cent,  of  crystalline  bile,  free  from  ether  and  alcohol,  should  be  used.  The 
work  should  be  done  in  a  dark  room  that  is  lighted  with  sodium  flame.  The 
retinae  must  be  fresh,  especially  when  taken  from  warm-blooded  animals. 
According  to  Ayres,6  they  are  longer  utilizable  if  placed  in  a  ten  per  cent, 
salt  solution,  as  this  probably  retards  coagulation  of  the  myosin  bodies. 
The  filtrate  is  best  filled  with  rock  salt,  which  carries  off  the  suspended 
fuscin  and  albuminous  corpuscles.  A  clear,  beautiful  purple  solution, 

1  Allgem.  Wien.  Med.  Zeit.,  May  13,  1877. 

2  Journal  of  Physiology,  ii.  p.  38. 

3  Centralb.  f.  d.  Med.  Wiss.,  1877,  No.  24. 

4  Fuchs  and  Walponer,  loc.  cit. ;  Kuehne,  Hermann's  Handbuch  d.  Physiol..  iii.  S. 
236  ;  Albertini,  Centralb.  f.  d.  Med.  Wiss.,  1880,  No.  40. 

5  H.  U.,  iii.  3  and  4,  S.  269. 
«H.  U.,ii.4,  S.  444. 


PHOTO-CHEMISTRY   OP   THE   RETINA. 


627 


which  in  daylight  turns  red,  yellow,  and  finally  becomes  colorless,  is  thus 
obtained.  If  diluted  with  water  it  becomes  rose-colored;  if  it  is  more 
strongly  diluted  it  turns  lilac  :  tints  which  never  arise  from  a  pure  red.  If 
these  solutions  are  dialyzed  the  membrane  contains  a  deep  purple  myeliu- 
like  magma  which  is  as  sensitive  to  light  as  the  retina.1  Kuehne  has 
exhibited  pure  visual  purple  with  a  trace  of  neurokeratin  in  decayed  retinae 
by  subjecting  them  to  trypsin  digestion,  bile,  acetic  acid  of  five  per  cent, 
and  water,  and  by  purifying  the  remnants  with  sal  ammoniac  and  benzol. 

Boll  having  in  later  articles  described  the  normal  color  of  the  retina  as 
a  pure  red,  Kuehne  lays  great  stress  upon  spectral  analysis  of  the  visual 
purple.  The  examination  is  made  with  a  spectroscopic  apparatus  with 
double  hollow  prisms,2  one  of  which  is  filled  with  the  purple  solution  and 
the  other  with  colorless  bile  possessing  the  same  refractive  power.  In  this 
way  the  spectrum  of  the  visual  purple,  which  soon  passes  over  into  that 
of  the  visual  yellow,  can  be  rapidly  observed.  (Fig.  3.)  This  apparatus 

FIG.  3. 


a,  spectrum  of  the  visual  purple ;  b,  spectrum  of  the  visual  yellow ;  c,  solar  spectrum.    (Kuehne.) 

enables  the  visual  purple  to  be  examined  in  solutions  of  different  thickness. 
Thin  sections  exhibit  a  shading  into  yellow-green  before  the  E line  appears, 
whilst  thicker  ones  show  both  spectra  with  diffuse  absorption-lines  without 
striated  bands. 

Retinae  or  drops  of  the  purple  solution  can  be  examined  in  the  objective 
spectrum.  In  yellow-green  they  appear  black  up  to  the  beginning  of  pure 
violet.  In  violet  they  are  yellow  and  orange-gray.  In  red  the  retinae 
cauuot  be  distinguished  from  those  which  are  bleached,  nor  the  drops  of 
purple  solution  from  drops  of  water. 

Kuehne  has  discovered  that  when  the  image  of  a  fresh  retina  in  an 
eye  is  physiologically  blended  with  a  color  by  one  of  the  methods  suggested 
by  Helmholtz,  only  those  colors  which  are  complementary  to  purple  give 
sensation  of  neutral  gray.  He  has  also  found  that  the  retinal  color  in 
mixtures  of  spectral  colors  (partially  white  and  others)  behaves'like  purple, 
and  that  in  genuine  purple  (red  and  violet)  it  appears  extremely  bright ; 
in  the  pseudo-purple  (red  and  blue)  it  is  fire-red. 

Visual  purple  being  a  lac  dye,  but  little  of  it  can  be  seen  in  the  ivtina 
in  front  of  the  dark  pigment  of  the  fuudus  of  the  eye.  It  is  somewhat 

1  Ewald  and  Kuehne,  H.  U.,  i.  S.  454. 

2  Centralb.  f.  d.  Med.  Wiss.,  1877,  S.  194,  and  H.  U.,  i.  S.  60;  also  H.  U.,  i. 
139-166. 


628  PHOTO-CHEMISTRY   OF   THE   RETINA. 

better  seen  in  front  of  the  white  or  yellow  tapetum  of  the  dog  or  the  cat, 
and  can  be  remarkably  well  seen  in  fishes  with  a  retinal  tapetum  (Abramia 
brama),  or  even  through  the  pupil  of  the  fish  when  dying  in  the  dark.1  O. 
Becker2  has  shown,  in  opposition  to  Boll,  as  well  as  Dietl  and  Pleuck,* 
that  the  visual  purple  cannot  be  seen  in  the  living  human  eye. 

The  visual  purple  is  visible  under  the  microscope  wherever  the  cylin- 
ders stand  erect,  and  those  that  lie  obliquely  may  show  a  complementary 
green  against  the  mosaic  of  the  rest.  Recumbent  rods  appear  rose-colored 
when  thickly  clustered  (frog,  salamander).  In  tritons  the  position  for 
observation  is  more  favorable.  This  is  so  because  these  transition  struc- 
tures between  rods  and  cones  contain  the  purple  in  a  layer  of  connective 
tissue. 

PHOTO-CHEMICAL  DECOMPOSITION  OF   THE   VISUAL   PURPLE. 

When  light  acts  upon  the  visual  purple  (rhodopsin)  it  first  produces 
visual  yellow  (xanthopsin)  and  then  visual  white  (leukopsin).  The  colors 
which  appear  during  the  bleaching  vary  according  to  the  entrance  of 
the  visual  yellow.  In  daylight  the  retinal  color  passes  over  into  redder 
purple,  pure  red,  orange,  yellow,  and  "  chamois,"  before  it  entirely  disap- 
pears. It  may  pass  through  pale  lilac  to  total  loss  of  tint.  The  fresher  the 
retina  the  more  marked  is  the  latter  course.  The  action  of  light  is  direct. 
Protected  districts  remain  circumscribed  with  color.  The  bleaching  depends 
on  the  intensity  of  the  light.  No  secondary  effect  of  a  bleaching  once 
begun  has  yet  been  observed.  A  bleaching  that  might  not  be  noticeable  by 
itself  is  recognized  when  the  retina  in  our  own  eye  is  mixed  physiologically 
with  white.  Fresh  retinae  then  appear  rose-colored  ;  slightly  bleached  ones, 
"  chamois."  This  is  also  so  when  we  observe  it  in  spectrally  mixed  purple, 
in  which  case  fresh  retinse  appear  Flemish  purple  bleached  a  dull  tile  color 
(a  shading  off  between  carmine  and  cinnabar  red). 

Examination  of  the  action  of  the  purple  by  monochromatic  light  is 
made  with  the  aid  of  tinted  glasses,  or  of  the  colors  of  the  spectrum,  in 
which,  for  light  of  short  vibrations,  flexion-spectra  are  preferable.  All 
colored  light  affects  the  purple  more  or  less,  although  with  weaker  tints 
hours  and  even  days  are  needed  to  complete  the  bleaching.  Fresh  retinae 
extend  in  their  initial  portions  as  far  as  green  before  bleaching  towards 
yellow ;  in  their  final  portions  more  to  lilac,  this  being  so  because  visual 
yellow  is  more  rapidly  destroyed  in  these  portions  than  in  the  initial  por- 
tions. In  a  good  light,  the  retina  or  the  purple  solution  exhibits  great 
alteration.  In  yellow-green  the  alteration  appears  in  an  incredibly  brief 
period,  passing  in  greenish  yellow  to  indigo  in  from  two  to  ten  minutes, 
in  yellow  in  twenty  minutes,  in  violet  and  orange  in  thirty  minutes,  in 
ultra-violet  in  forty-five  minutes,  and  in  red  in  a  little  longer  period  of 

1  Kuehne  and  Sewall,  H.  U.,  iii.  3  and  4,  S.  263. 

2  Klin.  Monatsb.  f.  Augenheilk.,  xv.  Jahrgang,  S.  145,  1877. 
8  Centralb.  f.  d.  Med.  Wiss.,  1877,  273. 


PHOTO-CHEMISTRY  OF  THE   RETINA.  629 

time.  So  far  as  the  action  of  monochromatic  light  upon  visual  purple  or 
visual  yellow  is  concerned,  it  has  been  established  that  the  law  of  the  action 
of  time  coincides  with  the  amount  of  absorption,  and  that  light  decomposes 
both  of  these  pigments  the  more  it  is  absorbed  by  them.  An  exception  in 
blue  and  green  light,  which  is  obtained  by  absorption  with  colored  glasses, 
in  favor  of  blue,  may  depend  upon  differences  in  intensity  between  such 
light  and  spectral  light. 

As  refrangible  light  acts  with  greater  effect  upon  visual  yellow,  and 
less  refrangible  light  acts  more  rapidly  upon  visual  purple,  blended  light 
of  both  sorts  is  best  for  bleaching.  The  most  powerful  mixture,  how- 
ever, need  not  appear  white.  A  combination  of  cyanide  blue  and  greenish 
yellow  to  greenish  white  is  the  most  dangerous  for  the  retina.  On  the 
whole,  each  individual  color  of  such  mixture  acts  for  itself,  and  only  better 
so  far  as  total  bleaching  of  the  purple  is  concerned  when  it  is  united  with 
the  others.  On  account  of  the  greater  intensity  of  mingled  light,  the  result 
of  the  union  is  best  confirmed  by  successive  mixtures. 

CHEMICAL   ACTION   OF   THE   VISUAL   PURPLE. 

Visual  purple  does  not  seem  to  contain  iron.1  Many  chemical  agents 
bleach  it.  This  is  so  even  in  the  dark,  and  when  so  bleached  it  cannot  be 
restored.  It  is  destroyed  by  lime  water  and  baryta  water,  alkalies,  and 
most  acids.  (According  to  Van  Geuderen  Stort,  the  retina  of  the  Perca 
fluviatilis  remains  red  after  treatment  with  thirty-five  per  cent,  nitric  acid.) 
It  is  further  destroyed  by  alcohol,  chloroform,  chloral  hydrate,  acetone, 
aldehyde,  acetic  ether,  oil  of  mustard,  thymol,  oil  of  bitter  almond,  oil  of 
turpentine,  solutions  of  soap,  subchloride  salts,  chlorine,  sulphuric  acid, 
iodine,  and  bromine.  Some  of  these  act  rapidly,  others  slowly  ;  a  few  even 
when  greatly  diluted.  In  many,  the  purple  passes  through  yellow  to  bleach- 
ing. Absolute  glycerin  acts  so  far  only  as  to  make  the  purple  of  dry 
retinae  yellow.  Visual  purple  remains  unaltered  in  sal  ammoniac,  carbon- 
ates, alkalies,  salt  solutions  of  every  intensity,  alum,  cyanide  of  potassium, 
sulphuric,  subchloride,  and  nitrogenous  alkalies,  sulphate  of  ammonium, 
sulphuretted  hydrogen,  ammoniacals,  acetates  containing  solutions  of  oxide 
of  tin,  chloride  of  iron,  acetate  of  lead,  superoxide  of  hydrogen,  ozone,  car- 
bonic acid,  carbonic  oxide,  boric  acid,  hydrocyanic  acid,  aqueous  glyconin, 
absolutely  pure  ether,  benzol,  petroleum  ether,  carbonic  dichloride  and 
tetrachloride,  sulphuret  of  carbon,  the  fats  and  balsams,  oleic  acid,  oil  of 
bergamot,  santoninic  acid  and  soda  santonate,  and,  finally,  urea.  Trypsin 
digestion  does  not  attack  the  purple  in  the  retina,  but  only  in  a  cholate 
solution.2  On  the  whole,  with  the  exception  of  the  oxidates  of  hyperosmic 
acid  and  permanganate  of  potassium,3  powerful  oxidizing  and  reducing  sub- 
stances have  but  little  effect  upon  the  visual  purple. 

.    !  Ewald  and  Kuehne,  H.  UM  i.  S.  488. 

2  C.  W.  Ayres,  H.  U.,  ii.  S.  446. 

3  Dreser,  Zeitsch.  f.  Biologic,  xxii.  8.  23. 


630  PHOTO-CHEMISTRY   OF   THE    RETINA. 

The  visual  purple  is  also  destroyed  by  the  influence  of  higher  tempera- 
tures. Retinae  are  instantaneously  bleached  at  76°  C.  (168.8°  F.) ;  solu- 
tions of  purple  at  72°  C.  (161°  F.).  The  lowest  temperature  that  alters  the 
purple  is  51°  C.  (123.8°  F.).  The  addition  of  sal  ammoniac  and  carbonate 
of  sodium  reduces  this  a  little  lower.  The  fact  that  the  purple  of  retinae 
dry  or  softened  in  concentrated  glycerin  or  saturated  salt  solutions  is  more 
slowly  destroyed  by  increased  temperature  is  suggestive  of  that  of  the 
coagulation  of  albumin.  Rabbit's  purple  is  earlier  destroyed  than  that  of 
the  frog,  suggesting  a  chemical  difference  between  the  purple  of  various 
animals. 

Temperature  exerts  some  influence  upon  the  rapidity  of  bleaching  by 
light.  Frozen  retinas  bleach  very  slowly.  In  the  frog,  the  sensibility  to 
light  increases  slowly  from  0°  C.  (32°  F.)  up  to  40°  C.  (104°  F.),  though 
very  noticeably  from  45°  C.  (113°  F.).  Inasmuch  as  the  solution  of  visual 
purple  obtained  from  rabbits  is  much  more  sensitive  to  light  at  35°  C.  to 
38°  C.  (95°  F.  to  100°  F.)  than  at  75°  C.  (167°  F.),  it  is  right  to  presume 
from  this  that  the  purple  of  living  warm-blooded  animals  is  very  sensitive 
to  light. 

The  chemical  influences  upon  bleaching  by  light  are  retarded  by  the 
withdrawal  of  water,  without  being  stopped.  Oxidizing  and  reducing 
materials  exert  no  influence  upon  this  process.  Bleaching  seems  to  be  a 
destruction  with  loss  of  water.  Retinae  in  acetic  acid  in  the  light  exhibit 
a  greater  tendency  to  yellow  than  when  they  are  placed  in  carbonate  of 
sodium.  Sal  ammoniac  does  not  seem  to  exert  any  influence  upon  bleach- 
ing by  light.1  It  further  seems  that  in  light  of  short  vibrations  the  visual 
yellow  is  more  easily  produced  than  it  is  in  acetic  acid,  whilst  in  less  re- 
frangible light  its  destruction  is  delayed. 

Under  certain  circumstances  the  indolence  of  the  visual  pigment  is  a 
peculiar  phenomenon.  Eyes  from  corpses  that  have  been  kept  in  the  dark 
are  often  insensible  to  light,  many  hours  being  lost  before  the  visual  yellow 
is  totally  bleached.  As  this  is  generally  the  case  with  most  animals  kept 
in  dark  stables,  it  is  curious  that  visual  yellow  is  not  oftener  seen.  Melloini 
alone2  has  seen  it,  and  upon  this  fact  he  has  based  a  special  theory  of  light. 
The  longer  retinae,  especially  those  that  have  been  separated  from  the  epi- 
thelium, have  lain  in  the  dark,  the  more  pronounced  is  the  phenomenon  : 
the  same  occurs  after  drying.  Desiccated  purple  solution  being  but 
slightly  indolent,  it  seems  as  if  it  were  a  question  of  the  fixation  of  the 
purple  in  the  rods  (perhaps  of  neurokeratin).  Aluminized  retinae,  if  for 
a  long  time  dried  in  the  dark  over  sulphuric  acid,  become  yellow  when 
remoistened,  but  the  yellow  is  not  completely  bleached  even  by  the  direct 
sunlight.  Retinae  that  have  become  yellow  with  acetic  acid  do  not,  if  they 
have  long  been  kept  in  darkness,  lose  their  color  for  days.  According  to 

1  Compare  Kuehne's  remark,  H.  U.,  ii.  S.  446. 

*  Comptes-Rendus,  xiv.  p.  823,  and  Ann.  de  Physik,  Ivi.  p.  574. 


PHOTO-CHEMISTRY   OP  THE    RETINA.  631 

Puglia,1  various  acids  (acetic,  sulphuric,  and  nitric)  produce  a  more  constant 
yellow.  In  solutions  of  sublimate  the  retina  seems  to  assume  a  bright 
yellow  which  is  unalterable  by  light.  Under  certain  circumstances  thawing 
furthers  this  indolence. 

THE  RELATIONS  OF  THE  FLUORESCENCE  OF  THE  RODS  TO  THE  VISUAL 

PURPLE. 

The  whitish-green  fluorescence  of  the  retina  discovered  by  Helmholtz 2 
and  further  investigated  by  Setschenow  s  originates  from  the  layer  of  rods,4 
and  is  connected  with  the  visual  purple  and  its  bleaching.  The  whitish- 
blue  fluorescence  of  the  anterior  layers  of  the  retina  is  independent  of  the 
action  of  light.  In  man  and  animals,  fresh  retinae  fluoresce  pale  blue,  and 
bleached  retinae  fluoresce  greenish.  Rods  without  purple  in  the  region  of 
the  ora  serrata  show  scarcely  any  fluorescence.  Cones  without  purple  act 
in  the  same  way.  The  fovea  centralis  in  ultra-violet  appears  as  an  in- 
creasingly darker  spot  the  more  the  surrounding  rods  fluoresce.  A  retina 
deprived  in  life  of  its  purple  fluoresces  greenish  white  posteriorly.  It  is 
difficult  to  say  whether  the  visual  purple  itself  fluoresces,  since  cholate 
solutions  also  fluoresce  bluish.  Purple  dissolved  in  cholate-acid  alkalies 
appears  pale  blue  in  ultra-violet.  After  bleaching  by  light  it  becomes  a 
bright  greenish  blue. 

When  the  yellow  disappears,  or  when  it  has  hardly  become  yellow,  or 
when  visual  white  has  been  formed  with  small  clumps  of  purple,  the  layer 
of  rods  is  most  fluorescent  (very  green).  Visual  white  is  evidently  a 
greenish  fluorescent  substance.  Visible  yellow  does  not  possess  this  prop- 
erty, or  at  least  possesses  it  to  but  a  slight  degree.  Chloride  of  zinc  in- 
creases fluorescence,  tinting  the  retina  yellow  in  the  dark  and  then  depriving 
it  of  its  fluorescence.  If  the  retina  is  now  illuminated  till  it  bleaches,  the 
fluorescence  reaches  its  maximum.  Acetic  acid  acts  in  the  same  manner. 

Fluorescence  ceases  on  moistening  with  alcohol  or  caustic  alkalies.  If 
the  alcohol  is  applied  when  the  rods  are  bleached  by  light,  the  fluorescence 
is  retained.  Alcohol  does  not  seem  to  alter  the  visual  white,  but  from  visual 
purple  it  appears  to  produce  different  alterations  that  are  connected  with 
discoloration  from  those  that  are  produced  by  light. 

The  green  rods  discovered  by  Boll  were  found  by  Ewald  and  Kuehne 
in  the  turtle-dove.  They  are  identical  with  the  rods  provided  with  short 
external  processes  that  have  been  described  by  Schwalbe.  They  are  more 
slowly  bleached  by  light  than  the  purple  ones,  but  at  last,  as  far  as  light 
extends,  this  occurs  totally.  It  is  not  yet  known  whether  they  are  pig- 
mented  or  not, 

1  Sulla  Porporo  visuale,  Annal.  d.  Ottal.,  vii.  668,  and  Gaz.  Med.  de  Paris,  July  J2, 
1879. 

2  Ann.  d.  Physik.,  xciv. 
»  Arch.  f.  Oph.,  v.  2. 

*  Ewald  and  Kuehne,  H.  U.,  i.  S.  169. 


632  PHOTO-CHEMISTRY   OF   THE    RETINA. 


PIGMENTS   OF   THE   CONES. 

The  outer  members  of  the  cones  are  never  tinted ;  pigment  has  been 
discovered  only  in  the  inner  members  in  birds,  reptiles,  and  a  few  fish.1  In 
man  and  the  ape  the  fovea  centralis  is  never  pigmented.2  Horner's  obser- 
vation 3  of  an  evanescent  cherry-red  spot  in  the  fresh  eye  cannot  refer  to 
coloration  of  the  fovea  itself,  but  must  depend  upon  transmitted  light. 
The  yellow  pigment  of  the  macula,  which  in  Max  Schultze's  opinion  lies 
in  front  of  the  visual  cells,  disappears  after  a  few  days'  exposure  to  the 
sun. 

The  oil  globules  discovered  by  Hannover 4  on  the  border  between  the 
inner  and  outer  layers  of  the  cones  have  been  carefully  studied  only  in  hens 
and  pigeons.  They  are  purple  to  red,  orange-yellow,  and  greenish-yellow. 
Occasionally  they  are  pure  green  and  blue.  They  consist  of  fat,  with  pig- 
ment in  solution.  Kuehne  and  Ayres 5  have  isolated  three  pigments  from 
the  oil  globules, — chlorophane,  xanthophane,  and  rhodophane.  Kuehne 
thinks  that  there  is  a  fourth, — kyanophane.6  These  investigators  exhibited 
the  pigments  by  saponification  in  alcohol,  the  next  best  identification  being 
based  upon  the  exclusive  solubility  of  chlorophane  in  petroleum  ether, 
with  a  superfluity  of  alkali,  and  the  insolubility  of  rhodophane  in  alcohol 
in  the  presence  of  acids  or  ammoniac.  These  pigments,  when  pure,  are 
soluble  in  ether,  petroleum  ether,  chloroform,  sulphuretted  carbon,  and  fatly 
oils ;  also  in  alcohol,  with  the  exception  of  rhodophane,  which  is  soluble 
there  only  in  the  presence  of  acids  and  ammoniac.  Rhodophane  is  soluble 
in  acetic  ether.  All  the  pigments  are  insoluble  in  water,  alkalies,  and  am- 
moniac. Chlorophane  is  greenish  yellow,  soluble  in  alcohol  and  in  ether, 
giving  them  the  same  tint ;  in  sulphuretted  carbon  it  is  orange-yellow. 
When  evaporated  it  is  soluble  in  alcohol  and  in  ether,  with  the  same  tint  as 
before.  Xanthophane  dissolves  in  alcohol  and  in  ether,  leaving  an  orange- 
yellow  tinge.  In  sulphuretted  carbon  it  becomes  reddish  orange.  Rhodo- 
phane dissolves  red  in  chloroform  j  in  sulphuretted  carbon  it  becomes  violet. 
The  three  pigments  give  characteristic  spectra.  (Fig.  4.) 

Walchlis's 7  doctrine,  that  chromophane  is  a  post-mortem  product,  and 
Capranica's,8  that  the  three  pigments  are  identical,  have  been  refuted  by 
Kuehne. 

1  Leydig,  Lehrb.  d.  Hist. ;  also  in  the  Bombinator  igneus. 

5  Kuehne,  Centralb.  f.  d.  Med.  Wiss.,  April  24,  1877,  S.  109,  and  H.  U.,  i.  Ss.  34, 
105,  109,  and  H.  U.,  ii.  Ss.  69,  89,  378;  later  confirmed  by  Bonders,  Klinisch.  Monatsbl. 
f.  Augenheilkunde,  xv.  Jahrgang,  S.  156. 

s  Klinisch.  Monatsbl.  f.  Augenheilk.,  xv.  Jahrgang,  S.  156. 

•  Archiv  f,  Anat.  u.  Physiol.,  1840,  S.  320;  1843,  S.  314. 

•  H.  U.,  ii.  S.  341. 

•  H.  TL,  iv.  S.  246. 

7  Graefe's  Archiv  f.  Augenheilk.,  xxvii.  2,  S.  303;  xxii.  Jaar.  Vers.  b.  h.  Nederl. 
Gasth.  v.  Ooglijders,  S.  153;  Bonders,  Utr.  Phys.  O.  Lab.,  vi.  S.  297. 

8  Loc.  cit. 


PHOTO-CHEMISTRY   OP   THE    RETINA. 


833 


According  to  Capranica  (modified  by  Kuehne),  the  oil  globules  and 
chromophanea  become  blue  in  nitric  acid  and  concentrated  sulphuric  arid. 
In  nitric  acid  containing  nitrous  acid  they  become  a  bluish  green.  Rh,,d,,- 
phane  is  affected  least  of  all.  The  greenish-blue  to  blue  reaction  with 
iodine,  discovered  by  Schwalbe  in  the  summits  of  the  cones,  is  not  Men, 
according  to  Kuehne,  in  pure  pigments.  The  chromophanes  are  not  entirely 
immutable  in  the  dark.  They  are  most  stable  in  the  presence  of  alkali,  "s 
and  after  removal  of  their  genuine  vehicle,  the  fat.  In  solutions  of  chlo- 
roform they  are  most  sensitive  to  light,  and  least  so  in  sulphuretted  carbon 
and  when  slightly  acidified. 

It  happened  to  the  author  to  discover  that  pigeons'  retinae,  when  enclosed 


FIG.  4. 


s,  solar  spectrum;  a,  spectrum  of  chlorophane  in  ether  or  petroleum  ether;  6,  spectrum  of  cliloro- 
phane  in  bisulphide  of  carbon ;  c,  spectrum  of  xanthophane  in  ether;  d,  spectrum  of  xanthophane  in 
bisulphide  of  carbon;  e,  spectrum  of  rhodophane  in  benzole;  /,  spectrum  of  rhodophane  in  oil  of 
turpentine.  (Kuehne.) 

in  carbonic  acid,  bleach  more  slowly  than  in  air;1  but  this  may  be  an  action 
of  acid.  Ozone  bleaches  the  chromophanes  in  the  dark, — chlorophane  most 
rapidly,  rhodophane  the  slowest. 

The  pigments,  which  so  frequently  occur  among  invertebrates  that  they 
are  regarded  as  rudimentary  eyes,  are,  according  to  Kuehne2  (partly  in 
opposition  to  Chopin's  views3)  and  Kru  ken  berg,4  but  slightly  sensitive  to 
light. 


1  H.  U.,  loc.  cit. 

3  Comptes-Rendus,  xxv.  p.  447. 


'  H.  U.,  i. 

«  H.  U.,  ii.  S.  68. 


634  PHOTO-CHEMISTRY   OF   THE   RETINA. 

ALTERATIONS   IN   THE   RETINA    DURING   VISION. 

Frogs  held  for  ten  to  fifteen  minutes  towards  full  sunlight,  or  rabbits 
with  enlarged  pupils  kept  for  a  time  in  the  light,  show  colorless  retinae. 
Bleaching  of  the  visual  purple  is,  therefore,  an  effect  of  light,  but  in  its 
course  significant  variations  arise  from  the  condition  of  things  in  isolated 
retinas. 

The  best  proof  of  the  direct  local  action  of  light  is  that  the  images 
thrown  by  the  refracting  media  on  the  fundus,  when  well  defined,  leave 
well-defined  designs  in  the  purple  layer  of  the  rods, — photographs,  as  it 
were,  or  optograms.     (Fig.  5.)     Optograms  can  be 
FIG.  5.  obtained  in  the  eyes  of  fresh  rabbits  or  cattle  by 

placing  them  in  a  black  box  twenty-five  centimetres 
a  high,  covered  with  a  ground-glass  plate  provided  with 

6  black  stripes  four  or  five  centimetres  wide  and  the 

c  same  distance  apart.     The  experiments  are  made  in 

the  open  air.    After  an  exposure  of  from  two  to  seven 
Rabbit's    retina   con-    minutes,  according  to  the  intensity  of  the  light,  the 
retinae  are  prepared  before  a  sodium  flame,  laid  over- 


lary  nerve-fibres;  6,  the     night  in  a  four  per  cent,  solution  of  alum,  and  then 

visual  ridge;  c,  optogram.  ,  nil  i    •  I 

(Kuehne.)  shelled  out  over  small  glazed  porcelain  cups,  where 

they  can  be  at  once  examined  by  daylight  or  after 

drying.  Where  many  experiments  are  to  be  made,  it  is  best  to  have  a 
darkened  room  with  a  movable  shutter. 

There  are  many  difficulties  in  the  way  of  exhibiting  optograms  in  frogs' 
eyes,  the  alum  treatment  being  useless.  In  order  to  remove  the  retina  un- 
injured, the  optic  insertion  has  to  be  excised.  Another  difficulty  lies  in  the 
pigment  epithelium  clinging  to  the  exposed  locality  (epithelial  optogram), 
so  that  either  the  image  must  be  made  minute  to  keep  the  epithelia  in 
continuity,  or  a  long  exposure  in  very  moderate  light  must  be  obtained. 
The  best  plan  is  to  make  the  frogs  oedematous  by  letting  them,  when 
curarized,  remain  for  some  time  in  water.  Pseudo-optograms  occur  in  the 
frog  when  the  rods  strip  off  from  the  illuminated  spot  and  only  the  cones 
remain,  thus  producing  white  designs.  Genuine  optograms  are  character- 
ized by  the  exhibition  of  bleached  rods  under  the  microscope.  By  follow- 
ing these  suggestions,  optograms  which  under  one  hundred  diameters  are 
scarcely  diffuse  may  be  obtained. 

Optograms  obtained  in  rabbits,  with  stripes  five  centimetres  in  breadth, 
and  at  a  distance  of  twenty-five  centimetres,  are  one  and  a  half  millimetres 
wide.  In  frogs  with  an  object  distance  of  fifteen  centimetres,  they  are  six- 
tenths  of  a  millimetre  wide. 

The  course  of  the  photo-chemical  process  is  best  followed  when  it  is 
arranged  that  the  optograms  in  rabbits  shall  fall  partly  upon  the  tinted 
visual  stripe  and  partly  on  both  sides  of  it,  as  the  various  stages  of  the 
bleaching  can  then  be  compared.  Furthermore,  the  images  ought  to  fall 


PHOTO-CHEMISTRY  OF   THE   RETINA.  635 

upon  that  portion  of  the  retina  which  lies  below  the  horizon,  because  it  is 
better  tinted  and  does  not  contain  medullary  nerve-fibres. 

The  changes  of  color  in  the  living  retina  run  through  the  same  intervals 
as  in  the  isolated  retina, — pure  red,  tile-red,  orange,  rose,  chamois,  yellow. 
Care  in  the  time  of  the  exposure  produces  colorless  stripes  with  purple 
cross-bands  of  equal  width  (as  in  the  object).  If  the  eyes  are  exposed  for 
too  short  a  time,  pure  red  stripes  on  a  purple  ground  are  obtained.  If  they 
are  exposed  too  long,  colorless  stripes  on  a  pure  red  ground  (for  the  opto- 
grams  occur  precisely  on  the  visual  ridge  at  the  stage  of  exact  exposure) 
are  obtained.  Later,  colorless  stripes  and  yellowish  ones,  which  always 
grow  narrower,  are  seen. 

Kuehne  found  that  the  action  of  monochromatic  light  upon  the  purple, 
in  life,  varied  in  many  respects  from  what  Boll  had  asserted.  Its  intensity, 
for  instance,  is  less  than  that  of  white  light.  Well-isolated  spectral  colors 
or  colored  glasses  or  solutions  whose  tints  must  be  verified  by  the  spectral 
apparatus  are  employed.  Whilst  Boll  had  at  first  believed  that  no  mono- 
chromatic light  could  totally  bleach  the  retina,  he  later  granted  it  for  light 
with  short  vibrations,  assuming  various  stages,  which  remind  one  of  the 
shades  of  light  employed  in  bleaching.  So  far  as  yellow  was  concerned,  he 
conceded  that  it  contained  pure  red,  which  he  regarded  as  the  normal  color 
of  the  retina.  He  asserted  that  common  red  deepened  it  to  a  brownish- 
purple.  Ewald  and  Kuehne,  on  the  contrary,  discovered  that  all  colors 
discolored  the  retina,  though  with  different  degrees  of  rapidity.  Red,  for 
example,  in  frogs,  in  the  height  of  summer  discolored  in  two  hours,  yellow- 
green  in  half  an  hour  or  more.  This  latter,  however,  is  difficult  to  recog- 
nize, on  account  of  the  obstinate  adherence  of  the  pigment  epithelium. 
Even  in  frogs  with  curare  oedema,  pigment  existed  between  the  rods,  and 
produced  what  Boll  had  regarded  as  its  characteristic  discolorations. 

Finally,  Kuehne  and  Ewald  have  advanced  the  following  theories : 

1.  Photo-chemical  decomposition  in  the  isolated  as  well  as  in  the  living 
retina  originates  a  single  pigmented  product,  the  visual  yellow,  the  pro- 
portion of  which  to  the  still  undecomposed  purple  decides  the  retinal  color 
before  the  bleaching  by  light  is  terminated. 

2.  Where  the  visual  yellow  is  decomposed  as  rapidly  as,  or  even  more 
so  than,  the  visual  purple  (in  light  of  short  vibrations),  the  retina  becomes 
rose  or  lilac.     Where  the  opposite  occurs  (in  light  of  long  vibrations),  the 
retina  becomes  in  turn  red,  orange,  chamois,  and  yellow. 

Having  discovered  the  influence  of  oxygen  upon  the  bleaching  of  the 
chromophane,  and  the  oxidation  in  the  living  body  proceeding  with  an  in- 
tensity that  under  equal  chances  could  not  be  chemically  imitated,  the  author 
felt  the  need  of  investigating  how  this  substance,  which  is  slightly  sensi- 
tive to  light,  behaved  during  life.  Kuehne  has  shown l  that  after  removing 
the  cornea  and  lens  in  birds  the  pupil  can  be  kept  open  by  a  speculum 

1  H.  U.,  ii.  S.  89. 


636  PHOTO-CHEMISTRY   OF   THE   RETINA. 

without  loss  of  vitreous,  and  the  eyes  illuminated  for  hours  with  a  heliostat. 
In  pigeons,  the  result  is  entirely  different  from  what  was  expected.  The 
colors  of  the  tips  of  the  cones  are  deepened,  and  some  become  brighter. 
The  yellowish-green  inclines  to  green,  the  red  to  rose  and  ruby  red.  The 
xanthophane  tips  do  not  change  at  all.  The  inner  members  of  the  cones 
with  chlorophaue  tips  often  contain  deposits  of  a  diffuse,  finely  granular, 
yellowish-green  pigment,  which  is  never  seen  in  animals  that  have  been 
kept  in  the  dark.  In  two  examples  of  Butea  vulgaris,  one  of  which  was 
kept  in  the  dark,  whilst  the  other  remained  in  the  light,  Kuehne  found  in 
the  former  the  tips  of  the  cones  colorless,  red,  orange,  and  yellowish  green 
of  moderate  saturation.  In  the  latter,  none  were  colorless,  and,  except  a 
few  of  deep  orange  and  dark  red,  none  were  greenish  yellow  and  only  a  few 
were  bluish  green.  All  these  discoveries  impress  us  with  the  idea  that  chro- 
mophane  is  a  neoplastic  formation  that  is  due  to  the  action  of  light.  At 
most,  the  cone-tips  may  have  a  yellow  pigment  which  yields  slowly  to 
light. 

The  oil  globules  of  the  epithelium  containing  lipochrin  exhibit  in  the 
frog,  after  exposure  to  bright  light,  a  peculiar  segmentation  into  smaller  and 
brighter  globules  which  is  chemically  different  from  the  myeloidin  gran- 
ules. The  latter  vary  so  much  in  frequency  that  it  is  difficult  to  decide  how 
they  act  under  illumination,  but  their  appearance  and  disappearance  seem 
to  be  connected  with  processes  which  ensue  upon  illumination  of  various 
length  and  intensity.  The  peculiar  appearance  of  striated  and  bristly  con- 
tents of  the  epithelial  summits  of  eyes  exposed  to  the  light  is  probably  due 
to  bleached  fuscin. 

REGENERATIVE   PROCESSES. 

All  the  rods  that  are  bleached  by  vision  resume  their  maximal  color 
when  the  eyes  have  been  kept  sufficiently  long  in  darkness.  The  purple 
from  frogs,  which  had  in  life  been  totally  bleached,  is  completely  regener- 
ated in  the  course  of  three  hours  in  complete  darkness,  no  matter  whether 
the  eye  remains  in  the  living  creature  or  is  enucleated.  In  either  case,  the 
restoration  passes  through  pale  lilac  and  rose  to  purple.  Even  the  halves  of 
frogs'  eyes  from  which  the  vitreous  has  been  removed  are  partially  regener- 
ated. The  regeneration  is  produced  by  the  retinal  epithelium ;  for  when 
two  living  frogs  are  bleached,  one  of  which  is  in  a  state  of  curare  oedema, 
so  that  the  retina  peels  off  without  the  epithelium,  and  the  other  animal  is 
not  curarized,  so  that  the  epithelium  clings  to  the  retina,  the  purple  is 
restored  in  the  second  retina,  but  not  in  the  first.  In  this  regeneration  lies 
the  reason  for  the  difference  between  the  time  required  for  bleaching  the 
living  retina,  or  the  retina  remaining  in  situ  after  life,  and  that  required  for 
bleaching  the  isolated  retina.  It  also  accounts  for  the  apparent  indolence 
of  the  purple  in  life,  which  is  greater  in  the  frog  than  it  is  in  warm-blooded 
animals.  This  epithelial  function  can  be  well  shown  by  elevating  the  retina 
one-half  (without  its  epithelium)  and  then  exposing  the  eye  to  the  light ; 


PHOTO-CHEMISTRY   OF   THE   RETINA.  637 

the  elevated  half  after  removal  of  the  retina  is  colorless,  the  other  half 
remains  red  or  yellow.  The  swifter  bleaching  of  the  isolated  retina  is  not 
due  to  cadaveric  processes,  but  is  dependent  upon  the  absence  of  regener- 
ation. The  epithelium,  however,  must  be  alive,  or  in  a  state  of  existence 
to  produce  regeneration ;  for  if  a  frog's  eye  be  destroyed  in  the  dark  in 
water  at  a  temperature  of  45°  C.  (113°  F.)  the  bleaching  will  be  as  rapid  as 
in  an  isolated  retina ;  or  if  the  frog  be  dazzled  whilst  alive  to  the  point  of 
bleaching  the  retina,  and  the  eye  destroyed  in  a  similar  manner,  regeneration 
will  be  absent.  It  is  not  necessary  to  regeneration  that  the  retina  should 
remain  in  its  natural  connection  with  the  epithelium,  for  it  may  be  half 
detached,  or  detached  to  the  zone  of  Zinn,  or  it  may  hang  down  like  a  sac 
filled  with  vitreous,  or  be  removed  in  toto  and  bleached  and  again  restored 
to  its  place  whilst  regeneration  still  remains  perfect.  In  the  first  case,  both 
halves  of  the  retina  will  exhibit  the  normal  tinge  of  purple  without  any 
line  of  demarcation.  When,  however,  the  rods  are  bleached  in  life,  the 
purple  is  not  regenerated  in  this  manner. 

It  is  difficult  to  be  convinced  of  any  traces  of  regeneration  in  warm- 
blooded animals,  in  which  the  epithelium  rapidly  dies.  By  working 
quickly,  however,  it  can  be  seen  that  the  same  bleaching-time  as  in  life 
produces  at  once,  and  for  a  few  minutes  after  death,  optograms  that  are 
successively  more  and  more  developed.  This  is  so  because  even  in  life, 
and  for  a  short  period  after  life,  regeneration  retards  the  bleaching  process. 
So  in  albinotic  rabbits  in  which  the  folded  fundus  of  the  eye  can  still  be 
seen,  the  visual  purple,  and  the  coloring  matter  of  the  blood,  pieces  of  retina 
removed  directly  after  enucleation  bleach  much  more  rapidly  than  those 
that  remain  protected  by  regeneration  in  the  eye.  This  distinction  dis- 
appears after  ten  minutes  in  the  dark. 

It  is  worth  noticing  that  regeneration  in  living  animals  occupies  a 
very  long  time.  Coccius  has  observed l  a  remarkable  pallor  of  the  retina 
in  rabbits  which  had  remained  for  a  long  time  in  the  open  air  and  then  a 
half-hour  in  darkness.  Ewald  and  Kuehne  have  established,  by  means 
of  the  optographic  method,  that  regeneration  in  the  frog  begins  after 
twenty  minutes  and  is  completed  in  from  one  to  two  hours  (longer  still 
if  the  temperature  is  low).  In  rabbits  the  same  steps  require  but  seven 
minutes  and  thirty  minutes  respectively. 

These  variations  in  regeneration  or  rhodogenesis  have  recently  been  ex- 
plained. Retina?  that  have  been  bleached  to  loss  of  all  color,  or  even  longer 
in  life,  do  not  show  visual  yellow,  but  exhibit  lilac,  rose,  and  purple  in 
any  stage  of  regeneration.  In  life,  the  products  of  bleaching  are  swept 
away.  That  this  is  so  is  suggested  by  the  circumstance  that  retinae  bleached 
in  life  do  not  fluoresce  whitish  green,  and  the  purple  must  be  formed  anew, 
which  happens  in  the  sequence  of  colors  previously  mentioned.  This 
process  has  been  called  neogenesis.  Furthermore,  and  particularly  in  iso- 

1  Acad.  Program.,  Leipzig,  1877. 


638  PHOTO-CHEMISTRY   OF   THE    RETINA. 

lated,  bleached  retinae,  and  in  life,  with  even  a  weak  illumination,  another 
regeneration  of  the  bleaching  products  that  have  not  been  swept  away 
occurs  in  the  usual  succession  of  colors, — yellow,  chamois,  orange,  red, 
purple.  This  is  called  anagenesis.  Both  processes  are  recognizable  by 
optograms  according  as  they  are  produced  by  longer  or  by  shorter  illumina- 
tion. Neogenesis  is  slow,  anagenesis  is  more  rapid.  In  this  way  the  dif- 
ferences in  regeneration  can  be  explained.  The  occurrence  of  rapid  ana- 
genesis in  life  also  is  proved  by  experiments  with  intermittent  light,  which 
give  either  no  optograms  at  all,  or  imperfect  ones.  This  will  occur  if  in 
the  pauses  of  about  three-quarters  of  a  second  between  the  individual  irri- 
tations of  light  the  purple  can  be  again  restored  from  its  bleaching  products. 

There  is  still  another  regeneration  which  is  independent  of  the  life  of 
the  cells.  This  is  known  as  auto-regeneration.  Attention  was  first  called 
to  it  by  the  fact  that  the  dependent  portions  of  retinae  free  from  fuscin 
bleached  more  slowly,  as  if  they  were  preserved  from  decomposition  by 
something  that  was  flowing  from  above  upon  the  purple.  In  the  dark, 
every  isolated  bleached  retina  shows  a  certain  return  of  color,  which  process 
can  be  repeated  several  times.  The  retina  may  be  actually  dead,  and  yet 
the  auto-regeneration  still  persist.  Even  bleached-out  solutions  of  purple 
show  a  slight  but  undeniable  return  of  color,  and  an  artificial  rhodogene- 
sis  can  also  be  discovered.1  Inasmuch  as  solutions  of  retinae  bleached 
during  life  and  separated  from  the  epithelium  by  curare  oedema  do  not  color 
in  the  dark,  a  rhodophylactic  property  must  be  attributed  to  the  epithe- 
lium. It  would  be  also  necessary  to  conclude  that  it  contained  a  substance 
called  rhodophane,  if  the  regeneration  of  solutions  of  purple  could  be  in- 
creased by  solutions  of  epithelium.  This,  however,  would  meet  witli  tech- 
nical difficulties,  since  epithelial  solutions  can  hardly  be  obtained  without 
haemoglobin.  In  addition,  together  with  blood-pigment  they  also  exhibit 
a  purple  the  bleaching  and  regeneration  of  which  can  be  easily  followed. 
In  fact,  it  seems  as  if  the  neogenetic  purple  were  here  formed.  Neverthe- 
less, in  frogs  kept  on  ice  in  the  dark,  we  succeed  in  obtaining  from  the 
retina  and  epithelium,  with  bile,  extracts  of  purple  that  are  free  from 
haemoglobin.  When  these  are  compared  with  extracts  obtained  from  the 
retina  alone,  a  slower  bleaching  and  a  more  rapid  and  intense  return  of 
the  color  in  the  dark  are  found  in  the  former.  There  must,  therefore,  be 
a  rhodophyllin  which  is  soluble  in  bile. 

The  slight  exudation  of  material  attributed  to  the  epithelial  cells  may 
explain  the  slowness  of  neogenesis,  whilst  the  illumination  itself  seems  to 
delay  the  process,  because  the  illuminated  and  naked  epithelium  has  less 
capacity  than  before  of  recoloring  the  rods.  So  long  as  the  light  reaches 
the  epithelium  through  the  purple  rods,  the  latter  cause  has  less  foundation, 
since  moderated  red  light,  as  shown  by  experiments  on  rabbits  and  frogs, 
does  less  harm  to  the  regenerative  process.  This,  however,  is  the  case  only 

1  Ewald  and  Kuehne,  Centralblatt  f.  d.  Med.  Wiss.,  1877,  S.  753 ;  H.  U.,  i.  S.  248. 


PHOTO-CHEMLSTItY    OF   THE    RETINA.  639 

after  illumination  of  the  rods  in  which  the  neogenesis  first  begins  in  its 
full  extent. 

Regeneration  in  warm-blooded  animals  seems  to  cease  either  with  the  cir- 
culation of  the  blood,  or  a  little  later.  It  is  retarded  by  pressure  upon  the 
eyeball,  by  excessive  loss  of  blood,  and  by  violent  electrical  shocks  which 
contract  the  blood-vessels.  It  is  not  influenced  by  stimulating  or  paralytic 
conditions  in  the  nerves  of  the  eye.  Holmgren l  has  demonstrated  the  visual 
purple  in  rabbits,  and  Langendorf 2  the  same  in  frogs,  in  all  of  which  the 
optic  nerve  had  long  before  been  severed.  Rabbits  prepared  by  Holmgren's 
method  are  said  to  exhibit  normal  bleaching  and  regeneration  for  a  long 
time.3  Division  of  the  trigeminus  or  of  the  cervical  sympathetic  or  of  the 
oculo-motorius  in  the  skull  has  no  effect  on  regeneration ;  nor  is  there  any 
difference  after  large  doses  of  curare  or  atropine.  Irritation  of  the  second 
eye  by  light  or  illumination,  of  adjacent  regions  fails  to  show  any  change 
in  the  normal  course  of  the  process.  On  the  contrary,  small  doses  of  mus- 
carin  and  pilocarpine,  which  increase  glandular  activity  in  the  dog,  rabbit, 
and  frog,4  hasten  regeneration,  so  that  the  epithelial  cells  must  possess  secre- 
tory activity. 

THE    IMPORTANCE   OF   THE   PHOTO-CHEMICAL   PROCESS    FOR   VISION. 

A  number  of  photo-chemical  processes  in  the  eye,  a  part  of  which  are 
very  rapid  and  a  part  rather  slow,  have  now  been  described.  This  knowl- 
edge has  led  to  the  establishment  of  a  photo-chemical  hypothesis  which  is 
intended  to  explain  the  actual  course  of  these  processes.  The  hypothesis 
assumes  the  existence  of  photo-chemical,  decomposable,  visual  matters s  in 
the  visual  cells,  which  do  not  cause  excitation  so  long  as  they  remain  un- 
decomposed.  The  moment  they  are  decomposed  by  the  action  of  light 
they  give  off  products  which  are  to  be  regarded  as  exciters  of  vision. 
When  the  secondary  action  of  light  is  considered,  a  material  view  of  the 
excitatory  matter  seems  to  be  the  preferable  one.  In  accordance  with  this, 
the  irritation  is  supposed  to  be  produced  by  transformatory  processes,* 
which  cease  when  the  light  is  removed. 

Any  matter  to  be  rightly  called  visual  matter  must  lie  in  a  situation 
where  there  is  reason  to  place  the  excitation  by  light, — i.e.,  in  the  visual 
cells.  It  must  also  be  proved  that  this  matter  can  be  altered  by  light 
during  life,  and  that  a  slight  transformation  may  be  of  great  significance. 
Nevertheless,  there  is  no  compulsion  to  regard  any  matter  with  such  pecu- 
liarities as  visual  matter ;  and,  in  fact,  even  visual  purple  may  be  nothing 

!H.  U.,  ii.  S.  81. 

2  Archiv  f.  Anat.  u.  Phys.,  Phys.  Abth.,  1877,  S.  437. 

3  Ayres,  H.  U.,  ii.  S.  215. 

*  Ayres,  loc.  cit  ;  Dreser,  loc.  cit.,  S.  30. 

5  This  name  was  first  used  by  Exner,  Arch.  f.  d.  Gesammt.  Physiol.,  xvi.  S.  409. 

6  Bernstein,  Unters.  u.  d.  Erregungsvorgange  in  Nerv-  und  Muskelsystem,  Heidel- 
berg, 1871. 


640  PHOTO-CHEMISTRY   OF   THE    RETINA. 

more  than  a  mere  absorber  of  light.  From  this  point  of  view,  it  is  im- 
portant to  know  that  vision  can  take  place  without  visual  purple.  More- 
over, visual  purple  does  not  exist  in  the  cones  in  the  fovea  centralis  with 
which  we  see  best.  Further,  animals  that  have  been  deprived  of  the  purple 
do  not  act  differently  from  those  which  possess  it.  Frogs  without  the 
purple  search  for  their  favorite  color,  green ;  and  rabbits,  which  seem  to 
have  no  cones  or  visual  cells  with  or  without  purple,  see  well  with  yel- 
lowish rods.  Yet  it  is  not  justifiable  to  deny  that  purple  may  be  a 
visual  pigment  simply  because  it  is  impossible  to  tell  whether  the  vision 
of  animals  with  bleached-out  retinae  is  modified  in  any  particular  way. 
Personal  observations  would  help  to  an  opinion  in  this  respect  if  it 
could  be  decided  when  our  own  visual  purple  (which,  from  Kuehne's 
experiments,  is  very  resistant  in  life)  had  disappeared.  Unfortunately, 
this  cannot  be  demonstrated  with  the  ophthalmoscope,  and  the  possibility 
of  perceiving  the  purple  entoptically,  which  was  unknowingly  done  by 
Tait,1  later  suspected  by  Boll,2  and  more  accurately  described  by  Ewald,3 
is  of  but  little  utility,  because  the  experiment  is  successful  only  under 
special  conditions.  Vision  without  the  purple,  and  mostly  without  retinal 
pigment,  renders  it  certain  that  this  visual  matter  is  only  a  paradigm  for 
other  untinted  matters  which  must  act  in  a  similar  manner, — i.e.,  as  exciters 
of  vision  by  the  decomposition  of  light.  The  great  difference  between  the 
intensity  of  the  irritation  in  the  visual  organ  must  lead  us  to  suppose  that 
the  sensibility  to  light  of  these  untinted  matters  varies. 

Exner  believed 4  that  he  discovered  in  his  own  eye  the  presence  of  visual 
matter,  which  disappeared  after  stopping  the  circulation  by  pressure  on  the 
organ.  When  Kuehne  objected 5  that  this  phenomenon  might  just  as  well 
be  due  to  alterations  in  the  conducting  apparatus,  especially  of  the  gray 
substance  of  the  retina,  Exner  replied 6  that  there  was  no  proof  in  favor 
of  either  view,  and  endeavored  by  new  experiments  to  support  his  former 
hypothesis. 

MECHANICAL   ALTERATIONS   IN   THE    RETINA    PRODUCED    BY   LIGHT. 

1.  The  Pigment  Epithelium. — The  sliding  off  of  the  retina  in  illumi- 
nated frogs'  eyes,  first  seen  by  Exner,  during  decomposition,  or  even  when 
covered  with  epithelium,  does  not  depend  on  changes  in  consistence  or 
softening,  as  Boll  thought,  but  upon  the  mobility  of  the  protoplasm  of  the 
retinal  epithelial  cells  and  their  prolongations,  as  suggested  by  Czerny.7 
Kuehne  had  found  that  it  depended  on  migration  and  shifting  in  the 

1  Edinburgh  Proceedings,  1869-70,  vii.  p.  605. 

2  Arch.  f.  Anat.  u.  Physiol.,  Phys.  Abth.,  1877,  S.  4. 
8  H.  U.,  ii.  S.  241. 

*  Pflueger's  Archiv,  xvi.  S.  407. 
6  H.  U.,  ii.  S.  46. 

6  Pflueger's  Archiv,  xx.  S.  614. 

7  Sitzungsber.  d.  Wiener  Akad.,  Ivi. 


PHOTO-CHEMISTRY   OF  THE   RETINA. 


641 


Stratification  of  the  crystals  of  fuscin,1  in  connection  with  and  depending 
upon  the  amount  of  the  illumination.  He  later2  studied  the  phenomenon 
more  closely,  and  Angelucci  has  also  offered  some  opinions  concerning  it.1 
Illuminated  retinae  remain  slightly  covered  with  epithelium,  whil.-t  tli..-. 
without  pigment  and  kept  in  the  dark  slide  off. 

There  are  other  circumstances  which  must  be  taken  into  consideration 
so  far  as  the  adhesion  of  the  epithelium  and  the  sulnlivision  of  the  pigment 
are  concerned.  For  example,  a  quarter  or  half  an  hour  after  death  a  firmer 
adhesion  is  developed,  during  which  the  pigment  wanders  forward  in  small 
portions  as  far  as  the  external  limiting  membrane.  This  is  develo|x>d,  even 
in  frogs  that  have  been  kept  in  the  dark,  by  lower  temperature  without 
migration  of  pigment,  and  is  even  stronger  than  curare  oedema.  The 
latter  loosens  the  epithelium  at  30°  C.  (86°  F.),  but  makes  the  separate 
fuscin-needles  project  considerably.  The  prolongations  of  the  cells  are 
easily  torn  from  their  roots.  (Fxlema  also  produces  loosening  in  fi>ln- 
with  bleached  rods  when  exposed  to  the  sun,  in  which  case  only  small 
masses  of  pigment  remain  between  the  rods.  Fick  has  called  attention  to 
the  influence  of  suffocation,  to  which  the  writer  will  later  return.  These 

FIG.  6. 


Shifting  of  the  fuscin  in  the  frog's  retina.-^,  in  the  dark ;  B,  after  the  action  of  sunlight.    (Kuehne.) 

conditions  must  be  kept  in  mind  when  we  are  observing  the  migration  of 
fuscin  under  the  influence  of  light,     The  retina  of  frogs  that   have  Iron 
ke,,t   in  the  dark  loosen  easily  and  completely  from  the  epithelium,  I 
the  fuscin  projects  between  the  rods  to  one- third  or  one-half  of  their 

height.  . 

The  epithelial  cells  exhibit  short,  dark,  ninepiu-shaped  processes  win 
terminate  in  long,  fine  fibres  without  pigment  and  without  bulging  :.t 
end      The  base  of  the  cells  is  filled  with  pigment,  which  extends 
anterior  margin  of  the  nucleus,  but  on  the  summits  rises  into  a  slight  cleva- 

i  H.  U.,  i- Ss.  21,  101. 

3  Attid'.  H.^d.  d.  Lined,  3  Scric,  28, 1877-78;  see  also  Arch.  f.  Anat.  u.  Phyriol., 
Physiol.  Abth.,  1878,  S.  352. 
VOL.  I.— 41 


642  PHOTO-CHEMISTRY   OF   THE   RETINA. 

tion.  The  summits  of  the  rods  in  retinae  in  which  the  epithelium  has  not 
slid  off  (cooling  off)  have  but  little  fuscin. 

The  epithelial  layer  is  firmly  adherent  in  illuminated  frogs  ;  thick  ropes 
and  spindle-shaped  masses  of  pigment  rise  up  among  the  rods.  The  base 
of  the  cells  has  lost  much  fuscin,  but  the  pigment  covers  the  summits  of 
the  rods  more  extensively.  In  this  way,  with  pigment  in  the  base  and 
spindle-shaped  projecting  clumps  between  the  rods,  we  see,  as  it  were,  a 
double  zone  of  pigment.  Swelling  of  the  pigment-cells  processes  and  of 
the  rods  makes  the  epithelium  adhere.  After  an  hour  or  two  in  the  dark, 
we  discover  the  maximal  position  for  darkness, — that  is  to  say,  in  about 
the  same  time  that  it  is  necessary  for  neogenesis  of  the  purple.  Moderate 
light  enables  us  to  see  that  migration  of  the  pigment  begins  before  the 
purple  is  bleached,  whilst  all  illuminations  which  make  any  permanent 
demands  upon  the  regeneration  and  are  most  favorable  to  equilibrium 
between  the  bleaching  and  the  restoration  of  the  purple  act  most  forcibly 
upon  the  epithelium.  The  direct  relation  between  the  illumination  and 
the  migration  of  pigment  is  best  shown  by  the  epithelial  optograms  seen 
in  frogs  after  a  brief  but  brilliant  illumination.  The  migration  of  the 
fuscin  is  most  marked  in  red  light,  especially  in  such  intensities  as  leave 
the  fuscin  normal,  whilst  migration  decreases  considerably  when  permanent 
bleaching  is  attained.  The  actual  migration  of  the  fuscin  into  the  proto- 
plasm of  the  cells  is  best  seen  in  fish l  in  which  there  is  a  tapetum,  first 
described  by  Bruecke,2  in  a  certain  position  of  the  retina.  It  is  composed 
of  a  deposit  of  guauin  in  the  retinal  epithelial  cells.  Guanin  does  not  mi- 
grate under  the  influence  of  light,  but  is  always  seen  in  the  same  permanent 
form  of  cells,  whilst  the  fuscin,  mostly  granular  in  these  cells,  migrates  in 
the  usual  manner.  The  reason  that  guanin  does  not  migrate  may  be  that 
it  is  situated  in  a  firmer  portion  of  the  protoplasm. 

Epithelial  reaction  is  probably  common  to  all  vertebrates,  and  also  to 
man,  but  it  is  not  always  easy  to  observe.  Similar  processes  have  been 
seen  in  invertebrates  in  their  complicated  system  of  pigment-cells,  as,  for 
instance,  by  Exner3  and  Szczawinska4  in  Crustacea,  and  by  Rawitz5  in 
cephalopods. 

2.  The  Rods  and  Cones. — In  this  layer  we  must  first  call  attention  to 
the  swelling  of  the  rods,  as  suggested  above,  and  carefully  studied  by 
Hornbostel.6  This  may  be  so  extreme  that  the  rods  lie  flat  against  one 
another.  When  restored  to  darkness,  they  diminish  in  size  in  an  hour  or 
more.  The  swelling  is  not  noticed  in  red  light  until  the  purple  has  totally 
disappeared. 

1  Kuehne  and  Sewall,  H.  U.,  iii.  3  and  4,  S.  221. 

2  Mueller's  Archiv,  1845,  S.  387. 

3  Sitzungsber.  d.  Wiener  Aka<l.,  Naturw.  Abth.,  xcviii.,  March  3,  1889. 

4  Loc  cit. 

5  Zool.  Anzeiger,  No.  363,  1889,  S.  157. 
8  H.  U.,  i.  S/409. 


PHOTO-CHEMISTRY   OF  THE   RETINA. 


A  much  more  remarkable  sight  is  the  shifting  of  the  cones  under  the 
influence  of  light,  as  described  by  Van  Genderen  Stort,1  in  frogs  which  had 
been  kept  for  four  hours  in  the  dark  and  whose  retime  had  been  hardened 
by  Altmann's  method2  with  three  and  a  half  per  cent,  of  nitric  acid.  Here 
the  pigment  fell  back  and  the  cones  no  longer  rested  with  a  broad  base 
upon  the  liniitaus  externa,  but  were  higher  up  between  the  external  mem- 
bers of  the  rods.  Fearing  that  these  were  artificial  productions,  lie  verified 
the  condition  with  other  methods  and  convinced  himself  that  similar  results 
were  always  to  be  obtained,  depending  upon  the  rapidity  with  which  the 
specimens  were  hardened. 

Stort  discovered  that  this  change  of  position  was  due  to  the  protoplasmic 


FIG.  7. 


FIG.  8. 


Position  of  the  cones  and  of  the 
pigment  in  frogs  kept  in  the  dark. 
(Engelmann.) 


Position  of  the  cones  and  of  the 
pigment  in  the  illuminated  frog's  eye. 
(Engelmann.) 


portion  of  the  internal  member  (the  so-called  cono-myoidin)  which  expands 
in  darkness  and  contracts  in  light.  He  then  described  the  process  in  the 
frog,  triton,  Perea  fluviatilis,  and  Coluraba  Livia,an<l  sketch"!  it  in  the  pi-. 
In  the  frog  and  in  the  perch  the  ditVerei.ee  in  the  position  of  the  coin-  i> 
very  great,  though  not  the  same  in  all  the  cones  of  the  same  individual. 
In  these  animals  the  expansion  is  often  so  great  that  the  outer  members  of 
the  cones  may  be  higher  than  the  rods.  In  other  animals  the  change  is  less. 


'Eneelmann    and  Van    Gendon-n    Btort,   IVooV-vrhi.!   .1.   k.   Aks.,1.  ti   Amsfnhm. 
March  29,  June  28,  1884;  Engolmann,  Donders,  Phys.  Lab.,  Iftndift, 
141    142   145,  1884;  and  Pflue^r's  A.vl,iv.  1886,  S.  4«.ts  ;  ate)  Van  Gendoen 
Nederland.,  xxx.,  1887,  Donders,  I'hysiol.  Inst.,  Utrecht,  iii.,  Reeks  r.  £ 
V.  Graefe's  Archiv,  xxiii.,  Abth.  iii.  S.  229,  1887. 

2  Arch.  f.  Anat.  u.  Physiol.,  Anat.  Abth.,  1881,  S.  219. 


644  PHOTO-CHEMISTRY   OF   THE   RETINA. 

Engelmann  found  the  process  slight  in  the  Tropidonotus,  and  doubtful  in 
the  Testudo  grseca,  and  thought  that  the  displacement  probably  occurred 
in  man. 

Van  Genderen  Stort's  investigations  further  show  that  in  the  case  of 
twin  cones  only  one  migrates,  the  larger  internal  member  remaining  motion- 
less. An  exception  is  to  be  noted  in  the  twin  cones  of  the  perch,  where 
both  are  of  the  same  size  and  the  protoplasmic  portion  is  common  to  both. 
Here  the  twin  cones  stretch  in  toto  in  the  darkness  and  contract  in  the  light. 
So,  too,  in  the  rods,  Stort  describes  a  slight  motion  in  opposite  senses  in 
various  animals.  In  the  perch  he  acknowledges  only  that  the  internal 
members  of  the  rods  probably  swell  in  the  dark,  and  in  this  way  are 
drawn  as  a  whole  centripetally,  whilst  in  tritons  the  rods  are  pushed  in  the 
darkness  centrifugally  by  anterior  migration  of  the  granules  of  the  rods, 
which  in  the  darkness  project  a  few  microns  above  the  limitans  extcrna. 

According  to  Engelmann,  the  rapidity  of  the  reaction  is  so  great  that 
in  frogs  kept  in  the  dark  the  action  of  even  a  few  minutes  of  bright  dif- 
fused daylight  is  sufficient  to  reduce  to  a  minimum  extreme  elongation  of 
the  rods ;  if  the  light  is  dazzling,  the  action  is  more  rapid  still.  The 
elongation  in  the  dark  is  slower,  and  not  always  simultaneous  with  the 
migration  of  the  pigment.  Refrangible  rays  seem  to  act  more  powerfully 
than  others.  Since  green  light  in  the  pigeon  most  powerfully  contracts  the 
rods  with  red  summits,  in  which  case  but  little  light  falls  upon  the  exterior 
members,  the  locality  of  the  irritation  must  be  sought  for  inside  of  these ; 
and  since  the  cones  in  the  inner  layer,  which  possess  both  red  globes  and 
finely  divided  red  globules,  contract  less  in  green  light  than  the  cones  which 
are  free  from  pigment,  the  locality  of  the  irritation  miist  be  placed  in  the 
internal  members.  It  is  plain,  too,  that  absorption  in  the  globes  and 
globules  cannot  produce  the  irritation,  because  the  reaction  is  seen  in  cones 
without  a  ninepin-shaped  summit,  and  is  absent  in  turtles,  which  have 
intensely  colored  summits.  The  contraction  in  the  frog  happens  first  in  the 
inner  portion  and  the  elongation  first  in  the  outer  portion  of  the  inner 
member.  In  the  pigeon  it  is  more  nearly  uniform. 

If  but  one  eye  is  illuminated,  the  cones  and  pigment  in  both  are  in  the 
position  for  light.  The  same  occurs  in  beheaded  frogs  if  the  brain  is  pre- 
served. The  cones  assume  of  their  own  accord  at  a  later  date  the  contracted 
form,  like  other  contractile  tissues  under  spontaneous  dissolution.  After  the 
brain  is  destroyed,  the  action  is  limited  to  the  illuminated  eye.  Frogs  with 
bandaged  eyes,  in  which  the  skin  alone  is  illuminated,  show  the  cones  and 
pigment  close  to  the  limitans,  in  which  case  the  action  upon  the  pigment- 
cells  is  more  prompt.  Strychnine  and  tetanization  of  the  eye  with  induced 
currents  in  the  living  animal,  or  directly  after  extirpation  of  the  organ, 
produce  in  the  dark  the  light-position  of  the  cones  and  of  the  pigment. 
Curare  has  no  influence.  Hamburger  has  confirmed  l  the  influence  of  light 

1  FestLundel  F.  C.  Bonders,  Amsterdam,  1888,  p.  285. 


PHOTO-CHEMISTRY  OF  THE   RETINA.  645 

and  darkness  upon  the  cones  five  months  after  division  of  the  optic  nerve. 
According  to  Gradenigo,1  the  external  granules  elongate  when  illuminated, 
and  in  the  eye  of  a  frog  kept  in  the  dark  and  warmed  to  35°  C.  (95°  F.) 
the  inner  members  of  the  cones  and  rods  were  contracted.  A.  E.  Fick 
found 2  that  the  position  of  the  cones  and  pigment  for  light  has  changed 
in  the  dark  after  suffocation,  and  refers  the  results  found  in  Engelmann's 
experiments  on  frogs  with  bandaged  heads  to  the  same  condition.  He  has 
also  reported 3  that  some  fishes  after  exposure  to  the  direct  sun  show  the 
light-position  after  several  hours  in  complete  darkness.  If  Fick  desires 
to  conclude  from  this  experiment  that  these  processes  are  chiefly  indirect, 
that  they  have  nothing  directly  to  do  with  the  sight,  we  must  once  more 
refer  to  Kuehne's  proof  of  the  direct  action  of  light  upon  the  migration 
of  fusciii  (epithelial  optograms),  and  ask  him  to  remember  that,  even  if 
other  irritations  can  produce  the  same  phenomena,  irritation  by  light  is 
not  excluded. 

ELECTRICAL    PROCESSES   IN   THE   RETINA    PRODUCED   BY   LIGHT. 

Holmgren 4  first  observed  the  photo-chemical  reaction  of  the  retina.  He 
thought  that  he  could  best  obtain  the  retinal  induction-current  by  making 
connection  between  the  cornea  and  the  posterior  portion  of  the  globe,  since 
the  termini  of  the  visual  cells  are  turned  towards  the  latter,  whilst  to  the 
cornea  the  longitudinal  sections  of  the  optic  nerve  are  conducted  through 
the  indifferent  conductors  the  vitreous  and  the  lens.  In  this  manner  Holm- 
gren found  the  cornea  positive  in  relation  to  the  posterior  portion  of  the 
eyeball.  He  further  found  that  the  current  fluctuated  under  the  influence 
of  light, — in  the  frog,  for  example,  it  being  positive  to  oncoming  or  depart- 
ing light.  He  also  succeeded  in  demonstrating  the  same  phenomenon  in 
the  isolated  retina,  and  in  reptiles  and  mammals  he  established  a  negative 
fluctuation  to  oncoming  light  and  a  positive  one  to  departing  light. 

Dewar  and  McKendrick  5  passed  the  induction-current  through  the  cor- 
nea and  the  optic  nerve,  finding  the  same  reaction  as  Holmgren,  and  also 
studied  the  influence  of  light  in  various  animals.  They  insist  that  warmth 
must  be  avoided,  and  thus  discovered  for  the  influence  of  light  alone  the  fol- 
lowing conditions :  for  extirpated  eyes  of  animals,  with  oncoming  light,  sink- 
ing of  the  current ;  with  departing  light,  slight  sinking  or  else  no  change ; 
for  frogs,  with  oncoming  light,  an  increase ;  during  the  light,  a  slow  sink- 
ing, oftentimes  below  the  induction-current ;  on  removal  of  the  light,  an 
increase  ;  for  living  birds  and  mammals,  in  oncoming  light,  a  sinking ;  in 

1  Wien.  Med.  Zeitsch  ,  1885,  Nos.  28  and  30 ;  Mittheil.  a.  d.  Embryol.  Institut  v. 
Schenk,  1886. 

2  Vierteljahressch.  d.  Naturforsch.  Ges.  z.  Zurich,  Jahrg.  xxxv.,  Heft  i. 

3  Bericht.  d.  Oph.  Gesellsch.  z.  Heidelberg,  1889. 

*Upsala,  Lakaraforeninss  Forhandlinger,  Bd.  i  ,  1866,  S.  184,  Bd.  vi.,  1871 ;  H.  U.f 
iii.  3  and  4,  S.  278 

5  Trans,  of  the  Royal  Society  of  Edinburgh,  xxvii.,  1874,  p.  141. 


646  PHOTO-CHEMISTRY   OF   THE    RETINA. 

prolonged  light,  a  slow  increase  ;  in  departing  light,  a  sudden  increase ;  for 
fishes,  in  oncoming  light,  an  increase  ;  during  illumination,  at  first  station- 
ary, then  a  slow  decrease ;  in  departing  light,  a  rapid  decrease.  Very  weak 
light,  like  moonlight,  sufficed  to  produce  the  fluctuations.  Lethal  doses  of 
curare,  santonin,  belladonna,  morphine,  and  Calabar  extract  did  not  destroy 
the  fluctuations  to  light.  In  the  compound  eyes  of  Crustacea  the  current 
was  in  the  opposite  direction,  but  it  reacted  in  the  same  manner  to  light. 

Kuehne  and  Steiner  elevated  the  examination  exercised  off-hand  by 
Holmgren  upon  isolated  retinae  to  a  method  j1  for,  though  the  investigations 
previously  made  had  revealed  the  insignificance  of  the  anterior  segment  of 
the  eyeball,  the  examination  of  the  isolated  retina  still  remained  open  for 
the  decision  of  the  question  of  the  situation  of  the  current.  To  do  this 
the  greatest  care  in  reference  to  the  constancy  of  the  unpolarizable  elec- 
trodes has  been  found  necessary.  This  has  been  obtained  by  the  employ- 
ment of  tissues  which  had  been  hardened  in  alcohol  and  then  washed,  and 
by  avoiding  the  alkaline  vapors  of  the  sodium  flame  by  working  behind 
red  glass.  The  induction-current  of  the  isolated  retina  acted  as  follows. 
The  optic  nerve  section  was  positive  on  the  side  towards  the  rods,  in 
opposition  to  the  periphery.  Two  points  of  the  latter  gave  weak  variable 
currents  on  the  fibrous  side.  The  optic  nerve  section  was  negative  in  oppo- 
sition to  every  other  region.  The  conduction  that  proved  most  suitable 
for  the  investigation  was  from  the  side  of  the  rods  and  fibres,  and  in  this 
case  a  stronger  current,  which  passed  by  cathodal  arcs  from  the  side  of  the 
fibres  to  the  side  of  the  rods,  was  revealed.  This  current  gradually  sank 
to  a  medium  height,  where  it  generally  remained  constant.  In  some  cases 
it  would  sink  farther,  die  away,  and  even  return.  In  the  opinion  of 
Kuehne  and  Steiner,  the  variations  of  this  current  to  the  irritation  of 
light  are  the  same  whether  the  rod  side  or  the  fibre  side  is  illuminated. 
They  consider  that  the  radiating  heat  is  not  of  much  account. 

Even  extremely  weak  light,  like  that  of  fluorescing  powder  or  that 
produced  by  a  puff  at  a  cigarette,  suffices  to  cause  variations.  Cold,  heat, 
chloroform,  chlorate  of  sodium,  large  amounts  of  pilocarpine  and  salicylate 
of  sodium,  destroy  the  electric  reaction  of  the  retina  to  light.  Atropine 
and  curare  do  not  have  any  effect.  Cutting  off  or  admitting  light  produces 
but  slight  variations.  The  behavior  of  these  fluctuations  to  the  irritation 
of  light,  as  careful  performance  of  the  experiment  shows,  is  astonishingly 
constant  and  complicated. 

With  the  frog's  retina  sudden  oncoming  light  primarily  produces  a 
positive  fluctuation,  which  rapidly  attains  its  maximum.  This  is  followed 
by  a  rapid  retrogression  to  a  negative  fluctuation.  During  the  presence 
of  the  light  the  current  remains  for  half  a  minute  constant ;  then,  upon 
sudden  removal  of  light,  with  the  same  rapidity  as  at  the  first  oncoming 
it  passes  to  the  zero  point  and  beyond,  and  finally  attains  much  more 

1  H.  U.,  iii.  3  and  4,  S.  327  ;  iv.  1  and  2,  S.  1. 


PHOTO-CHEMISTRY   OF  THE   RETINA. 


647 


FIG.  9. 


slowly  the  zero  point  again.  The  negative  fluctuation  may  be  very 
great,  so  that  the  first  positive  fluctuation  may  be  considered  as  only  a 
positive  preliminary  impulse,  and  may  produce  reversal  of  the  current. 
On  the  contrary,  it  may  be  small,  and  should  then  be  regarded  as  tin- 
decrement  of  the  first  positive  fluctuation,  this  being  the  case  chiefly  in 
very  fresh  eyes.  When  the  illumination  is  instantaneous,  all  three  fluctua- 
tions are  liable  to  occur.  Flickering  light  tetanizes,  as  it  were,  the  retina, 
the  phenomenon  of  superposition  being  dis- 
cernible in  the  first  positive  fluctuation. 

The  variation  in  the  direction  of  the  cur- 
rent during  absence  of  light  has  no  influence 
upon  the  entrance,  course,  and  magnitude  of 
the  photo-electrical  fluctuations,  except  that 
they  all  obtain  the  opposite  symbol.  This 
condition  is  evidently  based  upon  the  fact 
that  the  anterior  and  posterior  surfaces  of  the 
retina,  in  a  condition  of  excitation,  invariably 
contract  to  the  same  differences  in  tension  as 
had  existed  before  the  illumination.  The  law 
of  the  constant  alteration  of  tension  suggests 
that  these  variations  are  due  to  chemical 
causes. 

In  order  to  solve  the  question  how  the 
"visual  matters"  act  towards  these  fluctua- 
tions, investigations  have  been  made  on  the  only  verifiable  matter,  the 
visual  purple,  to  see  how  it  behaved  with  and  how  without  the  fluctua- 
tions. In  doing  this  it  was  necessary  to  remember  that  the  unbleached 
retina  is  at  rest.  In  order  to  bring  the  bleached  retina  into  the  same  con- 
dition, animals  must  be  on  ice  in  the  dark,  so  that  regeneration  is  retarded, 
— a  treatment  which  is  without  any  influence  upon  frogs  kept  in  the  dark. 
The  magnitude  of  the  fluctuations  in  unbleached  retina  was  then  discovered 
to  be  greater  for  similar  irritations  than  in  the  bleached,  and  the  irritations 
in  the  latter  were  also  altered  in  quality,  being  negative  to  oncoming  light, 
or  rarely  with  the  slightest  possible  positive  preliminary  impulse. 

The  results  obtained  by  Kuehne  and  Steiner  in  isolated  retinae  of  frogs 
vary  greatly  from  those  which  Holmgren,  as  well  as  Dewar  and  MrKm- 
drick,  had  found  in  the  globe.  All  these  observers,  however,  coincided 
in  the  negative  fluctuation  to  oncoming  light  in  rabbits'  retinae.  The  ques- 
tion then  arose,  which  is  the  normal  condition  ?  It  was  discovered  in  the 
retina?  of  rabbits  that  this  simple  negative  fluctuation  probably  depended 
on  post-mortem  appearances;  for,  in  the  first  place,  the  retinae  used  had 
lately  rapidly  decayed,  and  in  the  second  place,  in  the  experiments  with 
the  frogs'  retina  that  had  been  employed  (and  which  had  long  remained 
in  situ,  resulting  in  alterations  which  probably  depended  on  sutfi>cati<m) 
a  stao-e  was  discovered  in  which  only  a  negative  fluctuation  to  oncoming 


Fluctuations  of  the  electrical  current 
in  the  isolated  retina  of  a  frog  when 
exposed  to  light.— a,  oncoming  light ; 
b,  departing  light.  (Kuehne.) 


648 


PHOTO-CHEMISTRY   OF   THE   RETINA. 


FIG.  10. 


light  could  be  discovered.  From  these  results  the  inference  is  that  it  is 
possible  that  fresh  retinae  from  rabbits  would  act  similarly  to  those  of 
the  frog. 

As  to  how  the  retinal  currents  act  towards  bulbar  currents,  the  fol- 
lowing conditions  were  discovered  by  Kuehne  and  Steiner.     The  bulbar 
current  of  the  frog  fluctuates  positively  to  oncoming 
and  to  departing  light.     During  the  continuance  of 
the  light  there  is  no  decrement  in  the  first  current 
I  (which  is  never  seen  in  the  isolated  retina)  in  a 

^^     course  that  is  for  minutes  parallel  to  the  abscissa. 
These  results  are  in  opposition  to  the  findings  of 
•  Dewar   and    McKendrick,  who   evidently   experi- 

mented upon  wearied  or  injured  eyes. 

Here,  then,  lies  a  decided  difference  in  compari- 
son with  the  current  of  the  isolated  retina.    Kuehne 
and  Steiner  found  that  the  anterior  segments  of  the 
globe  could  be  separated  without  altering  the  photo- 
electrical  condition,  but  if  the  retina  were  pulled 
in  the  slightest  and  any  vitreous  escaped,  the  same 
fluctuations  appeared  as  in  the  isolated  retina.     The  negative  fluctuation, 
therefore,  which  appears  in  the  latter  is  a  fluctuation  of  alteration,  the  cause 

of  which  may  be  sought  in  an  encroachment  of 
the  vitreous  between  the  visual  cells  and  the 
epithelium,  which  in  the  globe,  perhaps,  is  not 
possible  to  regenerative  processes.  As  shown 
above,  the  normal  fluctuations  of  the  retinal  cur- 
rent in  warm-blooded  animals  may  act  in  the 
same  way  as  those  of  the  frog.  The  same  may 
be  assumed  for  bulbar  currents,  for  the  results 
obtained  by  Dewar  and  McKendrick  on  living 
animals,  which  do  not  coincide  with  this,  cannot 
be  employed  for  comparison,  because  of  the  great 
mutilation  of  the  parts  that  they  permitted  in 
their  experiments. 

Genuine  fluctuations  are  to  be  seen  in  fishes. 
The  induction-current  has  the  same  direction  as 
in  frogs. 

Here  the  fluctuations  to  light  act  as  in  the 
hollow  shell  of  the  eye,  and  at  first  as  on  the  iso- 
lated retina.  Later  many  alterations  are  visible. 
There  is  no  alteration  when  the  vitreous  escapes, 
because  it  is  more  viscid  and  does  not  penetrate 
so  easily.  The  unaltered  fluctuations  have  the  fol- 
lowing course  :  with  oncoming  light  they  are  double  :  at  first  they  are  nega- 
tive, then  positive,  in  which  the  later  increase  surpasses  the  magnitude  of  the 


Fluctuations  of  the  electrical 
current  in  a  frog's  eye  when 
exposed  to  the  irritation  of 
light. — a,  oncoming  light;  b, 
departing  light.  (Kuehne.) 


FIG.  11. 
a  b 


Variations  of  the  electrical 
current  in  the  fresh  eyes  of  fish 
(Perca  fluviatilis)  when  exposed 
to  the  irritation  of  light.— 7.  on 
the  eyeball;  II,  on  the  hollow 
shell  of  the  eye ;  III,  on  the  iso- 
lated retina ;  a,  oncoming  light ; 
b,  departing  light.  (Kuehne.) 


PHOTO-CHEMISTRY   OF   THE   RETINA. 


649 


current  in  darkness  whilst  the  illumination  persists.  On  the  departure  of 
the  light  they  are  very  positive.  These  fluctuations  are  evidently  the  nor- 
mal ones,  for  those  which  differ  from  them  in  the  entire  globe,  and  which 
to  oncoming  light  are  positive,  but  increasing  very  slowly,  and  persisting 
during  the  illumination,  and  are  again  positive  (though  weakly  so)  on  de- 
parture, seem  to  owe  these  variations  to  unfavorable  conductive  conditions, 
which  are  easy  to  understand  when  the  numerous  de^wsits  in  the  back- 
grounds of  fishes'  eyes  are  considered.  The  first  hesitating,  positive  vibra- 
tion makes  one  feel  as  if  all  the  antagonistic  forces  which  occur  in  the 
fluctuation  above  regarded  as  normal  had  simultaneously  come  to  an  expres- 
sion at  this  point. 

As  was  known  to  Du  Bois-Reymond,1  the  optic  nerve  of  fishes,  like  all 
other  nerves,  gives  the  legitimate  induction-current.  Kuehne  and  Steiner 
have  further  found  that  it  gives  normal  negative  fluctuations  to  electrical 
irritation.  The  optic  nerve  of  the  frog  acts  in  the  same  manner.  In  the 
frog,  however,  on  account  of  the  minuteness  of  the  nerve,  the  electrical  irri- 
tation must  be  brought  close  to  the  peripheral  terminal  organs  in  the  eye. 
It  is  easy,  by  rapid  work  during  the  action  of  light,  to  observe  on  the  optic 
nerve  of  the  frog  nothing  but  a  negative  fluctuation  during  oncoming  and 
departing  light,  as  by  every  other  irritation.  The  optic  nerve  reacts  to 
continuous  light  like  any  other  nerve  that  is 
exposed  to  constant  irritation.  The  negative 
fluctuation  is  permanent,  and  the  nerve  is  in 
a  state  of  phototonus. 

Some  have  desired  to  decide  the  question 
whether  normal  nerves  in  the  eye  are  free 
from  a  current  or  not.  They  have  attempted 
this  by  regarding  the  visual  cells  as  the  natu- 
ral section  of  the  expansion  of  the  optic 
nerve.  The  peculiar  photo-electrical  vibra- 
tions of  the  rods  and  cones  decidedly  re- 
fute such  a  view.  Nevertheless,  the  electrical  vibrations  are  really  to  be 
placed  in  the  rods  and  cones.  That  they  do  not  lie  in  the  pigment  epithe- 
lium has  been  proved  by  Dewar  and  McKendrick  and  by  Kuehne  and 
Steiner.  Owing  to  the  fact  that  the  options  must  be  considered  to  extend 
as  far  as  the  layer  of  ganglionic  cells,  the  differences  in  the  photo-electrical 
processes  in  the  retina  and  optic  nerve  compel  us  to  place  those  processes  at 
least  behind  the  ganglionic  cells.  Kuehne  and  Steiner  have  found  that  the 
fluctuations  of  the  optic  current  in  frogs  and  fishes  cease  much  sooner  than 
those  of  the  retina.  From  this  it  might  be  assumed  that  the  retinal  expan- 
sion of  the  optic  nerve— i.e.,  the  anterior  layers  of  the  retina — no  longer 
reacts  to  light,  and  that  the  still  existing  fluctuations  must  be  ascribed  to  the 
visual  cells  alone.  This  conclusion,  however,  does  not  hold  good  in  cold- 


Fio.  12. 


lopticus 


120 


Variation  of  the  electrical  current, 
under  the  influence  of  light,  in  the 
optic  nerve  of  the  frog.— a,  oncoming 
light;  6,  departing  light.  (Kuehne.) 


1  Untersuch.  ii.  thier.  Elec.,  ii.  S.  256. 


650  PHOTO-CHEMISTRY   OF   THE    RETINA. 

blooded  animals,  since  their  optic  nerve  (and  in  fishes  its  expansion  into 
the  retina)  remains  for  a  long  time  electrically  excitable  when  the  anterior 
layers  of  the  retina  no  longer  react  to  light.  It  thus  may  happen  that 
the  fluctuations  to  the  irritation  of  light  are  also  situated  in  the  anterior 
layers  of  the  retina,  and,  owing  to  the  death  of  something  interpolated 
between  them,  can  no  longer  be  conducted  to  the  optic  nerve.  The  gan- 
glionic  cells  of  the  retina  might  possibly  be  regarded  as  such  an  interpola- 
tion. Kuehne  and  Steiner,  however,  have  found  in  pigeons,  in  whom 
neither  the  ganglionic  cells  nor  the  nerve-fibres  in  excised  eyes  long  exist, 
that  they  could  demonstrate  that  the  retinal  fluctuations  persist  at  a  time 
when  the  above-mentioned  connective  tissue  elements  are  dead.  This  has, 
moreover,  been  confirmed  by  them  by  special  experiments.  In  such  ex- 
periments nothing  is  left  as  the  seat  of  the  vibrations  in  question  but 
the  rods  and  the  cones.  In  an  experiment  of  Kuehne  and  Steiner's  in 
which  the  retina  was  split  into  two  layers  and  gently  pressed  between  two 
silk  papers,  it  was  observed  that  the  external  members,  though  fallen  into 
considerable  disorder,  showed  no  electrical  reaction  to  the  irritation  of 
light,  while  the  other  half  of  the  retina  still  reacted.  From  this  it  may 
be  assumed  that  the  photo-electrical  processes  play  their  rdle  chiefly  in  the 
interior  members  of  the  visual  cells. 


INDEX  TO  VOLUME  I. 


A. 


Abducens,  development  of,  53,  59. 
Aberration,  chromatic,  469,  499. 
monochromatic,  470. 
spherical,  470. 
Ablation  experiments,  410. 

Abnormalities,  congenital,  in  the  ocular  appen- 
dages, 425. 
of  the  chorioid,  443. 
of  the  conjunctiva,  430. 
of  the  cornea,  431,  443. 
of  the  eyeball,  419. 
of  the  eyelids,  427. 
of  the  iris,  157,  434. 
of  the  lacrymal  apparatus,  427. 

gland,  427. 
of  the  lens,  448. 
of  the  optic  nerve,  448. 
of  the  retina,  446. 
of  the  retinal  vessels,  447. 
of  the  vitreous,  454. 

of  the   human    eye,   congenital    malforma- 
tions and,  417. 
of  the  pupil,  435. 
Absence  of  ocular  muscles,  430. 
Accessibility  of  the  eyeball  for  operation,  1 1 8, 1 19. 
Accommodation,  172. 
in  astigmatism,  503. 
loss  of,  with  age,  504. 
power  of,  501. 
region  of,  502,  503. 
total,  502. 
Accuracy  of  perception,  methods  for  studying, 

517. 
Achromatic  cells  of  the  retina,  312,  313. 

color-blindness,  600. 
Acid  reaction  of  fresh  retinae,  619. 
Actinic  rays,  583. 

effects,  583. 
Adipose  body  of  the  orbit,  123. 

capsule  of  the  eye,  123. 
Adventitia  oculi,  124. 
After-images,  complementary,  530. 

complication  of  positive  and  negative, 

532. 

negative,  530,  609. 
oscillations  of,  532. 
positive,  528,  529,  609. 
Age,  loss  of  accommodation  with,  504. 
Albumin  in  the  retina,  178,  619. 
Aleuronoid  particles,  293. 
Alkaline  reaction  of  the  vitreous,  618. 
Alterations  in  the  retina  during  vision,  634. 

mechanical,  in  the  retina,  produced  by  light, 

640. 

Alternate  stimulations,  534. 
Amacrinal  cells  of  Ram6n  y  Cajal,  45. 
Ainacrine  cells,  309. 
diffuse,  310. 
stratiform,  310. 


Ametropia,  489,  490. 
Ametropic  eye,  489. 

form  of  ciliary  muscle  in,  267. 
Ainmon,  filaments  of,  165. 
Amnion,  the,  8. 
Amphioxus,  eye  in  the,  9. 
Anabolic  change,  613. 
Anagenesis,  637,  638. 
Anatomy,  microscopical,  of  the  eyeball,  217. 

of  the  base  of  the  orbit,  78. 

of  the  eyeball,  109. 

of  the  intra-craninl  portion  of  the  visual  ap- 
paratus, 383. 

of  the  intra-orbital  portion  of  the  optic  nerve, 
109. 

of  the  lacrymal  canals,  92. 
ducts,  92. 
gland,  92. 
papillae,  92. 
sac,  93,  94. 

of  the  nasal  duct,  94. 

of  the  optic  chiasm,  389. 
nerve,  387. 

of  the  orbicularis  palpebraruui,  80. 

of  the  orbit  and  appendages  of  the  eye,  70. 

of  the  orbital  muscles,  95. 

of  the  region  of  the  eye,  79. 

of  the  septum  orbitale,  78,  79. 
Anchyloblepharon,  congenital,  428. 
Angle,  alpha,  501. 

gamma,  501. 

irido-corneal,  152. 

of  deviation,  467. 

of  incidence,  461. 

of  reflection,  461. 

of  the  anterior  chamber,  152. 

of  the  iris,  152. 
Angular  gyrus,  the,  406. 
Angulus  iridis,  152. 

Animal's  endowments,  curtailment  of,  411. 
Aniridia,  440. 

Annular  ligament  of  the  iris,  151. 
Annulus  senilis,  139. 

tendineus  communis,  213. 

tendinosus,  151. 

Zinnii,  213. 

Anomalies  of  refraction,  489. 
Anophthalmos,  419,  426. 
Anterior  basement  membrane,  221. 

boundary  layer,  221. 
of  the  iris,  177,  273. 

chamber.  48,  111,  254. 

ciliary  arteries,  101,  103,  270. 

dental  canal,  75. 

dichotomy,  42-1. 

rl:i~tif  membrane,  321. 

endothelium  of  the  iris,  177,  273. 

i-tliiiutidal  arteries,  101,  103. 
foramen,  73. 

focal  line,  496. 

homogeneous  lamina,  147. 

G51 


652 


INDEX    TO    VOLUME    I. 


Anterior  hyaloid  artery,  34. 

layer  of  epithelium  of  the  iris,  177. 
limiting  layer  of  the  cornea,  141,  142,  147, 

218,  221. 

or  external  basement  membrane,  147. 
polar  cataracts,  452. 
principal  focus,  484. 
scleral  foramen,  130. 
synechise,  congenital,  424. 

of  the  iris,  439. 
temporal  vein,  86. 
Antrum,  the,  75,  76. 
Apex  of  a  prism,  466. 

of  the  orbit,  73. 
Aphakia,  448. 
Aphakic  eye,  483. 

hyperopia,  490. 

Aponeurosis  orbito-ocularis,  123. 
Apparatus,    lacrymal,    congenital    abnormalities 

of  the,  426. 
stereoscopic,  547. 
Appendages,  development  of  the  eyeball  and  its, 

417. 

ocular,  congenital  abnormalities  in  the,  425. 
of  the  eye,  anatomy  of  the,  70. 
Aqueous  chamber,  the,  48,  111,  380. 

development  of  the  endothelium  of  the, 

49. 
humor,  111,  217. 

derivation  of  the,  49. 
formation  of  the,  198. 
production  of  the,  by  ciliary  processes, 

267. 

Arachnoidal  sheath  of  the  optic  nerve,  348. 
Arcus  juvenilis,  431. 

senilis,  139. 
Area,  germinal,  7. 

Martegiani,  209,  368. 
retinal,  21. 

visual,  of  the  cerebral  cortex,  404. 
extent  of,  410. 
of  the  cortex,  effect  of  removal  of,  upon 

the  field  of  vision,  410. 
Arteria  centralis  retinae,  37,  337. 

intrusion  of  the,  into  the  chorioid 

fissure,  22. 
macularis  inferior,  135,  338. 

superior,  338. 
nasalis,  338. 
temporalis  inferior,  338. 

superior,  338. 

Arteries,  ciliary,  development  of  the,  36. 
of  the  chorioid,  259. 
of  the  eyelids,  85. 
of  the  iris,  190. 
of  the  retina,  195. 
Artery,  anterior  ciliary,  101,  103,  270. 

hyaloid,  34. 
capsular,  34. 

central,  of  the  retina,  101,  103. 
ethmoidal,  102,  103. 
anterior,  102,  103. 
posterior,  102,  103. 
facial,  85. 
hyaloid,  197,  331. 

persistent,  454. 
infra-orbital,  85. 
lacrymal,  36,  85,  101,  103,  107. 
long  ciliary,  101,  103,  269. 
middle  temporal,  85. 

muscular  branches  of  ophthalmic,  102,  103. 
ophthalmic,  85,  101,  107. 
palpebral,  102,  103. 
posterior  ciliary,  101,  103. 
supra-orbital,  102,  103,  107. 
temporal,  85. 
transverse  facial,  85. 


Association  fibres,  413. 
Astigmatism,  accommodation  in,  503. 
correction  of,  498. 
irregular,  500. 
meridian  of,  497. 
regular,  495. 
Asymmetry  of  the  eyeball,  113. 

eyes,  84. 
Atresia  pupillae  congenita,  36. 

pupillaris,  causation  of,  52. 
Atrophy  of  the  disk,  congenital,  448. 
Auto-regeneration,  638. 
Axes  of  the  orbits,  72,  117. 

optic,  30. 
Axial  length  of  the  eyeball,  113. 

myopia,  493. 
Axis  cylinder,  479. 

fibres,  development  of  the,  248. 
of  cylindrical  lens,  determination  of,  480. 
of  the  eye,  111. 
external,  111. 
internal,  111. 
optic,  483. 

primary,  478. 
principal,  476. 
secondary,  476,  478,  484. 
Axoplasm  of  Schiefferdecker,  316. 

B. 

Baker,  Frank,  M.D.,  Ph.D.,  on  the  anatomy  of 
the  eyeball  and  of  the  intra-orbital  portion  of 
the  optic  nerve,  109. 
Basal  cells  of  the  retina,  304. 
Base  of  the  orbit,  anatomy  of  the,  78. 

of  a  prism,  466. 
Basilar  layer  of  the  iris,  177. 
Bay,  lacrymal,  description  of  the,  80. 
Binocular  color-mixture,  579. 

field  of  vision,  542. 

ophthalmoscope,  the,  550. 

otoscope,  the,  550. 

vision,  conflict  of  the  fields  of  vision,  ap- 
parent and  natural  size  of  objects, 
etc.,  539. 

mechanism  of,  392. 
perception  of  depth  by,  542. 
Bipolar  cells  of  the  retina,  288,  307. 
Blastocele,  the,  7. 
Blastodermic  vesicle,  7. 
Bleaching  of  the  visual  purple,  634. 
Blepharophimosis,  429. 
Blindness,  psychical,  412. 
Blind  spot,  193. 
Blinking,  reflex  act  of,  537. 
Blood-vessels  of  the  chorioid,  261. 

of  the  ciliary  body,  269. 

of  the  cornea,  155,  233. 

of  the  iris,  283. 

of  the  retina,  337. 

development  of  the,  45,  46. 

of  the  sclera,  245. 
Blue-blind,  603. 

sensitive  end-organs,  595. 
Bodies  of  visual  cells  of  the  retina,  288.. 
Body,  ciliary,  165.  166,  217,  255,  261. 

lenticular,  299. 

vitreous,  206,  217,  302. 
Bonnet's  capsule,  123. 
Borders  of  the  iris,  175. 
Boundary  layer,  anterior,  of  the  iris,  273. 

ring  of  Descemet's  membrane,  249. 

zone  of  the  chorioid,  259. 
Bowman,  membrane  of,  147. 
Brain,  phylogeny  of  the,  .J86. 

superposition  of  images  in  the,  393. 
Breadth  at  the  base  of  the  orbit,  77. 


INDEX   TO  VOLUME   I. 


653 


Brewster,  lens-stereoscope  of,  547. 

Bridge,  coloboma  with  a,  436. 

Brightness  of  color,  the,  589,  590. 

Broca's  orbital  index,  72. 

Brodhun,   Eugene,    M.D.,  on    binocular   vision, 

conflict  of  the  fields  of  vision,  apparent  and 

natural  size  of  objects,  etc.,  539. 
Bruch,  membrane  of,  270. 
Bruch's  layer,  165. 
Bulbar  fascia,  the,  123. 
Buphthalmos,  423. 

C. 

Calcarine  fissure,  406,  414. 
Camera  tertia  aquosa,  208. 
Canal,  anterior  dental,  75. 

central,  368. 

hyaloid.  206,  208. 

infra-orbital,  75. 

laorymal,  72,  75. 

anatomy  of  the,  92. 

medullary,  9. 

of  Cloquet,  368. 

of  Petit,  208,  397. 

of  Schlemm,  50,  135,  152,  247,  248. 
development  of  the,  50. 

of  Stilling,  368. 

of  the  sclera,  131. 

of  the  vitreous,  360. 

optic,  130. 

Canaliculi,  malformations  of  the,  421. 
Canalis  Cloqueti,  206. 

hyaloideus,  206,  368. 

Lauthi,  152. 

Petiti,  208. 

Schlemtuii,  152. 
Canals,  corneal,  225. 
Canthi,  the,  80. 

development  of  the,  29,  31. 
Capacity  of  the  orbit,  117. 
Capillary  loops  of  the  cornea,  233. 

zone  of  the  chorioid,  259,  260. 
Capranica's  doctrine,  632. 
Capsula  adiposa  bulbi,  123. 

aquea  cartilaginosa,  149. 

bulbi,  123. 

fibrosa,  125. 

preaquosa,  149. 
Capsular  artery,  34. 
Capsule  of  the  lens,  the,  203. 

development  of,  50,  418. 

of  Tenon,  89,  99,  123,  245. 

anatomy  of,  99. 
Caruncle,  lacrymal,  80. 

description  of,  80. 

malformations  of,  427. 
Caruncula  lacrymalis,  development  of,  32. 
Cataracts,  anterior  polar,  452. 

complete,  453. 

congenital,  452. 

dotted,  453. 

nuclear,  452. 

posterior  polar,  452. 

zonular,  452. 
Catoptrics,  460. 

Cattell,  J.  McKeen,  M.A.,  Ph.D.,  on  the   per- 
ception of  light,  505. 
Causation  of  atresia  pupillaris,  52. 
Cells,  collecting,  386. 

corneal,  226. 

distributive,  385. 
use  of,  623. 

pigments  of,  624. 

wandering,  of  the  cornea,  227,  230. 
Cellular. elements  of  the  vitreous,  368. 
Cement-substance  of  the  lens,  358. 


Central  artery  of  the  retina,  101,  103. 

fovea,  195. 

vein  of  the  retina,  104. 
Centre  of  rotation,  501. 
Centres,  lower  visual,  connection  of  the  occipital 

cortex  with  the,  413. 

Centrifugal  fibres  in  the  optic  nerve,  48,  388. 
Centring  of  dioptric  surfaces,  157,  500. 
Centripetal  fibres  in  the  optic  nerve,  48,  388. 
Cerebellum,  development  of  the,  24. 
Cerebral  cortex,  visual  area  of  the,  404. 

hemispheres,  26. 

layer  of  the  retina,  288. 

ventricles,  the  division  of  the  three  primary, 

lv« 

vesicle,  first,  13,  25. 

second,  25. 
Chamber,  anterior,  48,  254. 

posterior,  48,  181,  254. 

recesses  of  the,  207. 
Chambers  of  the  eye,  111. 

anterior,  111. 

aqueous,  48,  111. 

posterior,  111. 

vitreous,  111. 
Change,  anabolic,  613. 

catabolic,  613. 

Chemical  action  of  the  visual  purple,  629. 
Chiasm,  optic,  anatomy  of  the,  389. 

crossing  fibres  of  the,  389,  392. 

development  of  the,  48. 

maculary  fascicle  of  the,  389. 

Newton's  hypothesis  of  the,  390. 

uncrossed  fibres  of  the,  389,  392. 
Chlorophane,  632. 

Chorio-capillaris,  159,  164,  255,  258,  259. 
Chorioid,  arteries  of  the,  259. 

boundary  zone  of  the,  259. 

blood-vessels  of  the,  261. 

capillary  zone  of  the,  259. 

chorio-oapillaris  of  the,  160,  164. 

color  of  the,  160,  255. 

congenital  abnormalities  of  the,  443. 

defects  in  pigmentation  of  the,  446. 

fissure,  20,  22,  23,  47. 

results  of  imperfect  closure  of  the,  50. 

glassy  lamina  of  the,  260. 

lamina  vasculosa  of  the,  160,  163. 

lymphatics  of  the,  261. 

nerves  of  the,  261. 

proper,  255. 

structure  of  the,  255. 

thickness  of  the,  159. 

veins  of  the,  259. 
Chorioidal  epithelium,  development  of  the,  23. 

portion  of  the  cornea,  142,  143. 

retina,  192. 

ring,  194. 

stroma,  layer  of  the,  255,  257. 

tract,  254. 

vessels,  development  of  the,  44. 
Chorioidea,  159. 

propria,  163. 

Chromatic  aberration,  469,  499. 
Chromophilous  cells  of  the  retina.  312,  313. 
Ciliary  arteries,  the  long,  36,  101,  103,  269. 
anterior,  101,  103,  270. 
posterior,  101,  103. 

body,  165,  166,  217,  255,  261. 
blood-vessels  of  the,  269. 
motor  fibres  of  the,  271. 
nerves  of  the.  270. 
sensory  fibres  of  the,  271. 

folds,  168. 

ganglion,  development  of  the.  58. 

glands,  167,  268. 

muscle,  166,  168,  255,  262,  266. 


654 


INDEX   TO   VOLUME   I. 


Ciliary  muscle,  development  of  the,  51,  53. 
form  of  the,  in  ametropia,  267. 

in  emmetropia,  267. 
nerves  of  the,  173. 
of  Riolan,  87. 
nerves,  development  of  the,  36. 

short,  course  of  the,  30  fa 
plexus,  237. 
processes,  166,  262,  263,  264. 

development  of  the,  51,  53,  418. 
production  of  aqueous  humor  by,  267. 
region,  166. 
retina,  197. 

ridge  and  processes,  development  of  the,  44. 
ring,  262. 
veins,  104. 

Cilio-equatorial  fibres,  196,375,  378. 
Cilio-postero-capsular  fibres,  375,  378. 
Circles  of  diffusion,  489,  585. 
Circlet  of  Zinn,  133. 
Circular  sinus,  152, 153. 
Circulus  arteriosus  iridis  major,  191,  270,  283. 

minor,  191,  276. 
musculi  ciliaris,  92,  270. 
nervi  optici,  133. 
Petiti,  208. 
Schlemmii,  152. 
vasculosus  nervi  optici,  133. 
venosus  ciliaris,  248. 
iridis,  152,  153. 
Schlemmii,  152. 
Zinnii,  245. 

Classes  of  fibres  in  the  optic  nerve,  388. 
Cleavage  figures  of  the  lens,  37. 
Cleft,  first  visceral,  28. 
Cloquet's  canal,  206,  368. 
Coalescence  of  the  eyelids,  31. 
Coat,  inner,  191. 
middle,  158. 
nervous,  191. 
of  the  eye,  external,  125. 

fibrous,  125. 
sclerotic,  242. 
vascular,  109. 
Coats  of  the  eye,  109. 
Collateral  fissure,  406. 
Collecting  cells,  386. 
Colliculus  opticus,  the,  193. 
Collins,  E.   Treacher,  F.R.C.S.,  Eng.,  and  Wil- 
liam Lang,  F.R.C.S.,  Eng.,  on  congenital  mal- 
formations and  abnormalities  of  the  human 
eye,  417. 
Coloboma,  420. 

chorioidea,  causation  of,  50. 
iridis,  causation  of,  50. 
of  the  chorioid,  443. 
of  the  eyelid,  427. 
of  the  iris,  434,  441. 
of  the  lens,  449. 
of  the  macula,  445. 
of  the  sheath  of  the  optic  nerve,  445. 
with  a  bridge,  436. 
Color-blindness,  600. 

achromatic,  600. 
dichromatic!,  600. 
monochromatic,  600. 
uniocular,  593. 
-blind,  totally,  604. 
Color,  complementary,  588. 
intensity  and,  519. 
in  the  iris,  development  of,  52. 
-mixture,  587. 

binocular,  579, 
of  the  chorioid.  160. 
of  the  eyes,  177. 
of  the  iris,  variation  in,  434. 
-perception,  correlative  theory  of,  594. 


Color-perception,  Ebbinghaus's  theory  of,  614. 
Ilelmholtz's  theory  of,  598. 
Hering's  theory  of,  613. 
normal,  581. 
-sensation,  588. 
-vision  in  the  peripheral  parts  of  our  retina, 

606. 
Colors,  natural,  589. 

primary,  591. 

Combinations  of  cylindrical  lenses,  498. 
Commissural  fibres  of  the  optic  tract,  development 

of,  62. 

Commissures,  long  intra-hemispherical,  413. 
Comparison  of  magnitudes  of  light,  513. 
Complementary  after-images,  530. 

color,  588. 

Complication   of    positive    and   negative    after- 
images, 532. 
Compound  light,  583. 
Compressor  lentie,  171. 
Concave  lenses,  refraction  by,  472. 
mirror,  463. 

spherical  mirror,  reflection  by  a,  462. 
Concavo-convex  lens,  473. 
Cone,  bipolar,  308. 
fibres,  301. 

ganglion  cells  of  the  retina,  291. 
granules,  301. 
Cones,  double,  302. 

pigments  of  the,  632. 
retinal,  shifting  of  the,  643. 
Congenital  abnormalities  in  the  ocular  append- 
ages, 425. 

of  the  chorioid,  443. 
of  the  conjunctiva,  430. 
of  the  cornea,  431. 
of  the  eyeball,  419. 
of  the  eyelid,  427. 
of  the  lacrymal  apparatus,  427. 
of  the  lens,  448. 
of  the  optic  nerve,  448. 
of  the  pupil,  435. 
of  the  retina,  446. 
of  the  retinal  vessels,  447. 
of  the  vitreous,  454. 
anchyloblepharon,  428. 
anterior  synechiae,  424. 
cataracts,  452. 
conical  cornea,  432. 
defects  of  movements  of  the  eye  and  eyelids, 

429. 

dermoid  growths  of  the  cornea,  432. 
glaucoma,  423. 
iridodialysis,  436. 
malformations    and    abnormalities    of    the 

human  eye,  417. 
nystagmus,  430. 
opacities  of  the  cornea,  431. 
pigmentation  of  the  sclerotic,  431. 
ptosis,  426,  430. 
symblepharon,  429. 
variations  in  size  of  the  cornea,  432. 
Conical  cornea,  congenital,  432. 
Conjugate  foci,  463,  471. 
Conjunctiva,  88,  122. 
arteries  of  the,  90. 
congenital  abnormalities  of  the,  430. 
cornese,  147. 
fornix  of  the,  122. 
growths  of  the,  431. 
lymphatics  of  the,  90,  236. 
nerves  of  the,  90. 
ocular,  122. 
palpebral,  123. 
reflection  of  the,  88. 
Conjunctival  portion  of  the  cornea,  142. 
sac,  122. 


INDEX   TO    VOLUME    I. 


865 


Conjunctival  sac,  development  of  the,  31. 

glands  of  the,  33. 
Connections  of  the  occipital  cortex  with  the  lower 

visual  centres,  -I  I:;. 

of  the  optic  fibres  with  the  nuclei  of  the  eye- 
muscle  nerves,  403. 
tract  with  the  mid-brain,  397. 
Cono-myoidin,  185,  643. 
Conscious  vision,  409. 
Contact  theory,  315. 
Contents  of  the  eyeball,  197. 
Contours,  rivalry  of  visual  fields  and  of,  576. 
Contraction  of  the  pupil,  causes  of,  1 88. 
Contractor  pupilla:,  186. 
Contrast,  simultaneous,  611. 

successive,  611. 
Converging  lens,  473. 

Convex  spherical  lenses,  refraction  by,  469. 
mirror,  464. 

reflection  by  a,  463. 
Convexo-concave  lens,  473. 
Cord,  spinal,  development  of  the,  24. 
Corectopia,  435. 
Corium  of  the  cornea,  49. 
Cornea,  the,  110,  136,  217. 

and  sclerotic,  development  of  the,  48. 

annular  ligament  of  the,  151. 

anterior  limiting  layer  of  the,  141,  143,  147, 

218,  221. 

blood-vessels  of  the,  155,  233. 
capillary  loops  of  the,  233. 
capsular  portion  of  the,  132,  142. 
chorioidal  portion  of  the,  142,  143. 
congenital  abnormalities  of  the,  431. 
opacities  of  the,  431. 
variations  in  size  of  the,  432. 
conical,  congenital,  432. 
conjunctival  portion  of  the,  142. 
corium  of  the,  49. 
curvature  of  the,  138. 
cutaneous  portion  of  the,  142. 
deep  stroma  plexus  of  the,  156. 
dermoid  growths  of  the,  432. 
dimensions  of  the,  139. 
epidermis  of  the,  development  of,  38. 
epithelial  nerve-plexus  of  the,  238. 
examination  of  the,  by  polarized  light,  224. 
external  epithelium  of  the,  141,  142,  143, 

147,  218,  219. 
fixed  cells  of  the,  145. 
fundamental  nerve-plexus  of  the,  238,  241. 
ground-substance  of  the,  222. 
index  of  refraction  of  the,  137. 
internal  endothelium  of  the,  141,  150,  218, 

232. 

epithelium  of  the,  141,  150. 
interstitial  injections  into  the,  225. 
intra-epithelial  nerve-plexus  of  the,  239. 
lacunae  of  the,  227. 
layers  of  the,  141,  218. 
lymphatics  of  the,  234. 
lymph-channels  of  the,  234. 

-passages  of  the,  1 44. 
negative  picture  of  the,  145. 
nerves  of  the,  237. 
nutrition  of  the,  156. 
opaca,  126. 
pellucida,  136. 
positive  pictures  of  the,  1  l.">. 
posterior  endothelium  of  the,  218. 

limiting  layer  of  the,  141,  149,218,  231. 
prickle  cells  of  the,  147,  220. 
primary  plexus  of  the,  156. 
V roper  substance  of  the.  141,  218,  222. 
scleral  portion  of  the,  142. 
specific  gravity  of  the,  141. 
subepithelial  plexus  of  the,  157. 


Cornea,  superficial  stroma  plexus  of  the,  157. 

supporting  fibres  of  the,  JJI. 

thickness  of  the,  139,  21». 

transparency  of  the,  136. 

wandering  cells  of  the,  145,  227,  230. 

weight  of  the,  141. 
Cornea!  canals,  225. 

cells,  226. 

conjunctiva,  147. 

corpuscles,  227. 

epithelium,  147. 

groove,  131. 

interval,  130. 

lamella,  218. 

nerves,  perforating  branches  of,  238. 
terminal  fibres  of,  239. 

spaces,  fluids  of,  234. 

negative  pictures  of,  225. 
positive  pictures  of,  225. 

M P. ma.  222. 

tubes,  144,  226. 
Cornealfalz,  131. 
Corneo-scleral  junction,  131. 
Corona  ciliaris,  52. 

radiata,  fibres  of  the,  166,  414. 
Corpora  bigemina,  development  of  the,  24. 

quadrigemina  anterior,  tubercles  of  the,  398. 

399. 

development  of  the,  24. 
Corpus  adiposuin  orbitaj,  123. 

bigeminum,  386. 

callosum,  interhemispherical  fibres  of  the, 
414. 

orystallinum,  200. 

geniculatum  late  rale,  395,  396. 

hyaloideuin,  206. 

Luys,   part  played   by,   in    forming    optic 
tracts,  61. 

aubthalarnicum,  part  played  by,  in  forming 
optic  tracts,  61. 

vitreum,  206. 
Corpuscles,  corneal,  227. 

sclerotic,  243,  244. 
Correcting  hyperopia,  491. 

lens,  491,  493. 

Correction  of  astigmatism,  498. 
Correlation  theory  of  color-perception,  594. 
Corresponding  points  of  the  retina,  553. 

retinal  points,  390. 
Corrugator  supercilii,  88. 
Cortex,  cerebral,  visual  area  of  the,  404. 

occipital,  connection  of  the,  with  the  lower 

visual  centres,  413. 
region  of,  topography  of,  405. 

visual  area  of,   effect  of  removal  of,  upon 

field  of  vision,  410. 
Cortical  substance  of  the  lens,  204. 
Course  of  the  optic  nerves,  387. 
Cranial  flexure,  the,  17. 
Crassum  et  durum  involucrum  oculi,  126. 
Cribroso  lamina,  132. 
Cribrum,  132. 

Crick-dots  of  the  retina,  446. 
Crossing  fibres  of  the  optio  chiasm,  389,  392. 
Crown  and  flint  glass,  indexes  of  refraction  of, 

468. 

Cryptophthalmos,  425,  427. 
Crystalline  lens,  111,  200,  217,  349. 
Cuneus,  406. 

Cup,  optic,  the,  18,  19,  20. 
Curtailment  of  the  animal's  endowments,  411. 
Curvature  of  the  cornea,  137. 

hyperopia  of,  490. 

myopia  of,   I'.'.".. 
Curves.   l.i-saj>m>'.i.  535. 
ryi'lopia.  -I •_'»'.   IL1.".. 
Cylinder,  axis,  479. 


656 


INDEX   TO   VOLUME   I. 


Cylindrical  lens,  refraction  by  a,  478. 
lenses,  498. 

combinations  of,  498. 
Cysts,  dermoid,  of  the  eyebrows,  425. 

of  the  eyelids,  429. 
of  the  eyeball,  421. 


Dacryon,  the,  in  ascertaining  the  orbital  index, 

72. 

Decortication,  duration  of  effects  of,  412. 
Deep  stroma  plexus  of  the  cornea,  1 56. 
Defects  in  pigmentation  of  the  chorioid,  446. 
Demours,  membrane  of,  149,  231. 
Deposit  of  guanin,  642. 

Depth  in  binocular  vision,  perception  of,  542. 
in  monocular  vision,  perception  of,  539. 
of  the  orbit,  77. 

Derivation  of  the  aqueous  humor,  49. 
Dermoid  cysts  of  the  eyebrows,  425. 

of  the  eyelids,  429. 
growths  of  the  cornea,  432. 
Desceinet,  membrane  of,  149,  231. 
Descemet's  membrane,  boundary  ring  of  the,  249. 
Descending  root  of  the  optic  nerve,  398. 
Determination  of  axis  of  a  cylindrical  lens,  480. 
of  the  optical  centre  of  a  lens,  480. 
of  the  strength  of  a  lens,  479. 
Development  of  abducens,  53,  59. 
of  Homer's  muscle,  70. 
of  the  axis-cylinder  fibre,  24. 
of  the  blood-vessels  of  the  retina,  45. 
of  the  capsule  of  the  lens,  50,  418. 
of  the  caruncula  lacrymalis,  32. 
of  the  cerebellum,  24. 
of  the  chorioid  vessels,  44. 
of  the  ciliary  arteries,  36. 
ganglion,  58. 
muscles,  51,  53. 
nerves,  36. 

processes,  51,  53,  418. 
ridge  and  processes,  44. 
of  the  color  in  the  iris,  52. 
of  the  commissural  fibres  in  the  optic  tracts, 

62. 

of  the  conjunctival  sac,  31. 
of  the  corium  of  the  cornea,  49. 
of  the  corpora  bigemina,  24. 
of  the  corpora  quadrigemina,  24. 
of  the  endothelium  of  the  aqueous  chamber, 

49. 

of  the  epidermis  of  the  cornea.  38. 
of  the  eyeball  and  its  appendages,  417. 
of  the  eyelashes,  32. 
of  the  eyelids,  28,  30,  419. 
of  the  fifth  nerve,  53,  58. 
of  the  fourth  nerve,  53,  58. 

ventricle,  24. 

of  the  ganglionic  tracts,  24. 
of  the  Gasserian  ganglion,  58. 
of  the  geniculate  bodies,  24. 
of  the  glands  of  the  conjunctival  sac,  33. 
of  the  iris,  51,  418. 
of  the  lacrymal  artery,  36,  37. 
ducts,  419. 
glands,  32,  419. 
of  the  lacus  lacrymalis,  32. 
of  the  lamina  suprachorioidea,  49. 
of  the  lens,  25,  34,  37. 
of  the  Meibomian  glands,  32,  419. 
of  the  membrana  nictitans,  28. 

pupillaris,  49. 
of  the  motor  root  of  the  trigeininal  nerve, 

59. 

of  the  muscles  of  the  eyeball,  64. 
of  the  face,  69,  70. 


Development  of  the  neuroblasts,  56. 

of  the  oculo-motor  nerve,  53,  57. 

of  the  optic  chiastn,  48. 
nerve,  46,  47,  48. 
stalk,  47,  48. 
tract,  24,  59. 

of  the  ora  serrata,  51. 

of  the  pars  iridis  retinae,  43. 

of  the  patheticus,  53,  58. 

of  the  pectinate  ligament,  53. 

of  the  plica  semilunaris,  28. 

of  the  puncta  lacrymalia,  29,  31. 

of  the  pupil,  28. 

of  the  retina,  33,  34. 

and  the  optic  nerve,  42. 

of  the  rods  and  cones,  44. 

of  the  sclera  and  cornea,  48. 

of  the  sixth  nerve,  53,  59. 

of  the  space  of  Tenon,  49. 

of  the  spinal  cord,  24. 

of  the  spongioblasts,  56. 

of  the  stroma  of  the  iris,  52. 

of  the  suspensory  ligament,  418. 

of  the  tear-duct,  28,  29. 

of  the  tensor  tarsi,  70. 

of  the  third  nerve,  51,  53. 
ventricle,  24. 

of  the  trochlearis,  53,  58. 

of  the  vitreous  humor,  50. 
space,  34. 

of  the  zonula  Zinnii,  50. 
Deviation,  angle  of,  467. 

minimum,  467. 

position  of,  467. 
Diaphragma  bulbi,  174. 
Dichotomy,  anterior,  424. 

posterior,  424. 

Dichromatic  color-blindness,  600. 
Dichromic  imperfect  vision,  600. 

perfect  vision,  600. 
Diffuse  amacrine  colls,  310. 
Diffusion,  circles  of,  489,  585. 
Dilatation  of  the  pupil,  causes  of,  189. 
Dilatator  pupillse,  186. 

iridis,  186. 
Dilator  pupillae,  274,  278. 

muscle  of  the  iris,  279. 
Dimensions  of  the  eyeball,  111 

of  the  orbit,  116. 
Dioptre,  474. 
Dioptric  medium,  459. 

surface,  500. 

surfaces,  centring  of,  500. 

system,  474. 
Dioptrics,  464. 

of  the  eye,  459. 
Disk,  congenital  atrophy  of  the,  448. 

optic,  excavation  of  the,  193. 
Disparate  points  of  the  retina,  553. 
Dispersion  of  light,  468,  469. 
Distance,  focal,  470. 
Distributive  cells,  385. 
Diverging  lens,  473. 
Division  of    medullary  sheath  of  fibres  of  the 

optic  tracts,  61. 
Doctrine,  Capranica's,  632. 

Walchlis's,  632. 
Dorsal  nucleus,  anterior,  401. 

posterior,  401. 
Dotted  cataracts,  453. 
Double-concave  lens,  473. 
Double  cones,  302. 

origin  of  fibres  of  the  optic  tracts,  61. 
Ducts,  lacrymal,  anatomy  of,  92. 
course  of,  84. 
description  of,  84. 
.  development  of,  419. 


INDEX    TO    VOLUME    I. 


657 


Ducts,  lacrymal,  Luschka's  line  for,  84. 

Merkel's  line  for,  84. 
nasal,  anatomy  of,  94. 

valves  of,  94. 

Ductus  oculi  abducentes,  163. 
Duddell,  membrane  of,  149. 
Dura  inembrana,  126. 

oculi,  125. 

Dural  sheath  of  the  optic  nerve,  348. 
Duration  of  effects  of  decortication,  412. 
Dwight,  Thomas,  M.D.,  LL.D.,  on  the  anatomy 
of  the  orbit  and  appendages  of  the  eye,  70. 

E. 

Ebbinghaus's  theory  of  color-perception,  614. 
Ectochoroidea,  the,  161. 
Ectocornea,  the,  147. 
Ectoderm,  the,  8. 

derivatives  of  the,  8. 
Ectodermic  portions  of  the  eyeball,  217. 
Ectopia  of  the  lens,  450. 
Ectropion  of  uveal  pigment,  434. 
Edge  of  a  prism,  466. 
Effect  upon  the  field  of  vision  of  removal  of  the 

visual  area  of  the  cortex,  410. 
Effects,  actinic,  583. 

of  decortication,  duration  of,  412. 
Egg-tooth,  the,  28. 
Electrical  processes  in  the  retina  produced   by 

light,  645. 
transformation  of  light  into,  617. 

rays,  583. 
Embryology,  anatomy,  and  physiology   of  the 

eye,  7. 

Embryonic  area  of  the  blastoderm,  8. 
Eminences,  superciliary,  79. 
Emmetropia,  114,  488,  489. 

form  of  the  ciliary  muscle  in,  267. 
Emmetropic  eye,  483,  489. 
End-arteries  of  the  retina,  338. 

-bulbs  of  the  corneal  nerves,  239. 

-organs,  blue-sensitive,  595. 
green-sensitive,  595. 
red-sensitive,  595. 
Endothelium,  anterior,  of  the  iris,  273. 

of  the  membrane  of  Descemet,  150. 
Enophthalmos,  120. 
Entocornea,  the,  149. 
Entoderm,  the,  8. 

derivatives  of  the,  8. 
Entrance,  optic,  335. 
Entropion,  429. 

congenital,  429. 
Epicanthus,  426. 
Epidermis  of  the   cornea,  development  of  the, 

38. 

Episclera,  124. 
Episcleral  net-work,  145. 
Epithelial  nerve-plexus  of  the  cornea,  238. 

optograms,  645. 
Epithelium  humoris  aquei,  150. 

of  the  cornea,  external,  141,  142,  218,  219. 
internal,  141,  143,  150,  218. 

of  the  lens,  355. 

of  the  membrane  of  Descemet,  150. 

retinal,  619. 

fuscin  of,  622. 
Equator  of  the  eye,  111. 

of  the  lens,  113. 
Errors  of  refraction,  489. 
Ethmoidal  arteries,  101,  103. 
anterior,  101,  103. 
posterior,  101,  103. 

foramina,  anterior  and  posterior,  73. 
Excavatio  papillae  nervi  optici,  193. 

physiologica,  193. 
VOL.  I.— 42 


Excavation  of  the  optic  disk,  193. 

physiological,  192,  336. 
Exophthalmos,  120. 
Experiments,  ablation,  410. 
Extent  of  the  visual  area,  ill). 
External  coat  of  the  eye.  1  '.';>. 

epithelium  of  the  cornea,  141,  142,  147,  218. 

219. 

fibrous  tunic  of  the  eyeball,  217. 
palpebral  ligament,  79. 
plexiform  layers  of  the  retina,  288,  303. 
Eye,  ametropio,  489. 

anatomy  of  the  appendages  of  the,  70. 
of  the  region  of  the,  79. 

and   eyelids,   congenital   defects  of    move- 
ments of  the,  429. 

axis  of  the,  111. 
external,  111. 
internal,  111. 

coats  of  the,  109. 

development  of  the,  7,  28. 

dioptrics  of  the,  459. 

embryology,  anatomy,  and  physiology  of  the, 
7. 

emmetropic,  489. 

equator  of  the,  111. 

external  coat  of  the,  125. 

fibrous  coat  of  the,  125. 

human,  congenital  malformations   and  ab- 
normalities of  the,  417. 

hyperopic,  490. 

lateral  position  of  the,  25. 

median  position  of  the,  10. 

meridians  of  the,  111. 

middle  tunic  of  the,  254. 

myopic,  far  point  of,  493. 

nervous  tunic  of  the,  287. 

normal  emmetropic,  483. 

ophthalmometric  measurements  of  the,  485. 

parietal,  the,  10. 

poles  of  the,  111. 

primary  lateral  position  of  the,  17. 

reduced,  483,  487. 

refraction  of  the,  181. 

schematic,  483,  485. 

trichromic,  597. 

tunics  of  the,  109. 

vascular  tunic  of  the,  254. 

Eyeball,  accessibility  of  the,  for  operation,  118, 
119. 

anatomy  of  the,  109. 

and   its   appendages,  development  of    the, 
417. 

asymmetry  of  the,  113. 

axial  length  of  the,  115. 

congenital  abnormalities  of  the,  419. 

contrasts  of  the,  197. 

cysts  of  the,  421. 

development  of  muscles  of  the,  64. 

dimensions  of  the,  113. 

ectodermic  portions  of  the,  217. 

external  fibrous  tunic  of  the,  217. 

first  indication  of  the,  15. 

foundation  of  the,  18,  19. 

general  characters  of  the,  109. 

humors  of  the,  197. 

inner  nervous  tunic  of  the,  217. 

mesodermic  portions  of  the,  217. 

microscopical  anatomy  of  the,  217. 

middle  vascular  tunic  of  the,  217. 

nasal  hemisphere  of  the,  113. 

position  of  the,  during  sleep,  121. 

relations  of  the,  116. 

situation  of  the,  116,  117. 

temporal  hemisphere  of  the,  113. 

ti'ii-inn  of  tin-,  I9',i. 

variation*  of  the,  1 1-. 


658 


INDEX   TO   VOLUME   I. 


Eyeball,  volume  of  the,  116. 

weight  of  the,  115. 
Eyebrows,  dermoid  cysts  of  the,  425. 

description  of  the,  79. 

piebald,  425. 

variability  of  the,  425. 
Eyelashes,  description  of  the,  80. 

development  of  the,  32. 
Eyelids,  arteries  of  the,  85. 

coloboma  of  the,  427. 

congenital  abnormalities  of  the,  427. 

dermoid  cysts  of  the,  425,  429. 

description  of  the,  80. 

development  of  the,  30,  31,  419. 

growths  of  the,  429. 

layers  of  the,  88. 

lymphatics  of  the,  86. 

movements  of  the,  80,  81. 

naevi  of  the,  429. 

nerves  of  the,  86. 

third,  123. 

veins  of  the,  86. 

Eye-muscle  nerves,  connection  of  the  optic  fibres 
with  nuclei  of,  403. 

nuclei  of,  399. 
Eyes,  asymmetry  of  the,  84. 

color  of  the,  177. 

multiple,  424. 

of  birds,  maximum  volume  of,  25. 

of  different  animals,  comparison  of,  115. 

of  fishes,  maximum  volume  of,  25. 

of  mammals,  maximum  volume  of,  25. 

F. 

Facial  artery,  85. 

nerve,  86. 

vein,  86. 

Falx  cerebri,  development  of  the,  25. 
Far  point  of  hyperopic  eye,  490. 

of  myopic  eye,  493. 
Fascia,  bulbar,  123. 

Tenoni,  123. 
Fasciculus  occipitalis  perpendicularis,  413. 

transversus  lobuli  lingualis,  414. 
Fatigue,  526. 
Fechner's  law,  514. 
Fenestra  oculi,  the,  182. 
Fibres  arcuatae,  144,  149. 

corneae,  224. 

propriee,  413. 
Fibres,  association,  413. 

centrifugal,  388. 

centripetal,  48,  388. 

cilio-equatorial,  375,  378. 

cilio-postero-capsular,  375,  378. 

classes  of,  388. 

crossing,  of  the  optic  chiasm,  389,  392. 

interhemispherical,  of  the  corpus  callosum, 
414. 

intra-ciliary,  379. 

number  of,  in  the  optic  nerve,  387. 

of  eye-muscle  nerves,  403. 

of  Miiller,  319. 

of  the  corona  radiata,  414. 

of  the  optic  nerve,  centrifugal,  48. 

optic,  connection  of,  with  nuclei,  106. 

orbiculo-antero-capsular,  375,  376. 

orbiculo-ciliary,  379. 

orbiculo-postero-capsular,  375. 

projection,  414. 

radiating,  414. 

retinal,  connected  with  fore-brain,  394. 
with  mid-brain,  394. 

size  of,  in  optic  nerve,  387. 

suturales,  144. 

tangential,  414. 

tapetal,  414. 


Fibres,  terminal,  of  corneal  nerves,  239. 

zonular,  374. 
Fibrous  coat  of  the  eye,  125. 

cordage  of  Bowman,  149. 

layer  of  the  cornea,  143. 

tissue  of  the  sclera,  243. 

tunic  of  the  eyeball,  217. 
Field  of  view,  541. 

binocular,  542. 

of  vision,  effect  of  removal  of  visual  area  of 

cortex  upon,  410. 
monocular,  542. 
stereoscopic,  391. 

Fifth  nerve,  development  of  the,  53,  55. 
Filaments  of  Arnmon,  165. 
First  cerebral  vesicle,  25. 

distinct  tracts  of  the  eyes,  12. 

embryonic  cerebral  vesicle,  11. 

focal  line,  496. 

nodal  point,  476. 

plane  of  lens,  476. 

principal  focus,  484. 

visceral  cleft,  28. 
Fissure,  calcarine,  406,  414. 

chorioid,  20,  22,  47. 

collateral,  406. 

of  Rolando,  406. 

of  Sylvius,  404. 

parallel,  406. 

parieto-occipital,  406. 

sphenoidal,  76. 

spheno-maxillary,  73,  74. 
Fistula  of  the  lacrymal  sac,  426. 
Fixed  cells  of  the  cornea,  145. 
Flexure,  the  cranial,  17. 
Flight  of  steps,  Schroder's,  540. 
Floor  of  the  orbit,  73. 
Fluctuation  of  the  retina,  negative,  647. 

positive,  647. 

Fluids  of  the  corneal  spaces,  234. 
Fluorescence,  592. 

of  the  rods,  631. 
Focal  distance,  principal,  462,  470,  472. 

interval  of  Sturm,  497. 

line,  first  or  anterior,  496. 

second  or  posterior,  496. 
Foci,  conjugate,  463,  471. 
Focus,  principal,  462,  463,  470,  472,  478. 

first  or  anterior,  484. 

real,  472. 

second  or  posterior,  484. 

virtual,  463. 

Fold,  semilunar,  122,  123. 
Folds,  ciliary,  168. 

Fontana,  spaces  of,  152,  236,  249,  250. 
Foramen  centrale,  195. 

corneae,  130. 

malar,  73. 

optic,  73,  76,  130,  131. 

opticum  sclerotica,  130. 

sclerae  anterius,  130. 

spheno-malar,  73. 

supra-orbital,  71. 
Foramina,  anterior  and  posterior  ethmoidal,  73. 

optic,  position  of  the,  83. 
Fore-brain,    retinal   fibres    connected   with    the, 

394. 
Formation  of  images  by  mirrors,  463. 

of  the  aqueous  humor,  198. 

of  the  central  nervous  system,  385. 

of  the  pupil,  51. 

of  the  tear-duct,  26. 

of  the  upper  lip,  26. 
Forms,  geometrical,  of  lenses,  473. 
of  Jastrow  and  Oliver,  525. 
of  Snellen,  225. 
Fornix  of  the  conjunctiva,  122. 


INDEX   TO   VOLUME   I. 


859 


Fossa,  hyaloid,  206. 

hyaloidea,  206. 

lenticularis,  206. 

patellaris,  2()6,  362. 
Fourth  nerve,  course  of  the,  105,  107. 

development  of  the,  53,  58. 

nucleus  of  the,  400,  401. 
posterior,  401. 

ventricle,  24,  29. 
Fovea  centralis,  195,  327. 

externa,  332. 

interna,  331. 
Franklin,  Christine  Ladd,  translator  of  Dr.  Brod- 

hun's  article,  539. 
Fraunhofer's  lines,  584. 
Frontal  muscle,  79. 

nerve,  course  of  the,  106,  107. 

sinus,  70,  73,  75. 
"    vein,  86,  103. 
Fronto-nasal  process,  26. 
Fuchs's  coloboma,  445. 

Fundamental  nerve-plexus  of  the  cornea,  238. 
Funiculus  soleras,  135. 
Fuscin,  migration  of  the,  642. 
Fusion  of  sensation,  535. 


G. 


Ganglion  cells  of  the  retina,  44,  45,  288,  312. 
ciliary,  development  of  the,  58. 
Gasserian,  development  of  the,  58. 
retinal,  307,  340. 

Ganglionic  tracts,  development  of  the,  24. 
Gasserian  ganglion,  development  of  the,  58. 
Gastrula,  384. 

Geniculate  bodies,  development  of  the,  24.. 
Geometrical    forms    of    Jastrow    and    Oliver, 

525. 

of  Snellen,  525. 

Germ-cells  of  the  nervous  system,  384. 
Germinal  area,  7. 
Giant  bipolars,  309. 
Girdle,  visual,  626. 
Gland,  uveal,  269. 
Glands,  ciliary,  167,  268. 
lacrymal,  92, 109. 

abnormalities  of  the,  427. 
anatomy  of  the,  92. 
development  of  the,  32,  419. 
Meibomian,  88,  92. 

development  of  the,  32,  419. 
of  Moll,  88. 
of  the  conjunctival  sac,  development  of  the, 

33. 
Glass,  crown  and  flint,  indexes  of  refraction  of, 

468, 

Glaucoma,  congenital,  423. 
Granules,  myeloidin,  621. 

of  the  retinal  rods,  migration  of,  644. 
Gratiolet,  optic  radiations  of,  414. 
Green-blind,  600,  603. 

sensitive  end-organs,  595. 
Groove,  cornea),  131. 

external  lacrymo-nasal,  29. 

closure  of  the,  30. 
infra-orbital,  73. 
lacrymal,  72,  75. 
lacrymo-nasal,  26. 
medullary,  8. 

Ground-substance  of  the  cornea,  222. 
Growths,  congenital,  of  the  conjunctiva,  430. 
of  the  cornea,  dernioid,  432. 
of  the  eyelids,  429. 

congenital,  429. 
Guanin,  deposit  of,  642. 
Gyrus,  angular,  406. 


H. 

Haploscope,  the,  553. 

mirror,  the,  557. 
Hare-lip,  production  of,  26. 
Head-cavities,  64. 

mandibular,  66. 

premandibular,  68. 
Height  of  the  orbit,  77. 

in  the  infant,  78. 
Helmholtz's  stereoscope,  548. 

telestereoscope,  549. 

theory  of  color-perception,  598. 
Hemisphere  of  the  eyeball,  nasal,  113. 

temporal,  113. 

Bering's  theory  of  color-perception,  613. 
Heterochromia  of  the  iris,  434. 
Heteronymous  images,  572. 
Hill,  Alex,  M.A.,  M.D.,  on  the  anatomy  of  the 
intra-cranial  portion  of  the  visual  apparatus, 
383. 

His,  zones  and  plates  of,  54,  55. 
Histogeny  of  the  nervous  system,  384. 
Histology  of  the  optic  nerve,  215,  216. 
Homogeneous  light,  582. 
Homonymous  images,  572. 
Horizontal  cells  of  the  retina,  304. 
Homer's  muscle,  87,  93,  94. 
development  of,  70. 
Horopter,  the,  558. 
Hue,  589. 

Human  eye,  congenital  malformations  and  ab- 
normalities of  the,  417. 
Humor,  aqueous,  111,  198,  217,  380. 
formation  of,  49,  198. 

crystallinus,  200. 

vitreus,  111,  206,  209,  362. 

development  of,  50. 
Hyaloid  artery,  197,  337. 
persistent,  454. 

canal,  208,  368. 

fossa,  206. 

membrane,  50,  206,  363,  370. 

vessels,  33. 
Hyaloidea,  206. 

interna,  206. 

Hydrophthalmos  congenitus,  423. 
Hypermetropia,  114. 
Hyperopia,  490. 

correcting,  491. 

of  curvature,  490. 
Hyperopic  eye,  far  point  of,  490. 

I. 

Idio-retinal  light,  506. 
Image,  real,  477. 

virtual  and  erect,  477. 
Images,  formation  of,  by  mirrors,  463. 

heteronymous,  572. 

homonymous,  572. 

of  Purkinje,  172. 

of  Sanson,  172. 

superposition  of,  in  the  brain,  393. 
Imperfect  normal  vision,  606. 
Importance  of  the  photo-chemical  process   for 

vision,  639. 

Incidence,  angle  of,  461. 
Incident  rays,  459. 
Index  myopia,  493. 

of  refraction  of  the  cornea,  137. 

of  refraction  of  the  crystalline  lens,  482. 

orbital,  of  Broca,  72. 
Indexes  of  refraction  of  crown  and  flint  glass, 

468. 

Induction  current  of  the  optio  nerve,  649. 
Inertia,  principles  of,  528. 


660 


INDEX   TO   VOLUME   I. 


Inferior  oblique  muscle,  98. 

rectus  muscle,  91,  92. 

tarsus,  92. 
Infra-orbital  artery,  85. 

canal,  75. 

groove,  73. 
Infra-trochlear  nerve,  86. 

course  of  the,  106,  107. 
Inner  coat,  191. 

nervous  tunic  of  the  eyeball,  217. 

nuclear  layer  of  the  retina,  288,  307. 

reticular  layer  of  the  retina,  288. 
Insertions  of  the  recti,  128, 129. 

of  oblique  muscles,  129. 
Intensity  and  color,  519. 

and  field  of  vision,  521. 

and  sharpness  of  sight,  521. 

of  light,  505. 

unit  of,  566. 
Interfascial  space,  123. 
Interhemispherical  fibres  of  the  corpus  callosum, 

414. 

Intermittent  stimulation,  532. 
Internal  basement  membrane,  149,  231. 

endothelium  of  the  cornea,   141,   143,   150, 
232. 

epithelium  of  the  cornea,  141,  143,  150. 

maxillary  vein,  86. 

palpebral  ligament,  79. 

plexiforin  layer  of  the  retina,  288,  292,  311. 
Interstitial  injections  into  the  cornea,  225. 
Intervaginal  space,  214. 
Intervals  in  the  solera,  130. 
Intra-cerebral  optic  tract,  415. 

-ciliary  fibres,  379. 

-cranial   portion   of   the   visual   apparatus, 

anatomy  of  the,  383. 
segment  of  the  optic  nerve,  341. 

-epithelial  nerve-plexus  of  the  cornea,  239. 
plexus  of  the  cornea,  157. 

-hemispherical  commissures,  long,  413. 

-ocular  segment  of  the  optic  nerve,  341. 
tension,  159. 

-orbital  portion  of  the  optic  nerve,  210,  213, 

341. 

Intrinsic  light  of  the  retina,  609. 
Irideremia,  434,  440. 
Irido-corneal  angle,  1 52. 
Iridodialysis,  congenital,  436. 
Iridodonesis,  434,  440. 
Iris,  the,  111,  174,  254,  272,  522. 

angle  of  the,  152. 

anterior  boundary  layers  of  the,  177,  273. 
endothelium  of  the,  177,  273. 
layer  of  epithelium  of  the,  177. 
synechiae  of  the,  439. 

basilar  layer  of  the,  177. 

blood-vessels  of  the,  283. 

coloboma  of  the,  434. 

congenital  abnormalities  of  the,  434. 

development  of  the,  51. 

lesser  circle  of  the,  177. 

lymphatics  of  the,  284. 

muscular  tissue  of  the,  276. 

nerves  of  the,  190,  285. 

pectinate  ligament  of  the,  151. 

pigment-layer  of  the,  273,  275,  276,  281. 

pillars  of  the,  151. 

posterior  limiting  lamella  of  the,  273,  280. 

posterior  layer  of  epithelium  of  the,  177. 

retinal  portion  of  the,  176. 

stroma  of  the,  177. 

development  of  the,  52. 

thickness  of  the,  174. 

tremulous,  440. 

uveal  portion  of  the,  176. 

variations  in  color  of  the,  434. 


Iris,  vascular  stroma  layer  of  the,  273,  274. 
Irregular  astigmatism,  500. 
reflection,  460. 

J- 

Jackson,  Edward,  A.M.,  M.D.,  on  the  dioptrics 

of  the  eye,  459. 
Janitrix  oculi,  the,  182. 

Jastrow  and  Oliver,  geometrical  forms  of,  325. 
Junction,  corneo-scleral,  131. 
Juncture,  sclero-corneal,  246. 


K. 


Katabolic  change,  613. 
Keratoiditis,  136. 
Kyanophane,  632. 

L. 

Lacrymal    apparatus,    congenital    abnormalities 

of  the,  426. 
arteries,  101,  103,  107. 
artery,  85. 

development  of  the,  36,  37. 
bay,  description  of  the,  80. 
canaliculi,  malformations  of  the,  426. 
canals,  anatomy  of  the,  92. 
caruncle,  the,  80. 
ducts,  anatomy  of  the,  92. 
course  of  the,  84. 
description  of  the,  84. 
development  of  the,  419. 
gland,  92,  1 07. 

abnormalities  of  the,  427. 
anatomy  of  the,  92. 
development  of  the,  32,  419. 
groove  and  canal,  72,  75. 
nerve,  86,  90. 

course  of  the,  106,  107. 
papilla;,  anatomy  of  the,  92. 

description  of  the,  83,  84. 
sac,  79. 

anatomy  of  the,  93,  94. 
description  of  the,  84. 
fistula  of  the,  426. 
stricture  of  the,  427. 
vein,  104. 
Lacrymo-nasal  groove,  the,  26,  29. 

closure  of  the,  30. 

Lacunae,  lymphatic,  of  the  cornea,  227. 
Lacus  lacrymalis,  development  of  the,  32. 
Lagophthalmos,  429. 
congenital,  429. 
Lame  cartilagineuse,  149. 
Lamella,  corneal,  218. 

posterior  limiting,  of  iris,  273,  280. 
pupillary,  218. 

Lamellated  tissue  of  the  cornea,  143. 
Lamina  basalis,  165. 
basilaris,  165. 
cribrosa,  132,  242,  345. 
elastica  anterior,  147,  148,  221. 

chorioidea,  165. 
fusca,  126,  244. 
glassy,  of  the  chorioid,  260. 
suprachorioidea,  161,  256. 

development  of  the,  49. 
vasculosa,  160,  163. 
vitrea  chorioidea,  165. 

Lang,  William,  F.R.C.S.,  Eng.,  and  Collins,  E. 
Treacher,  F.R.C.S.,  Eng.,  on   congenital   mal- 
formations and    abnormalities  of  the  human 
eye,  417. 
Lateral    anterior   small-celled   nucleus  of  third 

nerve,  402. 

nucleus  of  third  nerve,  402. 
Law  of  relativity,  517. 


INDEX   TO  VOLUME   I. 


661 


Law,  Fechner's,  574. 

Talbot's,  535. 

Weber's,  514. 
Laxator  pupillaa,  186. 
Layer  of  chorioidal  stroma,  255,  257. 
Layers  of  the  cornea,  141,  218. 

of  the  eyelids,  88. 

of  the  iris,  273. 

of  the  retina,  288. 
Lens,  the,  200. 

-capsule,  38,  203,  351,  353. 

development  of  the,  50,  418. 

cement-substance  of  the,  358. 

cleavage-figures  of  the,  37. 

coloboma  of  the,  449. 

concavo-convex,  473. 

congenital  abnormalities  of  the,  448. 

converging,  473. 

convexo-concave,  473. 

correcting,  491,  493. 

cortical  substance  of  the,  204. 

crystallina,  111,  200,  217,  349. 

crystalline,  index  of  refraction  of  the,  482. 

cylindrical,  determination  of  the  axis  of  a, 

480. 
refraction  by  a,  478. 

determination  of  the  strength  of  a,  479. 

development  of  the,  25,  34,  35,  37. 

diverging,  473. 

double  concave,  473. 

double  convex,  473. 

ectopia  of  the,  450. 

epithelium  of  the,  355. 

equator  of  the,  113. 

-fibres,  40,  41. 

magnifying,  478. 

minifying,  478. 

nuclear  spindles  of  the,  37. 
zone  of  the,  356. 

nucleus  of  the,  204. 

nutrition  of  the,  206. 

optical  centre  of  the,  473. 

determination  of  the,  480. 

periscopic,  473. 

planes  of  a,  476. 
first  plane,  476. 
second  plane,  476. 

plano-concave,  473. 

plano-convex,  473. 

points  of  a,  476. 

principal  focal  distance  of   a,  to  ascertain, 
470. 

rudiment  of  the,  18,  19. 

-stars,  141,  205,  359. 

stereoscope  of  Brewster,  547. 

subcapsular  epithelium  of  the,  351. 

substance,  350,  356. 

suspensory  apparatus  of  the,  372. 
ligament  of  the,  207. 

development  of  the,  50. 

to  ascertain  the  principal  focal  distance  of  a, 
470. 

vesicle,  38,  39,  41,  417,  418. 

weight  of  the,  42. 

whorl,  355. 

zonular  lamella  of  the,  353. 
Lenses,  concave,  refraction  by,  472. 

convex  spherical,  refraction  by,  469. 

cylindrical,  498. 

combinations  of,  498. 

forms  of,  473. 

numbering  of,  473,  478. 
Lenticonus,  448. 
Lenticular  body,  299. 
Leptomeninx  ophthalmencephali,  158. 
Lesser  circle  of  the  iris,  177. 
Leuxopsin,  628. 


Levator  palpebrae,  91. 
Ligament,  external,  79. 

internal  palpebral,  79. 

large  des  paupieres,  78. 

Lockwood's  suspensory,  108. 

pectinate,  development  of  the,  53. 

suspensory,  of  the  lens,  207. 
Ligamentum  annulate,  151. 

pectinatum  iridis,  151,  232,  248 
Light,  582. 

chaos,  609. 

comparison  of  magnitudes  of,  513. 

compound,  583. 

dispersion  of,  468,  469. 

dust,  609. 

electrical  processes  in  the   retina  produced 
by,  645. 

homogeneous,  582. 

idio-retinal,  506. 

intensity  of,  505. 

intrinsic,  of  the  retina,  609. 

mechanical  alterations  in   the   retina  pro- 
duced by,  640. 

mixed,  583. 

monochromatic,  582. 

perception  of,  505. 

of  small  differences  of,  508. 

ray  of,  459,  582. 

reacting,  610. 

reaction-time  on,  and  the  time  of  percep- 
tion of,  536. 

reflection  of,  460. 

refraction  of,  464. 

time  of,  523. 

transformation  of,  into  electrical  processes, 
617. 

undulation  theory  of,  459,  582. 

vibrations  of,  459. 
Limbus  conjunctivas,  122. 
Limitans  ciliaris,  334. 
Limiting  angle  of  a  prism,  468. 

lamella,  posterior,  of  the  iris,  273,  280. 

layer  of  the  cornea,  anterior,  141,  143,  218, 

221. 

posterior,  141,  143,  149,  218,  231. 
Lines  of  fixation,  501. 

Fraunhofer's,  584. 

visual,  501. 

Lip,  upper,  formation  of  the,  26. 
Lipochrin,  620. 
Liquor  Morgagni,  205,  356. 
Lissajous's  curves,  535. 
Localization,  568. 

Lockwood's  suspensory  ligament,  108. 
Long  ciliary  arteries,  101,  103,  269. 

intra-heinispherical  commissures,  413. 
Loss  of  accommodation  with  age,  504. 
Lumen  oculi,  182. 

Luschka's  line  for  the  lacrymal  duct,  84. 
Lustre,  579. 
Lutein,  620. 

Lymphatic  lacunas  of  the  cornea,  227. 
Lymphatics  of  the  chorioid,  261. 

of  the  conjunctiva,  236. 

of  the  cornea,  234. 

of  the  eyelids,  86. 

of  the  iris,  284. 

of  the  optic  nerve,  349. 

of  the  retina,  340. 

development  of  the.  45,  46. 

of  the  solera,  246. 

orbital,  104,  145. 
Lymph-channels  of  the  cornea,  237. 

-passages  of  the  cornea,  144. 

-space,  supra-chorioid,  356. 

-spaces  of  the  solera,  244. 

-tracts,  perineural,  236. 


662 


INDEX   TO   VOLUME   I. 


M. 


Macula  albida,  193. 

arcuata,  139,  431. 

coloboma  of  the,  445. 

corneas,  139. 

lutea,  327. 

Macular  bundle  of  Michel,  137. 
Maculary  fascicle  of  the  optic  chiasm,  389. 
Magnifying  lens,  478. 
Magnitudes  of  light,  comparison  of,  513. 
Malar  foramen,  73. 

Malformations  and  abnormalities  of  the  human 
eye,  congenital,  417. 

of  the  caruncle,  427. 

of  the  lacrymal  canaliculi,  436. 
Mandibular  head-cavity,  the,  66. 
Mantle-layer  of  neuroglia,  56. 
Marasmus  senilis  corneae,  139. 
Marginal  looped  plexus  of  the  cornea,  155. 
Mariotte's  spot,  193. 
Maxillary  processes,  27,  29. 
Maximum    sensation,    the    threshold    and   the, 

523. 
Mays,  Carl,  M.D.,  on  the  photo-chemistry  of  the 

retina,  617. 
Measurements,    ophthalmometric,  of    schematic 

eye,  486. 
Mechanical  alterations  in  the  retina  produced 

by  light,  640. 

Mechanism  of  binocular  vision,  372. 
Median  eye,  10. 
Medium,  dioptric,  459. 
Medulla,  rudiment  of  the,  25. 
Medullary  canal,  9. 

groove,  8. 
Meibomian  glands,  92. 

development  of  the,  32,  419. 
Membrana  Bruchii,  165. 

capsularis,  35,  36. 

capsulo-pupillaris,  35,  36,  352. 

Demoursi,  149. 

Demoursiana,  149. 

Descemeti,  149. 

Descemetiana,  149. 

Duddelliana,  149. 

humoris  aquei,  149. 

hyaloidea,  206,  363,  370. 

intra-chorioidea,  165. 

limjtans  externa,  320. 

nictitans,  123. 

development  of  the,  28. 

pigmenti,  165. 

pro  humore  aqueo,  149. 

pupillaris,  35,  36,  49,  352. 

suprachorioidea,  161. 

villoso-glandulosa,  161. 

vitrea,  206. 
Membrane,  hyaloid,  50. 

of  Bowman,  147. 

of  Bruch,  260. 

of  Demours,  149,  231. 

of  Descemet,  149,231. 

endothelium  of  the,  150. 
epithelium  of  the,  150. 

of  Duddell,  149. 

of  Reichert,  147. 

of  the  aqueous  humor,  149. 

pupillary,  persistent,  437. 

vascular  pupillary,  218. 

vitreous,  203,  256,  260. 
Meniscus,  473. 
Meridians  of  astigmatism,  497. 

of  the  eye,  111. 

Merkel's  line  for  the  lacrymal  duct,  84. 
Mesial  anterior  small-celled  nucleus  of  the  third 
nerve,  402. 


Mesochorioidea,  163. 
Mesocornea,  143. 
Mesoderui,  68. 

derivatives  of  the,  8. 
Mesoderinic  portions  of  the  eyeball,  217. 
Methods  for  studying  the  accuracy  of  percep- 
tion, 517. 

Metric  system,  474. 
Michel,  macular  bundle  of,  317. 
Microcoria,  435. 
Microphthahnos,  420. 
Microscope,  stereoscopic,  551. 
Microscopical  anatomy  of  the  eyeball,  217. 
Mid-brain,  connection  of  optic  tract  with  the,  397. 

retinal  fibres  connected  with  the,  394. 
Middle  coat,  158. 

temporal  artery,  85. 

tunic  of  the  eye,  254. 

vascular  tunic  of  the  eyeball,  217. 
Migration  of  granules  of  retinal  rods,  644. 
Minifying  lens,  478. 
Minimum  deviation,  467. 
position  of,  467. 
Mirror,  concave,  463. 

convex  spherical,  464. 

-haploscope,  557. 

plane,  463. 

-stereoscope  of  Wheatstone,  547. 
Mixed  light,  683. 
Molecular  layers  of  the  retina,  21. 
Moll,  glands  of,  88. 
Monochromatic  aberration,  470. 

color-blindness,  600. 

light,  582,  583. 
Monocular  field  of  vision,  542. 

vision,  perception  of  depth  in,  539. 
Monro,  sulci  of,  55. 
Motor  fibres  of  the  ciliary  body,  271. 

root'of  the  trigeminal  nerve,  development  of 

the,  59. 
Movements  of  the  eye  and  eyelids,  congenital 

defects  of,  429. 
Miiller,  fibres  of,  319. 
Multiple  eyes.  424. 
Muscse  volitantes,  137,  211. 
Muscle,   ciliary,   51,  52,  123,  166,  168,  255,  262, 
266. 

corrugator  supercilii,  88. 

dilator,  of  the  iris,  279. 

external  rectus,  95,  96. 

Homer's,  87,  93,  94. 

inferior  oblique,  98. 
rectus,  91,  92,  97. 

internal  rectus,  97,  98. 

levator  palpebraa,  91,  92. 

Miiller's,  91,  92. 

of  Riolan,  ciliary,  87. 
Muscles,  oblique,  insertions  of,  129,  130. 

ocular,  absence  of,  430. 

of  the  eyeball,  development  of  the,  64. 

of  the  face,  development  of  the,  69,  76. 

orbital,  action  of  the,  90. 
anatomy  of  the,  95. 
development  of  the,  64. 

recti,  insertions  of  the,  128,  129. 
Muscular  tissue  of  the  iris,  276. 
Musculus  ciliaris,  168. 

circularis  iridis,  186. 

crystallinus,  200. 

radialis  iridis,  186. 
Muybridge's  photographs,  536. 
Myeloidin  granules,  621. 
Myograph,  pendulum,  524. 
Myopia,  114,  493. 

axial,  4!>3. 

index,  493. 

of  curvature,  493. 


INDEX   TO   VOLUME    I. 


663 


Myopic  eye,  far  point  of,  493. 

Myotomes  in  the  formation  of  the  orbital  mus 

cles,  64. 
relation  of  the  somites  to,  54. 

N. 

Naevi  iridis,  178. 

of  the  eyelids,  congenital,  429. 
Nasal  duct,  anatomy  of  the,  94. 
development  of  the,  29. 
hemisphere  of  the  eyeball,  113. 
membrane,  course  of  the,  106. 
process,  external,  27,  29. 
vein,  103. 

Naso-ciliary  nerve,  course  of  the,  106. 
Natural  colors,  589. 
Nature  of  Schlemm's  venous  canal,  250. 

of  the  spaces  of  Fontana,  250. 
Negative  after-images,  530,  609. 
fluctuation  of  the  retina,  647. 
picture  of  the  cornea,  145. 
pictures  of  the  corneal  spaces,  225. 
Neogenesis,  637. 
Nerve,  facial,  86. 

*     -fibres  of  the  retina,  opaque,  446. 
fifth,  development  of  the,  53,  58. 
fourth,  course  of  the,  105,  106,  107. 
development  of  the,  53,  58. 
nucleus  of  the,  400,  401. 

posterior,  401. 

frontal,  course  of  the,  106,  107. 
infra-trochlear,  86,  90,  106,  107. 
lacrymal,  86,  90. 

course  of  the,  106,  107. 
nasal,  course  of  the,  106. 
naso-ciliary,  106. 

oculo-motor,  nucleus  of  the,  400,  401. 
ophthalmic,  86. 

course  of  the,  106,  107. 
optic,  19,  109,  217. 

anatomy  of  the,  387. 
coloboma  of  sheath  of  the,  445. 
congenital  abnormalities  of  the,  448. 
course  of  the,  95. 
descending  root  of  the,  398. 
development  of  the,  46. 
histology  of  the,  215,  216. 
induction  current  of  the,  649. 
intra-cranial  segment  of  the,  341. 
intra-ocular  segment  of  the,  341. 
intra-orbital  segment  of  the,  210,  213, 

341. 

sheaths  of  the,  348. 
arachnoidal,  348. 
dural,  348. 
pial,  348. 
slack  of  the,  214. 
subarachnoid  space  of  the,  348. 
orbital,  course  of  the,  165. 
plexus,  fundamental,  238,  241. 
intra-epithelial,  239. 
of  the  cornea,  epithelial,  238. 
short  ciliary,  107. 
sixth,  course  of  the,  106. 

development  of  the,  53,  59. 
nucleus  of  the,  399. 
supra-orbital,  86. 

course  of  the,  106,107. 
supra-trochlear,  86,  90. 

course  of  the,  106,  107. 
third,  course  of  the,  105. 

development  of  the,  53,  57. 
nucleus  of  the,  400.  401. 
trigeminal,  development  of  the,  53,  58. 
trochlear,  nucleus  of  the,  400,  401. 
posterior,  401. 


Nerves,  ciliary,  development  of  the,  3«. 

eye-muscle,  connection  of  the   optic   fibre* 

with  the  nuclei  of  the,  403. 
nuclei  of  the,  399. 
of  the  chorioid,  261. 
of  the  ciliary  body,  270. 
of  the  ciliary  muscle,  173. 
of  the  cornea,  237. 
of  the  eyelids,  86. 
of  the  irig,  190,  285. 
of  the  solera,  246. 
optic,  course  of  the,  387. 

origin  of  the,  394. 
orbital,  105. 
Nervous  coat,  the,  191. 

system,  central,  phylogeny  of  the,  384. 
formation  of  the,  385. 
germ-cells  of  the,  384. 
histogeny  of  the,  384. 
ontology  of  the,  383. 
spongioblasts  of  the,  384. 
tunic  of  the  eye,  287. 
tunic  of  the  eyeball,  217. 
Net-work,  episcleral,  245. 
Neural  crest  in  relation  to  the  cranial  nerves  54 

of  Balfour,  20. 

Neuroblaste,  development  of  the,  56. 
Neuro-epithelial  layer  of  the  retina,  288,  296. 
Neurogha-cells  of  the  retina,  44. 
Neuroglia,  mantle-layer  of  the,  56. 
Neurokeratin,  623. 
Neuromeres,  relation  of  the,  to  the  cranial  nerves, 

54. 

Neuron,  394. 

Newton's  hypothesis  of  the  optic  chiasm,  390. 
Nigrum  oculi,  182. 
Nodal  point,  476,  478,  484. 
first,  476. 
second,  476. 

position  of  the,  491,  494. 
Normal  color -perception,  581. 

emmetropic  eye. 
Notch,  supra-orbital,  70,  82. 
Nuclear  cataracts,  452. 

spindles  of  the  lens,  37. 
zone  of  the  lens,  356. 
Nuclei  of  eye-muscle  nerves,  399. 

connection  of,  with  the  optic  fibres,  403. 
Nucleus  lentis,  350. 

of  the  fourth  nerve,  400,  401. 
of  the  lens,  204. 

of  the  oculo-motor  nerve,  400,  401. 
of  the  sixth  nerve,  399. 
of  the  third  nerve,  400,  401. 
of  the  trochlear  nerve,  400,  401. 
N'umber  of  fibres  in  the  optic  nerve,  387. 
Numbering  of  lenses,  473,  478. 
V  ut  ri tii >n  of  the  cornea,  156. 
Nystagmus,  congenital,  430. 

O. 

)tlique  muscles,  insertion  of  the,  12<J,  !•'  n. 
Occipital  cortex,  connection  of  the,  with  lower 

visual  centres,  413. 
region  of  the  cerebral  cortex,  topography  of 

the,  405. 
)ccipitalis  muscle,  79. 
Ocular  appendages,  congenital  abnormalities  in 

the,  425. 
conjunctiva,  122. 

corneal  portion,  122. 
scleral  portion,  ll"-'.  123. 
muscles,  absence  of  the,  430. 
Oculo-motor  nerve,  anterior  dorsal  nucleus  of  the, 

401. 
anterior  nucleus  of  the  ventral,  401. 


664 


INDEX   TO   VOLUME   I. 


Oculo-motor  nerve,  lateral  anterior  small-celled 

nucleus  of  the,  402. 
mesial  anterior  small-celled  nucleus  of 

the,  402. 
posterior  dorsal  nucleus  of  the,  401. 

ventral  nucleus  of  the,  401. 
true  ganglion  of  the,  57,  58. 
Olfactories,  first  traces  of  the,  18. 
Olfactory  pits,  23. 

Oliver,  Jastrow  and,  geometrical  forms  of,  525. 
Ontology  of  the  nervous  system,  383. 
Opacities  of  the  cornea,  congenital,  431. 
Opaque  nerve-fibres  of  the  retina,  446. 
Opening,  palpebral,  120,  121. 
Ophthalmic  artery,  85. 

anatomy  of  the,  101,  107. 
nerve,  86. 

course  of  the,  145. 
vein,  86. 

inferior,  104. 
superior,  103. 

Ophthalmo-facial  vein,  104. 
Ophthalmo-meningeal  vein,  104. 
Ophthalmometric  measurements  of  the  schematic 

eye,  486. 

.  Ophthalmoscope,  binocular,  550. 
Optic  axis,  483. 

primary,  485. 
canal,  130. 
chiasm,  anatomy  of  the,  389. 

crossing  fibres  of  the,  389,  392. 
development  of  the,  48. 
maculary  fascicle  of  the,  389. 
Newton's  hypothesis  of  the,  390. 
uncrossed  fibres  of  the,  389,  392. 
cup,  18-23,  27,  29,  30,  34,  38,  48. 
disk,  193. 

excavation  of  the,  193. 
entrance,  193,  335. 
fibres,  connection  of  the,  with  the  nuclei  of 

the  eye-muscle  nerves,  403. 
foramen,  73,  76,  130,  131,  213. 
foramina,  position  of  the,  83. 
globe,  27. 

nerve,  19,  109,  217,  341. 
anatomy  of  the,  387. 

of  intra-orbital  portion  of  the,  109. 
and  the  retina,  42. 
coloborna  of  the  sheaths  of  the,  445. 
congenital  abnormalities  of  the,  448. 
course  of  the,  95,  387. 
descending  root  of  the,  398. 
development  of  the,  42,  46. 
fibres  of  the  retina,  288. 
first  indication  of  the,  15. 
histology  of  the,  215,  216. 
induction  current  of  the,  649. 
intra-cranial  segment  of  the,  341. 
intra-ocular  segment  of  the,  341. 
intra-orbital  segment  of  the,  210,   213, 

341. 

portion  of  the,  213. 
lymphatics  of  the,  349. 
number  of  fibres  in  the,  387. 
sheaths  of  the,  348. 
arachnoidal,  348. 
dural,  348. 
pial,  348. 

size  of  fibres  in  the,  387. 
slack  of  the,  214. 
subarachnoidal  space  of  the,  348. 
nerves,  origin  of  the,  394. 
papilla,  193,  335. 

physiological  excavation  of  the,  336. 
radiations  of  Gratiolet,  414. 
stalk,  18,  19,  20,  21,  22,25. 

development  of  the,  47,  48. 


Optic  stalk,  embryonic,  14. 

thalatni,  rudiments  of  the,  23. 

thalamus,  395,  396. 

tract,  connection  of  the,  with  the  mid-brain, 

397. 

development  of  the,  24,  59. 
intra-cerebral,  415. 
tracts,  development  of  the  commissural  fibres 

of  the,  62. 

double  origin  of  the  fibres  of  the,  61. 
roots  of,  397. 

vesicles,  primary,  13,  17,  18. 
Optical  centre  of  the  lens,  determination  of  the, 

480. 

Optograms,  634. 
epithelial,  645. 
pseudo-,  634. 
Ora  serrata,  192,  255,  333. 

development  of  the,  51. 
Orbicularis  palpebrarum,  78. 
anatomy  of  the,  86. 
origin  of  the,  31. 

Orbiculo-antero-capsular  fibres,  375,  376. 
-ciliary  fibres,  379. 
-postero-capsular  fibres,  375. 
OrbicUlus  ciliaris,  166,  262. 

gangliosus,  156,  271,  285. 
Orbit,  adipose  body  of  the,  123. 
anatomy  of  the,  70. 
base  of  the,  78. 
bony  wall  of  the,  70. 
Orbital  aponeurosis,  123. 
fat,  the,  215. 
index  of  Broca,  72. 
lymphatics,  104,  105. 
margin,  palpation  of  the,  82,  83. 
muscles,  action  of  the,  98. 
anatomy  of  the,  95. 
development  of  the,  64. 
nerves,  105. 

course  of  the,  105. 
veins,  103. 
Orbito-ocular  aponeurosis,  123. 

fascia,  123. 

Orbits,  apex  of  the,  73. 
axes  of  the,  72,  117. 
borders  of  the,  71,  72. 
breadth  at  the  base  of  the,  77. 
capacity  of  the,  117. 
depth  of  the,  77. 
dimensions  of  the,  116. 
floor  of  the,  73. 
height  of  the,  77. 
roof  of  the,  72. 
shape  of  the,  116. 
size  of  the,  77. 

sympathetic  filaments  of  the,  105. 
Origin  of  the  fibres  of  the  optic  tracts,  double, 

61. 

of  sense-organs,  384. 
of  the  optic  nerves,  394. 
of  the  orbicularis  palpebrae,  31. 
Oscillations  of  after-images,  532. 
Otoscope,  binocular,  550. 
Outer  reticular  layer  of  the  retina,  288,  303,  311. 

P. 

Pachymeninx  ophthalmencephali,  125. 
Palpebra  tertia,  123. 
Palpebral  arteries,  101,  103. 

conjunctiva,  122. 

ligament,  external,  79. 
internal,  79. 

opening,  120,  121. 
Pannus,  156. 
Papilla  optica,  193,  335. 


INDEX   TO   VOLUME   I. 


666 


Papillae,  lacrymal,  anato'niy  of  the,  92. 

description  of  the,  83,  84. 
1'upillo-macular  bundle,  the,  216. 
Parallax,  stereoscopic,  544. 
Parallel  fissure,  406. 

surfaces,  refraction  by  a  plate  of  glass  with, 

466. 

Parieto-occipital  fissure,  406. 
Pars  chorioidealis  corneae,  143,  218. 

ciliaris  retina;,  288,  3.'54. 

conjunctivalis  corneas,  142,  218. 

cutanea  corneae,  142. 

iridica  retinas,  43,  272,  288,  334. 

optica  retinae,  192,  288. 

scleralis  corneae,  142,  218. 

uvealis  corneae,  143,  248. 
Path  of  visual  stimuli,  59,  60. 
Patheticus,  development  of  the,  50,  53. 
Pecten  sclerae,  132. 
Pectinate  ligament  of  the  iris,  151. 

development  of  the,  53. 
Pencil  of  rays,  459. 
Pendulum  myograph,  524. 
Perception,  413. 

methods  for  studying  the  accuracy  of,  517. 

of  depth  in  binocular  vision,  542. 
in  monocular  vision,  539. 

of  light,  505. 

of  small  differences  of  light,  508. 
Perfect  normal  vision,  606. 
Perforating  branches  of  the  corneal  nerves,  238. 
Pericapsular  membrane,  376. 
Perichorioid'al  space,  134. 
Perineural  lymph-tracts,  236. 
Peripheral  parts  of  our  retina,  color-vision  in, 

606. 

Periscopic  lens,  473. 
Persistent  hyaloid  artery,  454. 

pupillary  membrane,  437. 
Petit,  canal  of,  208,  379. 
Phosphorescence,  592. 

Photo-chemical    decomposition    of    the    visual 
purple,  628. 

process,  the  importance  of,  for  vision,  639. 
Photo-chemistry  of  the  retina,  617. 
Photogenic  rays,  583. 
Photographs,  Muybridge's,  536. 
Phototomes,  649. 
Phiylogeny  of  the  brain,  383. 

of  the  central  nervous  system,  384. 
Physiological  excavation,  193. 

of  the  optic  papilla,  336. 

retina,  192. 

Pial  sheath  of  the  optic  nerve,  348. 
Pictures,  stereoscopic,  543. 
Piebald  eyebrows,  425. 
Piersol,  George  A.,  M.D.,  on  the  microscopical 

anatomy  of  the  eyeball,  217. 
Pigment  layer  of  the  iris,  273,  275,  276,  281. 

layer  of  the  retina,  190,  288,  292. 

particles  in  the  retina,  migration  of  the,  294. 

uveal,  ectropion  of  the,  434. 
Pigmentation  of  the  chorioid,  defects  in,  446. 

of  the  sclerotic,  congenital,  431. 
Pigmented  areas  of  Necturus,  1 1 . 

connective-tissue  cells  of  the  sclera,  24-1. 

epithelium,  19. 
Pigments  of  the  cones,  632. 
Pillars  of  the  iris,  151. 
Pinna,  formation  of  the,  28,  30. 
Plane  mirror,  463. 

reflection  by  a,  461. 
Plano-concave  lens,  473. 

-convex  lens,  473. 
Plexus  annularis,  156,  237. 

ciliaris,  152. 
Plexuses,  vascular,  of  the  retina,  338. 


Plica  centralis,  195. 

semilunaris,  28,  34,  90,  123. 
Plicae  ciliares,  263,  264. 
Point,  nodal,  476,  478,  484. 
first,  476. 
second,  476. 
Polar  cataracts,  anterior,  452. 

posterior,  452. 
Polarized  light,  examination  of  the  cornea  by. 

224. 

Poles  of  the  eye,  111. 
Polycoria,  435,  436. 
Porus  opticus,  193. 
Position  of  the  eyeball  during  sleep,  121. 

of  minimum  deviation,  467. 

of  the  optic  foramina,  83. 

of  the  second  nodal  point,  491,  494. 
Positive  after-images,  528,  529,  609. 

fluctuation  of  the  retina,  647. 

picture  of  the  cornea,  145. 

of  the  corneal  spaces,  225. 
Posterior  chamber,  48,  111,  181,  254. 
recesses  of  the,  207. 

ciliary  arteries,  101,  103. 

dichotomy,  424. 

elastic  lamina,  149,  231. 

endothelium  of  the  cornea,  218,  232. 

ethmoidal  arteries,  101,  103.  i 

foramen,  73. 

focal  lines,  496. 

layer  of  epithelium  of  the  iris,  177. 

limiting  lamella  of  the  iris,  273,  280. 

layer  of  the  cornea,  141,  143,  149,  218, 
231. 

nucleus  of  the  trochlear  nerve,  401. 

polar  cataract*,  452. 

principal  focus,  484. 
Post-zonular  lymphatic  space,  208. 
Power  of  accommodation,  501. 
Precuneus,  406. 

Premandibular  head-cavity,  68. 
Presbyopia,  504. 
Primary  axis,  478,  485. 

colors,  591. 

germ-layers,  8. 

optic  vesicles,  13,  15,  38,  417. 

plexus  of  the  cornea,  156. 
Principal  axis,  476. 

anterior  or  first,  484. 

focal  distance  of  a  lens,  to  ascertain  the,  470. 

focus,  462,  463,  470,  472,  478. 
posterior  or  second,  484. 
Principles  of  inertia,  528. 
Prism,  apex  of  a,  466. 

base  of  a,  466. 

edge  of  a,  466. 

limiting  angle  of  a,  468. 

refracting  angle  of  a,  461. 

refraction  by  a,  466. 
Process,  external  nasal,  25,  27,  29. 

fronto-nasal,  27,  29. 

maxillary,  25. 
Processes,  ciliary,  51,  53,  166,  262,  264,  418. 

niiixillary,  27,  29. 

regenerative,  in  the  retina,  63fi. 

scleral,  internal  and  external,  -IT. 
Processus  peripherici,  151. 
Projection  fibres,  414. 
Proper  substance  of  the  cornea,  141,  142,  218, 

m. 

Protovertebrae  in  formation  of  the  orbital  muscles, 

64. 

Pseudo-optograms,  634. 
Pseudoscope,  Wheatstone's,  561. 
Psychical  blindness,  412. 
Ptosis,  congenital,  426,  430. 
Pulvinar,  395. 


666 


INDEX   TO   VOLUME   I. 


Puncta  lacrymalis,  development  of  the,  29. 
Punctum  csecum,  193. 
proximum,  502. 
remotum,  502. 
Pupil,  the,  182. 

abnormalities  of  the,  435. 
cause  of  contraction  of  the,  188. 
development  of  the,  28. 
dilatation  of  the,  189. 
formation  of  the,  51. 
Pupillary  lamella,  218. 

membrane,  persistent,  437. 

vascular,  218. 
Purity  of  color,  589,  590. 
Purkinje,  images  of  the,  172. 
Purple,  visual,  299,  625. 

bleaching  of  the,  634. 
chemical  action  of  the,  629. 
photo-chemical   decomposition  of  the, 
628. 

R. 

Radiating  fibres,  414. 

Radiations,  optic,  of  Gratiolet,  414. 

Radii  minores  iridis,  180. 

Ramus  ophthalmicus  profundus,  58. 

Raphe  sclerae,  135. 

Ray  of  light,  459,  582. 

Rays,  actinic,  583. 

electric,  583. 

incident,  459. 

pencil  of,  459. 

photogenic,  583. 

thermic,  583. 

ultra-red,  585. 

-violet,  585. 
Reacting  light,  610. 
Reaction,  acid,  of  fresh  retinae,  619. 

alkaline,  of  the  vitreous,  618. 

-time  on  light,  and  the  time  of  perception, 

536. 

Real  image,  477. 

Recesses  of  the  posterior  chamber,  207. 
Recti,  insertions  of  the,  128,  129. 

origins  of  the,  215. 
Red-blind,  600,  603. 
Red-green  blindness,  603. 
Red-sensitive  end-organs,  595. 
Reduced  eye,  483,  487. 
Reflecting  angle  of  a  prism,  466. 

surface,  459. 
Reflection  by  a  concave  spherical  mirror,  462. 

by  a  convex  spherical  mirror,  463. 

by  a  plane  mirror,  461. 

of  light,  460. 

of  the  conjunctiva,  88. 

regular,  460. 

Reflex  act  of  blinking,  537. 
Refraction  by  a  cylindrical  lens,  478. 

by  a  plate  of  glass  with  parallel  surfaces, 
466. 

by  a  prism,  466. 

by  concave  lenses,  472. 

by  convex  spherical  lenses,  469. 

errors  of,  489. 

index  of,  464. 

index  of,  of  a  crystalline  lens,  482. 

of  light,  451,  464. 

of  the  cornea,  index  of,  137. 

of  the  eye,  481. 

Regenerative  processes  in  the  retina,  636. 
Region,  ciliary,  166. 

of  accommodation,  502,  503. 

of  the  eye,  anatomy  of  the,  79. 
Regular  astigmatism,  495. 
Reichert,  membrane  of,  147. 


Relations  of  the  eyeball,  116. 

Relativity,  law  of,  517. 

Relief  telescope,  Zeiss's,  549. 

Removal  of  the  visual  area  of  the  cortex,  effect 

upon  the  field  of  vision  of,  410. 
Retina,  the,  217,  618. 

achromatic  cells  of  the,  312,  313. 

albumin  in  the,  619. 

alterations  in  the,  during  vision,  634. 

and  the  optic  nerve,  development  of  the,  42. 

arteries  of  the,  195. 

blood-vessels  of  the,  337. 

cells  of  the,  basal,  304. 
horizontal,  304,  305. 
stellate,  304. 

central  artery  of  the,  101,  103. 
vein  of  the,  104. 

chromophilous  cells  of  the,  312,  313. 

ciliary,  197. 

color-vision  in  peripheral  parts  of  our,  606. 

cone  ganglion-cells  of  the,  291. 

congenital  abnormalities  of  the,  446. 

corresponding  points  of  the,  553. 

development  of  the,  33,  34. 

disparate  points  of  the,  553. 

electrical  processes  in  the,  produced  by  light, 
645. 

embryonic,  inner  layer  of  the,  42. 
outer  layer  of  the,  43. 

end-arteries  of  the,  338. 

first  indication  of  the,  18,  19. 

fluctuation  of  the,  negative,  647. 
positive,  647. 

fresh,  acid  reaction  of  the,  619. 

ganglion-cells  of  the,  44,  45. 

intrinsic  light  of  the,  609. 

layers  of  the,  288. 

lymphatics  of  the,  340. 

mechanical  alterations  in  the,  produced  by 
light,  640. 

migration  of  pigment  particles  in  the,  394. 

neuro-epithelial  layer  of  the,  288. 

neuroglia-cells  of  the,  44. 

opaque  nerve-fibres  of  the,  446. 

photo-chemistry  of  the,  617. 

pigment  layers  of  the,  288. 

regenerative  processes  in  the,  636. 

spongioblasts  of  the,  309. 

stellate  neuroglia-cells  of  the,  325. 

zones  of  the,  42. 
Retinal  area,  21. 

cones,  shifting  of  the,  643. 

epithelium,  619. 

fuscin  of  the,  622. 

fibres  connected  with  the  fore-brain,  394. 
with  the  mid-brain,  394. 

points,  corresponding,  390. 

portion  of  the  iris.  176. 

rods,  migration  of  granules  of  the,  644. 

sustentacular  tissue,  319. 

tapetum,  628. 

vessels,  congenital  abnormalities  of  the,  447. 
Retractor  bulbi  muscle,  65. 
Rhodogenesis,  637. 
Rhodophane.  632. 
Rhodophyllin,  638. 
Rhodopsin,  299,  625. 
Ridge,  visual,  626. 
Rim,  scleral,  132. 
Rima  cornealis,  130. 
Ring,  chorioidal,  194. 

ciliary,  262. 

scleral,  194. 

Riolan,  ciliary  muscle  of,  87. 
Rivalry  of  visual  fields  and  of  contours,  576. 
Rod-bipolars,  308. 

-ellipsoid,  299. 


I1S7DEX   TO   VOLUME   I. 


Rod-fibre,  299. 

-granule,  299,  300. 
Rods  and  cones,  288,  296,  623. 

development  of  the,  21,  44. 

fluorescence  of  the,  631. 

retinal,  migration  of  granules  of  the,  644. 
Rolando,  fissure  of,  406. 
Roof  of  the  orbit,  72. 
Root  of  the  optic  nerve,  descending,  398. 
Roots  of  the  optic  tract,  397. 
Rotation,  centre  of,  501. 

Ryder,  John  A.,  Ph.D.,  on  the  development  of 
the  eye,  7. 

S. 

Sac,  conjunctiva!,  122. 

development  of  the,  31. 
lacrymal,  79. 

anatomy  of  the,  93,  94. 
description  of  the,  84. 
fistula  of  the,  426. 
stricture  of  the,  427. 
Sagittal  tract  of  Wernicke,  415. 
Sanson,  images  of,  172. 
Schematic  eye,  482,  485. 

ophthalmometric  measurements  of  the,  485 
Schiefferdecker,  axoplasm  of,  316. 
Schlemm,  canal  of,  135,  152,  247,  248. 
development  of,  50. 
nature  of,  250. 

Schroder's  flight  of  steps,  540. 
Schwalbe's  supra-vaginal  space,  100, 105. 
Sclera,  the,  110,  126. 

blood-vessels  of  the,  245. 
canal  of  the,  131. 

chemical  characters  of  the,  135,  136. 
density  of  the,  127. 
fibrous  tissue  of  the,  243. 
intervals  in  the,  130. 
lymphatics  of  the,  246. 
lymph-spaces  of  the,  244. 
nerves  of  the,  135,  246. 
pigmented  connective-tissue  cells  of  the,  244. 
thickness  of  the,  242. 
tint  of  the,  242. 
tissue-spaces  of  the,  243. 
vessels  of  the,  135. 
wandering  cells  of  the,  244. 
Scleral  nerves,  135. 

portion  of  the  cornea,  142. 
processes,  internal  and  external,  247. 
rim,  132. 
ring,  194. 
sinus,  152. 
sulcus,  111. 

Sclero-corneal  juncture,  246. 
Sclerophthalmos,  431. 
Sclerotic,  the,  126,  217. 

and  cornea,  development  of  the,  48. 
coat,  126,  242. 

congenital  pigmentation  of  the,  431. 
corpuscles,  243,  244. 
Sclerotica,  126. 
Second  cerebral  vesicle,  25. 
focal  line,  496. 
nodal  point,  476. 

position  of  the,  491,  494. 
plane  of  a  lens,  476. 
principal  focus,  484. 
Secondary  axis,  476,  478,  484. 

optic  vesicle,  38,  417. 
Semilunar  fold,  122,  123. 

plica,  123. 
Sensation,  413. 
fusion  of,  535. 

maximum,  the  threshold  and  the,  523. 
of  light  and  shade,  606. 


Sense-judgment,  413. 
Sense-organs,  origin  of  the,  384. 
Sensory  fibres  of  the  ciliary  body,  271. 
Septum  orbitale,  78. 

anatomy  of  the,  78,  79. 
Shade,  590. 

Shape  of  the  orbit,  116. 
Sharpness  of  sight,  intensity  and,  621. 
Sheath  of  the  optic   nerve,   coloboma  of   the, 

445. 

Sheaths  of  the  optic  nerve,  348. 
Shifting  of  the  retinal  cones,  643. 
Short  ciliary  nerves,  course  of  the,  107. 
Shortness  of  the  eyelids,  429. 
Sight,  intensity  and  sharpness  of,  521. 
Simultaneous  contrast,  611. 
Sinus  circularis  iridis,  152,  153. 
frontal,  70,  73,  75. 
Schlemmii,  152. 
sclerae,  152. 

venosus  corneae,  152,  153. 
iridis,  152,  153,  248. 
Schlemmii,  152,  248. 
sclerae,  152. 

Situation  of  the  eyeball,  116,  117. 
Sixth  nerve,  development  of  the,  53,  59. 

nucleus  of  the,  399. 
Size  of  the  cornea,  congenital  variations  in  the. 

432. 

of  the  fibres  in  the  optic  nerve,  387. 
of  the  orbit,  77. 

Slack  of  the  optic  nerve,  the,  214. 
Sleep,  position  of  the  eyeball  during,  121. 
Small  differences   of   light,    the   perception   of. 

508. 

Smallness,  congenital,  of  the  lens,  448. 
Snellen,  geometrical  forms  of,  525. 
Somites,  relations  of  the  cranial  nerves  to,  54. 
Space,  interfascial,  123. 
of  Fontana,  152. 
of  Tenon,  49,  245. 

development  of  the,  49. 
perichorioidal,  134. 
post-lenticular,  209. 
Schwalbe's  supra- vaginal,  100, 105. 
supra-chorioid,  255. 
Spaces,  corneal,  22. 

fluids  of,  234. 
negative  pictures  of,  225. 
positive  pictures  of,  225. 
lymph-,  of  the  solera,  244. 
of  Fontana,  236,  249,  250. 
tissue-,  of  the  solera,  243. 
zonular,  208. 
Spalding,  James  A.,  A.M.,  M.D.,  translator  of  Dr. 

Mays's  article,  617. 
Spatia  zonularia,  208. 
Spatium  interfasciale,  123. 
Specific  gravity  of  the  cornea,  141. 
Spectacles,  stenopaic,  500. 
Spectrum,  the,  584. 

visible,  583. 
phenoidal  fissure,  76. 
spheno-malar  foramen,  73. 
Spheno-maxillary  fissure,  73,  74,  76. 
Spherical  aberration,  470. 
Sphincter  iridis,  186. 

pupilhv,  186,  274,  276. 
Spinal  cord,  development  of  the,  24. 
"pindle  fibre-cells  of  the  iris  1'". 
Splenium,  414. 
~pongioblasts,  development  of  the,  56. 

of  the  retina,  309. 
Stars  of  Winslow,  !•;.">. 
Stellw  vasculosa.  \Vin.-lowii,  165. 
Stellate  cells  of  the  rftina,  304. 

neuroglia-cell  of  the  retina.  325. 


668 


INDEX   TO   VOLUME   I. 


Stenopaic  spectacles,  500. 
Steps,  Schroder's  flight  of,  540. 
Stereoscope,  Helinholtz's,  548. 
Stereoscopic  apparatus,  547. 

microscope,  551. 

parallax,  544. 

pictures,  543. 

vision,  fields  of,  391. 
Stereoscopy,  393. 
Stilling,  canal  of,  368. 
Stimulations,  alternate,  534. 

intermittent,  532. 
Stratiform  amacrine  cells,  310. 
Stratum  cunei  transversum,  414. 
•     nervosum,  147. 

proprium  cunei,  414. 

Strength  of  a  lens,  determination  of  the,  479. 
Stricture  of  the  lacrymal  sac,  427. 
Stroboscope,  the,  535. 
Stroma,  chorioidal  layer  of  the,  255,  257. 

chorioidea,  163. 

corneal,  222. 

layer,  vascular,  of  the  iris,  273,  274. 

of  the  cornea,  143. 

of  the  iris,  177. 

development  of  the,  52. 

vitrei,  209. 
Studying  the  accuracy  of  perception,  methods  of, 

517. 

Sturm,  focal  interval  of,  497. 
Subarachnoid   space   of    the    optic    nerve,    214, 

348. 

Subdural  space,  the,  214. 
Subepithelial  plexus  of  the  cornea,  157. 

stratum,  221. 
Substance  of  the  vitreous,  209. 

vitreous,  364. 
Substantia  corticalis  lentis,  350. 

fibrosa  corneae,  143. 

propria  cornese,  143,  222,  227. 
Successive  contrast,  611. 
Sulci  of  Monro,  55. 
Sulcus  sclerse  externus,  111. 

scleral,  111. 

Superciliary  eminences,  79. 
Superficial  stroma  plexus  of  the  cornea,  157. 
Superior  oblique  muscle,  98. 

rectus,  91. 

Superposition  of  images  in  the  brain,  393. 
Supporting  fibres  of  the  cornea,  224. 
Supra-chorioid  space,  255. 
Supra-chorioidal  lamina,  134. 

lymph-space,  256. 
Supra-chorioidea,  the,  161. 
Supra-orbital  arteries,  101,  103. 

foramen,  70. 

nerve,  86. 

course  of  the,  106,  107. 

notch,  70. 

Supra-scieral  space,  123. 
Supra-trochlear  nerve,  86,  90. 

course  of  the,  106,  107. 
Supra-vaginal  space,  Schwalbe's,  100,  105. 
Surface,  dioptric,  459. 

reflecting,  459. 
Suspensory  apparatus  of  the  lens,  372. 

ligament,  Lockwood's,  108. 
of  the  lens,  207. 

development  of  the,  50,  418. 
Sustentacular  tissue,  the  retinal,  319. 
Sylvius,  fissure  of,  406. 
Symblepharon,  congenital,  429. 
Sympathetic  filaments  of  the  orbit,  105. 
Synechiae,  congenital  anterior,  424. 

of  the  iris,  anterior,  439. 
System,  dioptric,  474. 

metric,  474. 


T. 

Tiilbot's  law,  535. 
Tangential  fibres,  414. 
Tapetal  fibres,  414. 
Tapetum  cellulosum,  164,  259. 
fibrosum,  164,  259. 
retinal,  628. 
yellow,  628. 
Tarsal  plates,  78. 
Tarsus,  inferior,  92. 
Tear-duct,  formation  of  the,  26. 
Telescope,  Zeiss's  relief,  549. 
Telestereoscope,  Helmholtz's,  549. 
Temporal  artery,  85. 
Tendon  of  Zinn,  the,  213. 
Tenon's  capsule,  89,  96,  99,  123,  245. 

anatomy  of,  99, 
Tenon,  space  of,  49,  123,  245. 
Tension,  intra-ocular,  159. 

of  the  eyeball,  199. 
Tensor  chorioidea,  170,  266,  267. 
tarsi,  development  of  the,  70. 
Terminal  fibres  of  the  corneal  nerves,  239. 
Thalami,  optic,  rudiments  of  the,  23,  24. 
Thalamus,  optic,  23,  24,  395,  396. 
Theories  of  the  vitreous  body,  363. 
Theory,  contact,  315. 

correlation,  of  color-perception,  594. 
of  color-perception,  Ebbinghaus's,  614. 
Helmholtz's,  598. 
Bering's,  613. 
undulation,  of  light,  582. 
Thermic  rays,  583. 
Thickness  of  the  chorioid,  159. 
of  the  cornea,  218. 
of  the  iris,  174. 
ot  the  solera,  242. 
Third  eyelid,  123. 

nerve,  anterior  dorsal  nucleus  of  the,  401. 
anterior  ventral  nucleus  of  the,  401. 
course  of  the,  105. 
development  of  the,  53,  57. 
lateral  anterior  small-celled  nucleus  of 

the,  482. 
mesial  anterior  small-celled  nucleus  of 

the,  482. 

nucleus  of  the,  400,  401. 
posterior  dorsal  nucleus  of  the,  401. 
posterior  ventral  nucleus  of  the,  401. 
Threshold,  the,  505. 

and  the  maximum  sensation,  523. 
values,  605. 
Thomson,    William,   M.D.,  and    Weiland,  Carl, 

M.D.,  on  normal  color-perception,  581. 
Time  of  light,  523. 

of  perception,  the  reaction-time  of  light  and, 

536. 
Tint,  590. 

of  the  sclera,  242. 
Tissue-spaces  of  the  sclera,  243. 
Topography  of  the  occipital  region  of  the  cere- 
bral cortex,  405. 
Total  accommodation,  502. 
Totally  color-blind,  604. 
Tract,  chorioidal,  254. 

optic,  connection  of  the,  with  the  mid-brain, 

397. 

development  of  the,  59. 
intra-cerebral,  415. 
sagittal,  of  Wernicke,  415. 
uveal,  254 
Tractus  spiralis  foraminosus,  132. 

uvealis,  158. 
Transformation  of  light  into  electrical  processes, 

the,  617. 
Transition-colors,  584. 


INDEX   TO   VOLUME   I. 


869 


Transparency  of  the  cornea,  136. 

Transverse  facial  artery,  85. 

Tremulous  iris,  440. 

Trichiasis,  congenital,  429. 

Trichrotnic  eye,  597. 

Trigeminal  nerve,  development  of  the  motor  roo 

of  the,  59. 
Trochlear  nerve,  nucleus  of  the,  400,  401. 

posterior  nucleus  of  the,  401. 
Trochlearis,  development  of  the,  53,  58. 
Tubercles  of  the  anterior  corpora  quadrieemina 

398,  399. 

Tubes,  corneal,  144,  226. 
Tunic  of  the  eyeball,  external  fibrous,  21". 
inner  nervous,  217. 
middle  vascular,  217. 
nervous,  of  the  eye,  2S7. 
Tunica  acinalis,  158. 
aeiniformis,  158. 
alba,  126. 
albuginea,  126. 

arachnoidea  oculi,  126,  161,  206. 
cellulosa,  161. 
chorioidea,  159. 
coerulea,  174. 
externa,  125. 
fibrosa,  125. 
hyaloidea,  206. 
interna,  191. 
media,  158. 
nervosa,  191. 
sclerotica,  126. 
secundina,  158. 
suprachorioidea,  181. 
uvaeformis,  158. 
urea,  158. 
vaginalis  bulbi,  123. 

oculi,  123. 
vasculosa,  158. 
Halleri,  163. 
lentis,  135. 
Twin  cones,  302. 

U. 

Ultra-violet  rays,  585. 

waves,  583. 

Uncrossed  fibres  of  the  optic  chiasm,  389,  392. 
Undulation  theory  of  light,  582. 
Undulatory  theory  of  light,  459. 
Uniocular  color-blindness,  593.  » 
Upper  lip,  formation  of  the,  26. 
Uveal  gland,  269. 

pigment,  ectropion  of  the,  434. 

portion  of  the  iris,  176. 

tract,  254. 

foundation  of  the,  43. 

V. 

Vaginal  tunic,  123. 
Valves  of  the  nasal  duct,  94. 
Variability  of  the  eyebrows,  425. 
Variations  in  color  of  the  iris,  434. 

in  size  of  the  cornea,  congenital,  432. 

of  the  eyeball,  112. 
Vasa  vorticosa,  36. 
Vascular  coat,  the,  158. 

plexuses  of  the  retina,  330. 

pupillary  membrane,  218. 

stroma  layer  of  the  iris,  273,  274. 

tunic  of  the  eye,  254. 

of  the  eyeball,  217. 

Vaso-motor  fibres  of  the  ciliary  body,  271. 
Vein,  anterior  temporal,  86. 

central,  of  the  retina,  104. 

ciliary,  104. 


Vein,  facial,  86. 
frontal,  86,  103. 
inferior  ophthalmic,  104. 
internal  maxillary,  86. 
lacrymal,  104. 
nasal,  103. 
of  the  ohorioid,  259. 
of  the  eyelids,  86. 
of  the  iris,  191. 
ophthalmic,  86. 
ophthaliuo-facial,  104. 
ophthalmo-meningeal,  104 
orbital,  103. 

superior  ophthalmic,  103. 
Velum  confine,  384. 
VenaB  ciliares  posticae,  163. 
stellatae,  133. 

vorticosa,  104,  133,  134,  163,  258. 
Ventral  nucleus,  anterior,  401. 

posterior,  401. 
Ventricle,  fourth,  24. 

third,  development  of  the,  24. 
Vesicle,  blastodermio,  7. 
first  cerebral,  25. 
lens,  417. 
optic,  15. 
primary,  13,  17. 
second  cerebral,  25. 
secondary,  17,  18. 
Vessels,  chorioid,  development  of  the,  49. 

retinal,  congenital  abnormalities  of  the,  447. 
Vibrations  of  light,  459. 
Virtual  focus,  463,  472,  478. 
Visceral  clefts,  17. 

first,  28. 
Visible  ether-waves,  583. 

spectrum,  583. 
Visio  oculi,  182. 

Vision,  alterations  in  the  retina  during,  634. 
conscious,  409. 
dichromic  imperfect,  606. 

perfect,  606. 
field  of,  391. 

effect  of  removal  of  the  visual  area  of  the 

cortex  upon  the,  410. 
importance  of  the   photo-chemical   process 

for,  639. 

mechanism  of  binocular,  392. 
normal  imperfect,  606. 

perfect,  606. 
psychical,  409. 
stereoscopic  field  of,  391. 
Visual  apparatus,  anatomy  of  the  intra-cranial 

portion  of  the,  383. 
area  of  the  cerebral  cortex,  404. 

effect  of  removal  of  the,  upon  the  field 

of  vision,  410. 
extent  of  the,  410. 
cells,  623. 

pigments  of  the,  624. 
centres,  lower,  connection  of  the  occipital 

cortex  with  the,  413. 
fields  and  of  contours,  rivalry  of,  576. 
girdle,  626. 
line,  501. 

purple,  bleaching  of  the,  634. 
chemical  action  of  the,  629. 
photo-chemical   decomposition   of   the, 

628. 

ridge,  626. 

stimuli,  path  of,  59,  60. 
zone,  500. 
'itellin,  624. 
'itreous,  th«,  266. 

alkaline  n-.n-ticm  of  the,  61S. 
Im.ly,  206.  217. 
cavity,  19. 


670 


INDEX    TO   VOLUME    J. 


Vitreous,  cellular  elements  of  the,  368. 

chamber,  111. 

congenital  abnormalities  of  the,  454. 

fibrillas,  366. 

humor,  24,  111,  206. 

development  of  the,  50. 

lamella  of  the  cornea,  149. 

membrane,  203,  256,  260. 

space,  development  of  the,  34. 

substance  of  the,  209,  364. 
Vitreum,  206. 
Vitrina  oculi,  209. 
Volume  of  the  eyeball,  116. 
Vorticose  veins,  133,  163. 

W. 

Walchlis's  doctrine,  632. 

Walls  of  tho  orbit,  anatomy  of  the,  70. 

Wandering  cells  of  the  cornea,  145,  227,  230. 

of  the  sclera,  244. 
Wave-length,  582. 
Waves,  ultra-violet,  583. 

visible  ether-,  583. 
Weber's  law,  514. 
Weight  of  the  cornea,  141. 

of  the  eyeball,  115. 

of  the  lens,  42. 
Weiland,   Carl,    M.D.,  and  Thomson*   William, 

M.D.,  on  normal  color-perception,  581. 
Wernicke,  sagittal  tract  of,  415. 
Wheatstone's  pseudoscope,  551. 


Wheel  of  life,  535. 
White  of  the  eye,  146. 
Winslow,  stars  of,  165. 


X. 


Xanthophane,  632. 
Xanthopsin,  628. 


Y. 

Yellow  spot,  193,  194,  327. 
tapetum,  628. 

Z. 

Zeiss's  relief  telescope,  549. 
Zinn,  circlet  of,  133. 

tendon  of,  213. 

zone  of,  372. 
Zone,  boundary,  of  the  chorioid,  259,  260. 

capillary,  of  the  chorioid,  259,  260. 

of  Zinn,  372. 

visual,  500. 
Zones  and  plates  of  His,  54,  55. 

of  the  retina,  42. 
Zonula  ciliaris,  the,  372. 

Zinnii,  development  of,  50,  53. 
Zonular  cataracts,  452. 

fibres,  374. 

lamella  of  the  lens,  353. 

spaces,  208. 


END  OF  VOL.    I. 


SYSTEM    OF   THE   EYE. 

EDITED   BY 

WILLIAM  F.  NORRIS  AND  CHARLES  A.  OLIVER. 

IN  POUR  VOLUMES. 

Volume  I.  now  ready.     The  remaining  Volumes  will  follow  at  intervals  of  a  few  month*. 
Price,  Cloth,  $20.00;  Sheep,  $24.00;  Half-Russia,  $26.00. 


BY    SUBSCRIPTION    ONLY. 


GENERAL    OUTLINE. 

THE  first  volume  of  this  System  includes  a  most  careful  study  of  the 
embryology  and  development  of  the  organ,  followed  by  a  systematic  and 
extended  account  of  its  general  anatomy,  passing  back  to  those  portions 
of  the  brain  which  are  in  relation.  No  book  of  a  similar  kind  has  ever 
made  so  careful  a  study  of  the  visual  apparatus  beyond  its  entrance  into 
the  cranial  cavity. 

Following  this  is  a  succinct  and  fully  illustrated  account  of  the  congeni- 
tal malformations  and  abnormalities. 

Sufficient  optics  are  given  in  a  graphic  way  to  make  the  physiology  of 
the  organ  comprehensible,  while  the  associated  and  more  complex  rela- 
tions arising  from  the  presence  of  two  similar  and  interrelated  organs  are 
naturally  considered  next. 

Color-perception,  embracing  a  careful  consideration  of  the  most  advanced 
and  certain  of  the  newest  theories,  and  a  thoroughly  practical  article  upon 
the  photo-chemistry  of  the  retina,  conclude  the  volume. 

It  will  thus  be  seen  that  this  the  first  part  of  the  contents  gives  the 
fundamental  principles  upon  which  the  entire  System  is  based. 

The  second  volume  treats  of  the  methods  of  technique  in  examination, 
taking  each  grouping  seriatim.  Beginning  with  the  methods  of  determina- 
tion of  the  acuity  of  vision  and  the  range  of  accommodation,  mydriatics 
and  myotics,  oblique  illumination,  the  ophthalmoscope,  the  retinoscope,  the 
ophthalmometer,  the  phorometer,  and  the  perimeter,  are  all  described  mi- 
nutely. The  plans  for  the  detection  of  color-blindness,  given  in  their  fullest 
detail,  are  next  considered,  followed  by  an  accurate  rfaumt  of  the  conditions 
underlying  the  hygiene  of  the  eyes  in  schools.  The  article  on  statistics  of 
blindness  is  carried  further  forward  and  made  more  general  than  any  other 
paper  or  monogram  upon  the  subject.  Antisepsis  and  bacteriological  re- 
search, including  much  experimental  work  and  giving  the  latest  views, 
conclude  the  volume. 

In  the  third  volume  are  considered  all  the  local  diseases  of  the  organ, 
given  in  strict  logical  order,  each  article  being  contributed  by  an  author 
who  has  made  an  international  reputation  in  his  department.  The  latter 
part  of  the  volume,  which  in  reality  constitutes  a  volume  in  itself,  deals 
with  wounds  and  injuries  of  the  eyeball,  sympathetic  diseases,  and  descrip- 
tions of  the  operations  usually  practised  in  eye  surgery. 

The  fourth  volume  considers  the  relationship  between  general  disturb- 
ances and  local  expressions  of  such  in  the  eye,  making  the  work  a  MOMBftjf 
to  every  general  practitioner  and  specialist  in  other  forms  of  disease. 


CONTRIBUTORS  TO   THE   "SYSTEM." 


JOHN  A.  RYDER,  Pn.D PHILADELPHIA,  PA.,  U.S.A. 

THOMAS  DWIGHT,  M.D.,  LL.D BOSTON,  MASS.,  U  S.A. 

FRANK  BAKER,  M.D.,  PH.D WASHINGTON,  D.C.,  U.S.A. 

GEORGE  A.  PIERSOL,  M.D PHILADELPHIA,  PA.,  U.S.A. 

ALEX   HILL,  M.A.,  M.D CAMBRIDGE,  ENGLAND. 

WILLIAM  LANG,  F.R.C.S.E LONDON,  ENGLAND. 

E.  TREACHER  COLLINS,  F.R.C.S.E LONDON,  ENGLAND. 

J.  McKEEN  CATTELL,  PH.D NEW  YORK  CITY,  N.Y.,  U.S.A. 

EUGEN  BRODHUN,  M.D BERLIN,  GERMANY. 

WILLIAM  THOMSON,  M.D PHILADELPHIA,  PA.,  U.S.A. 

CARL  MAYS,  M.D HEIDELBERG,  GERMANY. 

HERMAN  SNELLEN,  M.D UTRECHT,  HOLLAND. 

HERMAN  SNELLEN,  JR.,  M.D UTRECHT,  HOLLAND. 

L.  LAQUEUR,  M.D STRASSBURG,  ALSACE,  GERMANY. 

GEORGE  M.  GOULD,  A.M.,  M.D PHILADELPHIA,  PA.,  U.S.A. 

EDWARD  JACKSON,  A.M.,  M.D PHILADELPHIA,  PA.,  U.S.A. 

ADOLPHE  JAVAL,  M.D PARIS,  FRANCE. 

WILLIAM  S.  DENNETT,  A.M.,  M.D NEW  YORK  CITY,  N.Y.,  U.S.A. 

GEORGE  T.  STEVENS,  M.D.,  PH.D NEW  YORK  CITY,  N.Y.,  U.S.A. 

HERMAN  WILBRAND,  M.D HAMBURG,  GERMANY. 

SAMUEL   D.  RISLEY,  A.M.,  M.D PHILADELPHIA,  PA.,  U.S.A. 

I.  MINIS  HAYS,  A.M.,  M.D PHILADELPHIA,  PA.,  U.S.A. 

JOSEPH  A.  ANDREWS,  M.D NEW  YORK  CITY,  N.Y.,  U.S.A. 

JOSEPH  McFARLAND,  M.D PHILADELPHIA,  PA.,  U.S.A. 

SAMUEL  STRYKER   KNEASS,  M.D PHILADELPHIA,  PA.,  U.S.A. 

CHARLES  STEDMAN  BULL,  A.M.,  M.D NEW  YORK  CITY,  N.Y.,  U.S.A. 

GEORGE  C.  HARLAN,  A.M.,  M.D PHILADELPHIA,  PA.,  U.S.A. 

SAMUEL  THEOBALD,  M.D BALTIMORE,  MD.,  U.S.A. 

EDMUND  LANDOLT,  M.D PARIS,  FRANCE. 

SWAN  M.  BURNETT,  M.D.,  PH.D WASHINGTON,  D.C.,  U.S.A. 

J.  P.  NUEL,  M.D LIEGE,  BELGIUM. 

W.  A.  BRAILEY,  M.A.,  M.D.  (Cambridge) LONDON,  ENGLAND. 

SYDNEY  STEPHENSON,  M.B LONDON,  ENGLAND. 

CHARLES  A.  OLIVER,  A.M.,  M.D PHILADELPHIA,  PA.,  U.S.A. 

WILLIAM   F.  NORRIS,  A.M.,  M.D PHILADELPHIA,  PA.,  U.S.A. 

A.  HILL  GRIFFITH,  M.D.  (Aberd.) MANCHESTER,  ENGLAND. 

ISIDOR    SCHNABEL,  M.D VIENNA,  AUSTRIA. 

JOSEPH  SCHO'BL,  M.D PRAGUE,  AUSTRIA. 

JOHANN  DEYL,  M.D PRAGUE,  AUSTRIA. 

PRIESTLEY  SMITH,  ESQ BIRMINGHAM,  ENGLAND. 

EMIL  GRUENING,  M.D NEW  YORK  CITY,  N.Y.,  U.S.A. 

ROBERT  L.  RANDOLPH,  M.D BALTIMORE,  MD.,  U.S.A. 

HERMAN  KNAPP,  M.D NEW  YORK  CITY,  N.Y.,  U.S.A. 

O.  HAAB,  M.D ZURICH,  SWITZERLAND. 

HENRY  R.  SWANZY,  A.M.,  M.B.,  F.R.C.S.I DUBLIN,  IRELAND. 

JOHN  B.  STORY,  M.B.,  F.R.C.S.I DUBLIN,  IRELAND. 

J.  SANTOS-FERNANDEZ,  M.D HAVANA,  CUBA. 

M.  PARINAUD,  M.D PARIS,  FRANCE. 

JONATHAN  HUTCHINSON,  JR.,  ESQ LONDON,  ENGLAND. 

MYLES  STANDISH,  A.M.,  M.D BOSTON,  MASS.,  U.S.A. 

GEORGE  E.  DE  SCHWEINITZ,  A.M.,  M.D PHILADELPHIA,  PA.,  U.S.A. 

MAXIMILIAN  SALZMANN,  M.D VIENNA,  AUSTRIA. 

A.  GAYET,  M.D LYONS,  FRANCE. 


CONTENTS. 

VOLUME    I. 

Embryology,  Anatomy,  and  Physiology  of  the  Eye. 

DEVELOPMENT    OF    THE    EYE. 

John  A    Ryder    Ph.D.  Philadelphia,  Pa.,  U.S.A. 

Professor  of  Comparative  Embryology  in  the  University  of  Pennsylvania. 
THE  ANATOMY  OF  THE  ORBIT  AND  THE  APPENDAGES  OF  THE  EYE 

Thomas  Dwight,  M.D.,  LL.D Boston,  Mass.,  U.S.A! 

Parkman  Professor  of  Anatomy  at  Harvard  University. 

THE  ANATOMY  OF  THE  EYEBALL  AND  OF  THE  INTRA-ORBITAL 
PORTION  OF  THE  OPTIC  NERVE. 

Frank  Baker,  M.D.,  Ph.D Washington,  D.C.,  U.S.A. 

Professor  of  Anatomy  in  the  University  of  Georgetown  ;  Honorary  Curator  of  Anatomy  in  the  U  S 
National  Museum 

THE    MICROSCOPICAL   ANATOMY   OF   THE   EYEBALL. 

George  A.  Piersol,  M.D Philadelphia,  Pa.,  U.S.A. 

Professor  of  Anatomy  in  the  University  of  Pennsylvania. 

ANATOMY  OF  THE  INTRA-CRANIAL  PORTION  OF  THE  VISUAL    AP- 
PARATUS. 

Alex  Hill,  M.A.,  M.D Cnmbridge,  England. 

Master  of  Downing  College ;  late  Hunterian  Professor  at  the  Royal  College  of  Surgeons  of  England. 

CONGENITAL     MALFORMATIONS     AND     ABNORMALITIES    OF    THE 
HUMAN    EYE. 
William  Lang,  F.R.C.S.E London,  England. 

Surgeon  to  the  Royal  London  Ophthalmic  Hospital ;  Ophthalmic  Surgeon  to  and  Lecturer  on  Oph- 
thalmology at  the  Middlesex  Hospital. 

E.  Treacher  Collins,  F.R.C.S.E London,  England. 

Curator  and  Librarian  to  the  Royal  London  Ophthalmic  Hospital. 

THE   DIOPTRICS    OF   THE   EYE. 

Edward  Jackson,  A.M.,  M.D Philadelphia,  Pa.,  U.S.A. 

Professor  of  Diseases  of  the  Eye  in  the  Philadelphia  Polyclinic ;  Special  Lecturer  on  Physiological 
Optics  in  the  University  of  Colorado. 

THE    PERCEPTION    OF    LIGHT. 

J.  McKeen  Cattell,  M.A.,  Ph.D New  York  City,  N.Y.,  U  S.A. 

Professor  of  Experimental  Psychology  in  Columbia  College. 

BINOCULAR    VISION,  CONFLICT    OF    THE    FIELDS    OF   VISION,  AP- 
PARENT  AND    NATURAL   SIZE   OF    OBJECTS,  ETC. 
Eugen  Brodhun,  M.D Berlin,  Germany. 

Assistant  in  the  Frederick  William  University. 

Translated  by  Christine  Ladd  Franklin  .    .    .    .    '. Baltimore,  Md.,  U.S.A. 

NORMAL   COLOR-PERCEPTION. 

William  Thomson,  M.D Philadelphia,  Pa.,  U.S.A. 

Professor  of  Ophthalmology  in  the  Jefferson  Medical  College  ;  Attending  Surgeon  to  the  Wills  Eye 

Hospital. 
Assisted  by  Carl  Weiland,  M.D Philadelphia,  Pa.,  U.S.A. 

Clinical  Assistant  in  the  Eye  Department  of  the  Jefferson  Medical  College  Hospital. 
PHOTO-CHEMISTRY    OF    THE    RETINA. 

Carl  Mays,  M.D Heidelberg,  Germany. 

Assistant  in  the  Physiological  Laboratory  of  Heidelberg. 

Translated  by  James  A.  Spalding,  A.M.,  M.D Portland,  Me  ,  U.S.A. 

Ophthalmic  Surgeon  to  the  Maine  Eye  and  Ear  Infirmary,  and  to  the  Maine  General  Hospital. 


VOLUME    II. 

Examination  of  the  Eye,  School  Hygiene,  Statistics  of 
Blindness,  and  Antisepsis. 

ON  THE  METHODS  OF  DETERMINATION  OF  THE  ACUITY  OF  VISION 
AND  THE  RANGE  OF  ACCOMMODATION. 
Herman  Snellen,  M.D Utrecht,  Holland. 

Professor  of  Ophthalmology  in  the  University  of  Utrecht. 
Translated  by  George  A.  Berry,  M.B.,  F.R.C.S.  Ed. 

Ophthalmic  Surgeon  to  the  Edinburgh  Royal  Infirmary ;  Lecturer  on  Ophthalmology  in 
College  of  Surgeons,  Edinburgh. 

3 


MYDRIATICS    AND    MYOTICS. 

Herman  Snellen,  Jr.,  M.D Utrecht,  Holland. 

Ophthalmic  Surgeon  to  the  Netherlands  Hospital. 

OBLIQUE   ILLUMINATION    AND    THE   CORNEAL   MICROSCOPE. 

L.  Laqueur,  M.D Strassburg-in-Alsace,  Germany. 

Professor  of  Ophthalmology,  and   Director  of  the  Ophthalmological  Clinic  in  the  University  of 
Strassburg. 

Translated  by  Harry  Friedenwald,  A. B.,  M.D Baltimore,  Md.,  U.S.A. 

Associate  Professor  of  Ophthalmology  and  Otology  in  the  College  of  Physicians  and  Surgeons  ;  Oph- 
thalmic and  Aural  Surgeon  to  the  Baltimore  City  Hospital. 

THE  OPHTHALMOSCOPE  AND  THE  ART  OF  OPHTHALMOSCOPY. 

George  M.  Gould,  A.M.,  M.D Philadelphia,  Pa.,  U.S.A. 

Late  Ophthalmologist  to  the  Philadelphia  Hospital. 

SKIASCOPY    (THE    SHADOW-TEST,    RETINOSCOPY)    AND    ITS    PRAC- 
TICAL  APPLICATION. 

Edward  Jackson,  A.M..  M.D Philadelphia,  Pa.,  U.S.A. 

Professor  of  Diseases  of  the  Eye  in  the  Philadelphia  Polyciinic  and  College  for  Graduates  in  Aiea- 
icine  ;  Special  Lecturer  on  Physiological  Optics  in  the  University  of  Colorado. 

OPHTHALMOMETRY   AND    ITS   CLINICAL   APPLICATIONS. 

Adolphe  Javal,  M.D Paris,  France. 

(Translated. ) 

PRISMS   AND    PRISMOMETRY. 

William  S.  Dennett,  A.M.,  M.D.     .    .' New  York  City,  N.Y.,  U.S.A. 

Surgeon  to  the  New  York  Eye  and  Ear  Infirmary. 

METHODS     FOR    THE     ESTIMATION     OF    THE     BALANCE     OF     THE 
EXTRA-OCULAR   MUSCLES. 
George  T.  Stevens,  M.D.,  Ph.D.     . New  York  City,  N.Y.,  U.S.A. 

PERIMETRY   AND    ITS   CLINICAL   VALUE. 

Herman  Wilbrand,  M.D Hamburg,  Germany. 

Ophthalmic  Surgeon  to  the  General  Hospital  of  Hamburg. 
Translated  by  Thomas  R.  Pooley,  M.D New  York  City,  N.Y.,  U.S.A. 

Formerly  Professor  of  Ophthalmology  in  the  New  York  Polyciinic  ;  Surgeon  to  the  New  Amsterdam 

Eye  and  Ear  Hospital. 

DETECTION    OF   COLOR   BLINDNESS. 

William  Thomson,  M.D.    .        Philadelphia,  Pa.,  U.S.A. 

Professor  of  Ophthalmology  in  the  Jefferson  Medical  College ;   Attending  Surgeon  to  the  Wills  Eye 
Hospital. 

SCHOOL    HYGIENE. 

Samuel  D.  Risley,  A.M.,  M.D Philadelphia,  Pa.,  U.S.A. 

Attending  Surgeon  to  the  Wills  Eye  Hospital ;    Professor  of  Ophthalmology  in  the  Philadelphia 
Polyciinic  and  School  for  Graduates  in  Medicine. 

STATISTICS   OF   BLINDNESS. 

I.  Minis  Hays,  A.M.,  M.D Philadelphia,  Pa.,  U.S.A. 

ANTISEPSIS. 

Joseph  A.  Andrews,  M.D New  York  City,  N.Y.,  U.S.A. 

Ophthalmic  Surgeon  to  the  Charity  Hospital. 

MICRO-ORGANISMS    OF   THE   CONJUNCTIVAL    SAC   AND    LACRYMAL 
APPARATUS. 
Joseph  McFarland,  M.D Philadelphia,  Pa.,  U.S.A. 

Demonstrator  of  Pathological  Histology  in  the  University  of  Pennsylvania;  Adjunct  Professor  of 
Pathology  in  the  Philadelphia  Polyciinic  and  School  for  Graduates  in  Medicine. 

Samuel  Stryker  Kneass,  M.D Philadelphia,  Pa.,  U.S.A. 

Adjunct   Professor  of  Bacteriology  in   the   Philadelphia    Polyciinic   and   School   for  Graduates  in 
Medicine. 


VOLUMK    III. 

Local  Diseases  of  the  Eye. 


DISEASES    OF   THE   ORBIT. 

Charles  Stedman  Bull,  A.M.,  M.D New  York  City,  N.Y.,  U.S.A. 

Professor  of  Ophthalmology  in  the  University  of  the  City  of  New  York ;  Surgeon  to  the  New  York 
Eye  and  Ear  Infirmary. 

4 


DISEASES    OF    THE    EYELIDS. 

George  C.  Harlan,  A.M.,  M.D  .............   Philadelphia,  Pa.,  U.S.A. 

Surgeon  to  the  Wills  Eye  Hospital,  and  to  the  Eye  and  Ear  Department  of  the  Pennsylvania  Hospital. 

DISEASES    OF   THE    LACRYMAL    APPARATUS. 

Samuel  Theobald,  M.D  .................  Baltimore,  Md.,  U.S.A. 

Clinical  Professor  of  Ophthalmology  in  the  Johns  Hopkins  University  ;  <  )phthalmic  and  Aurai  Sur- 
geon to  the  Johns  Hopkins  Hospital. 

DISEASES    OF    THE    EXTRA-OCULAR    MUSCLES. 

Edmund  Landolt,  M.D  ......................  Paris,  France. 

Translated  by  C.  M.  Culver,  A.M.,  M.D  ...........     Albany,  N.Y.,  U.S.A. 

Uphthalmic  Surgeon  to  the  Albany  Orphan  Asylum. 

DISEASES    OF   THE    CONJUNCTIVA. 

Swan  M.  Burnett,  M.D.  ,  Ph.  D  ............     Washington.  B.C.,  U.S.A. 

Professor  of  Ophthalmology  and  Otology  in  the  University  of  Georgetown  ;  Director  of  the  Eye  and 
Ear  Clinic  at  the  Emergency  Hospital. 

DISEASES    OF    THE    CORNEA. 

J.  P.  Nuel,  M.D  ........................  Liege,  Belgium. 

Professor  of  Ophthalmology  in  the  University  of  Liege. 

Translated  by  Thomas  H.  Fenton,  M.D  .........  Philadelphia,  Pa.,  U.S.A. 

Medical  Director  and  Senior  Ophthalmic  Surgeon  to  the  Union  Mission  Hospital. 

DISEASES   OF   THE   IRIS   AND    CILIARY   BODY. 

W.  A.  Brailey,  M.A.,  M.D.  (Cambridge)  ...........   London,  England. 

Assistant  Ophthalmic  Surgeon  to  Guy's  Hospital  ;  Ophthalmic  Surgeon  to  the  Evelina  Hospital  for 
Children. 

Sydney  Stephenson,  M.B.,  F.R.C.S.  Ed  ............   London,  England. 

Surgeon  to  the  Ophthalmic  School,  Hanwell,  W. 

ERRORS   OF  REFRACTION  :    ETIOLOGY,  COURSE,  AND   TREATMENT. 

Charles  A.  Oliver,  A.M.,  M.D  .............  Philadelphia,  Pa.,  U.S.A. 

One  of  the  Attending  Surgeons  to  Wills  Eye  Hospital  ;  one  of  the  Ophthalmic  Surgeons  to  the 
Philadelphia  Hospital. 

DISEASES   OF   THE   LENS. 

William  F.  Norris,  A.M.,  M.D  .............  Philadelphia,  fa.,  U.S.A 

Professor  of  Ophthalmology  in  the  University  of  Pennsylvania  ;  one  of  the  Attending  Surgeons  to 
Wills  Eye  Hospital. 

DISEASES    OF    THE   CHORIOID    AND    VITREOUS. 

A.  Hill  Griffith,  M.D.  (Aberd.)      .............     Manchester,  England. 

Honorary  Surgeon  to  the  Royal  Eye  Hospital. 

THE   ANATOMY   OF   STAPHYLOMA    POSTICUM. 

Isidor  Schnabel,  M.D  .................  Vienna,  Austria. 

Professor  of  Ophthalmology  in  the  University  of  Vienna. 
Translated  by  Charles  H.  Reed,  M.D.  .        .  .....  Philadelphia,  Pa.,  U.S.A. 

DISEASES    OF   THE   RETINA. 

Joseph  Schobl,  M.D  ......................  Prague,  Austria. 

Professor  of  Ophthalmology  in  the  University  of  Prague. 

Translated  by  Adolf  Alt,  M.D.,  M.C.P.  and  S.  (Ont.)  .    .    .    .    St   Louis,  Mo  ,  U.S.A. 
Professor  of  Ophthalmology  and  Otology  in  the  Beaumont  Hospital  Medical  College  :  < 
Oculist  to  the  Northern  Pacific  Railway  System. 

DISEASES   OF   THE   OPTIC   NERVE. 

Johann  Deyl,  M.D  ...........    ...........  Prt^e>  Austna- 

Professor  of  Ophthalmology  in  the  Royal  Bohemian  Polychmc. 

Translated  by  Robert  Sattler,  M.D. 

GLAUCOMA:     PATHOGENESIS,    SYMPTOMS,    COURSE,    AND    TREAT- 
MENT. 
Priestley  Smith    Esq.  ...............    Birmingham,  England. 

Ophthalmic  Surgeon  to  the  Queen's  Hospital  ;  Lecturer  on  Ophthalmology  in  the  Queen's  1 
of  Medicine,  Mason  College. 
WOUNDS  AND  INJURIES  OF  THE  EYEBALL  AND   ITS  APPENDAGES. 

Fmil    Grneninp-    M  D  .......    New  York  C^'  N-Y-  U.S.A. 

Professor  of  Ophthalmology  'in  'th'e  New  York  Polyclinic  ;  Ophthalmic  Surgeon  to  the  New  York 
Eye  and  Ear  Infirmary. 
SYMPATHETIC    IRRITATION   AND   SYMPATHETIC   INFLAMMAT1 


.      nht.noandOto.o,  Johns      opknsUniversty. 

Aural  Surgeon  to  the  Johns  Hopkins  Hospital. 

A.    DESCRIPTION    OF    THE    OPERATIONS    USUALLY    PRACTISED     IN 

SU5n   nn  YM  D  New  York  Citv,  N  Y..  M  A. 

HarofesSofPSPh?h;fmo.ogv  in  the'  College  'of  Physicians  and  Surgeons  ;  Surgeon  to  ,he  New  York 
Ophthalmic  and  Aural  Institute.  5 


VOLUMK   IV. 

Medical  Ophthalmology. 


OCULAR  LESIONS  DEPENDENT  UPON  DISEASES  IN  THE  CIRCULA- 
TORY SYSTEM  (GENERAL  AND  PERNICIOUS  ANEMIA,  LEUCO- 
CYTH^EMIA,  CARDIAC  AFFECTIONS,  THROMBOSIS,  EMBOLISM, 
PERIVASCULITIS,  ENDARTERITIS,  RECURRENT  RETINAL  HEM- 
ORRHAGE IN  ADOLESCENTS,  ETC.). 
O.  Haab,  M.D Zurich,  Switzerland. 

Professor  in  the  University  of  Zurich,  and  Director  of  the  Eye-Clinic  at  Zurich. 
Translated  by  William  Zentmayer,  M.D Philadelphia,  Pa.,  U.S.A. 

Assistant  Surgeon  to  Wills  Eye  Hospital ;  Ophthalmic  Surgeon  to  St.  Mary's  Hospital. 

SENSORY     AND      MOTOR      OCULAR     CHANGES      RESULTING      FROM 
LESIONS    OF   THE   BRAIN    AND    SPINAL   CORD. 
Henry  R.  Swanzy,  A.M.,  M.B.,  F.R.C.S.I Dublin,  Ireland. 

Surgeon  to  the  National  Eye  and  Ear  Infirmary;  Ophthalmic  Surgeon  to  the  Adelaide  Hospital. 

OCULAR  LESIONS  IN  SPECIAL  DYSCRASI^E  (VARIOLA,  RUBEOLA, 
SCARLATINA,  ERYSIPELAS,  RELAPSING  TYPHUS  FEVER,  ETC.). 
John  B.  Story,  M.B.,  F.R.C.S.I Dublin,  Ireland. 

Surgeon  to  St.  Mark's  Ophthalmic  Hospital;  Professor  of  Ophthalmology  in  the  Royal  College  of 
Surgeons,  Ireland. 

OCULAR    LESIONS    IN     SPECIAL    DYSCRASI^    (INFLUENZA,    DYSEN- 
TERY, CHOLERA,  MALARIAL  FEVER,  DENGUE,  YELLOW  FEVER, 
ETC.). 
J.  Santos-Fernandez,  M.D Havana,  Cuba. 

Director  of  the  Histo-Bacteriological  Laboratory  of  the  Cronica  Medica-Quirurgica. 
Translated  by  Daniel  Guiteras,  M.D. 

Passed  Assistant  Surgeon,  U.  S.  Navy. 

EYE  AFFECTIONS  DUE  TO  HYSTERIA. 

M.  Parinaud,  M.D Paris,  France. 

Senior  Chief  of  the  Ophthalmological  Service  of  the  Salpetriere  Hospital. 

Translated  by  Casey  A.  Wood,  M.C.,  M.D Chicago,  111.,  U.S.A. 

Professor  of  Ophthalmology  in  the  Post-Graduate  Medical  School ;  Ophthalmic  Surgeon  to  the  Cook 
County  Hospital. 

EYE   AFFECTIONS    DUE    TO   GRAVES'S    DISEASE    AND    HERPES  ZOS- 
TER. 

Jonathan  Hutchinson,  Jr.,  Esq London,  England. 

Assistant  Surgeon  to  the  London  Hospital. 

MOTOR  CHANGES  IN  THE  OCULAR  APPARATUS  ASSOCIATED  WITH 
FUNCTIONAL   NEUROSES. 
Myles  Standish,  A.M.,  M.D Boston,  Mass.,  U.S.A. 

Ophthalmic  Surgeon  to  the  Massachusetts  Charitable  Eye  and  Ear  Infirmary;  Assistant  in  Ophthal- 
mology in  the  Harvard  Medical  School. 

TOXIC   AMBLYOPIAS. 

George  E.  de  Schweinitz,  A.M.,  M.D Philadelphia,  Pa.,  U.S.A. 

Clinical  Professor  of  Ophthalmology  in  the  Jefferson  Medical  College ;  Ophthalmic  Surgeon  to  the 
Philadelphia  Hospital. 

PARASITES   OF    THE   HUMAN    EYE. 

Maximilian  Salzmann,  M.D Vienna,  Austria. 

First  Assistant  in  Professor  Fuch's  Eye  Clinic,  Vienna,  Austria. 

Translated  by  H.  V.  Wiirdemann,  M.D Milwaukee,  Wis.,  U.S.A. 

Director  and  Secretary,  Wisconsin  General  Hospital  Association  ;  Oculist  and  Aurist  to  the  Children's 
Hospital. 

OCULAR    SIGNS   OF   DEATH. 

A.  Gayet,  M.D Lyons,  France. 

Professor  of  Ophthalmology  and  Chief  Ex-Surgeon  to  the  Hotel  Dieu. 
Translated  by  E.  C.  Ellett,  M.D Memphis,  Tenn.,  U.S.A. 

Assistant  Ophthalmologist  to  the  East  Side  Dispensary. 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 

Los  Angeles 

This  book  is  DUE  on  the  last  date  stamped  below. 


BIOMEO.  Ll& 

1*  JUN151971 

JUN    2tttt 


STACK 

JAN  12 1987 
ANNEX 


OCT141986 

NON-AfcNEVVAILE 

BIOMEOUB: 
OCT061986 

REC'D 

Form  L9-Series  4939 


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STACK 

OCT051987 


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